Contains questions and exercises on various technical topics, sometimes related to DevOps and SRE

This repo contains questions and exercises on various technical topics, sometimes related to DevOps and SRE :)

There are currently 1825 questions

To learn more about DevOps and SRE, check the resources in devops-resources repository

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DevOps

DevOps Exercises

Name	Topic	Objective & Instructions	Solution	Comments
Set up a CI pipeline	CI	Exercise
Deploy to Kubernetes	Deployment	Exercise	Solution

DevOps Self Assessment

What is DevOps?

You can answer it by describing what DevOps means to you and/or rely on how companies define it. I've put here a couple of examples.

Amazon:

"DevOps is the combination of cultural philosophies, practices, and tools that increases an organization’s ability to deliver applications and services at high velocity: evolving and improving products at a faster pace than organizations using traditional software development and infrastructure management processes. This speed enables organizations to better serve their customers and compete more effectively in the market."

Microsoft:

"DevOps is the union of people, process, and products to enable continuous delivery of value to our end users. The contraction of “Dev” and “Ops” refers to replacing siloed Development and Operations to create multidisciplinary teams that now work together with shared and efficient practices and tools. Essential DevOps practices include agile planning, continuous integration, continuous delivery, and monitoring of applications."

Red Hat:

"DevOps describes approaches to speeding up the processes by which an idea (like a new software feature, a request for enhancement, or a bug fix) goes from development to deployment in a production environment where it can provide value to the user. These approaches require that development teams and operations teams communicate frequently and approach their work with empathy for their teammates. Scalability and flexible provisioning are also necessary. With DevOps, those that need power the most, get it—through self service and automation. Developers, usually coding in a standard development environment, work closely with IT operations to speed software builds, tests, and releases—without sacrificing reliability."

Google:

"...The organizational and cultural movement that aims to increase software delivery velocity, improve service reliability, and build shared ownership among software stakeholders"

What are the benefits of DevOps? What can it help us to achieve?

Collaboration
Improved delivery
Security
Speed
Scale
Reliability

What are the anti-patterns of DevOps?

A couple of examples:

One person is in charge of specific tasks. For example there is only one person who is allowed to merge the code of everyone else into the repository.
Treating production differently from development environment. For example, not implementing security in development environment
Not allowing someone to push to production on Friday ;)

How would you describe a successful DevOps engineer or a team?

The answer can focus on:

Collaboration
Communication
Set up and improve workflows and processes (related to testing, delivery, ...)
Dealing with issues

Things to think about:

What DevOps teams or engineers should NOT focus on or do?
Do DevOps teams or engineers have to be innovative or practice innovation as part of their role?

Tooling

What are you taking into consideration when choosing a tool/technology?

A few ideas to think about:

mature/stable vs. cutting edge
community size
architecture aspects - agent vs. agentless, master vs. masterless, etc.
learning curve

Can you describe which tool or platform you chose to use in some of the following areas and how?

CI/CD
Provisioning infrastructure
Configuration Management
Monitoring & alerting
Logging
Code review
Code coverage
Issue Tracking
Containers and Containers Orchestration
Tests

This is a more practical version of the previous question where you might be asked additional specific questions on the technology you chose

CI/CD - Jenkins, Circle CI, Travis, Drone, Argo CD, Zuul
Provisioning infrastructure - Terraform, CloudFormation
Configuration Management - Ansible, Puppet, Chef
Monitoring & alerting - Prometheus, Nagios
Logging - Logstash, Graylog, Fluentd
Code review - Gerrit, Review Board
Code coverage - Cobertura, Clover, JaCoCo
Issue tracking - Jira, Bugzilla
Containers and Containers Orchestration - Docker, Podman, Kubernetes, Nomad
Tests - Robot, Serenity, Gauge

A team member of yours, suggests to replace the current CI/CD platform used by the organization with a new one. How would you reply?

Things to think about:

What we gain from doing so? Are there new features in the new platform? Does the new platform deals with some of the limitations presented in the current platform?
What this suggestion is based on? In other words, did he/she tried out the new platform? Was there extensive technical research?
What does the switch from one platform to another will require from the organization? For example, training users who use the platform? How much time the team has to invest in such move?

Version Control

What is Version Control?

Version control is the sytem of tracking and managing changes to software code.
It helps software teams to manage changes to source code over time.
Version control also helps developers move faster and allows software teams to preserve efficiency and agility as the team scales to include more developers.

What is a commit?

In Git, a commit is a snapshot of your repo at a specific point in time.
The git commit command will save all staged changes, along with a brief description from the user, in a “commit” to the local repository.

What is a merge?

Merging is Git's way of putting a forked history back together again. The git merge command lets you take the independent lines of development created by git branch and integrate them into a single branch.

What is a merge conflict?

A merge conflict is an event that occurs when Git is unable to automatically resolve differences in code between two commits. When all the changes in the code occur on different lines or in different files, Git will successfully merge commits without your help.

What best practices are you familiar with regarding version control?

Use a descriptive commit message
Make each commit a logical unit
Incorporate others' changes frequently
Share your changes frequently
Coordinate with your co-workers
Don't commit generated files

CI/CD

What is Continuous Integration?

A development practice where developers integrate code into a shared repository frequently. It can range from a couple of changes every day or a week to a couple of changes in one hour in larger scales.

Each piece of code (change/patch) is verified, to make the change is safe to merge. Today, it's a common practice to test the change using an automated build that makes sure the code can integrated. It can be one build which runs several tests in different levels (unit, functional, etc.) or several separate builds that all or some has to pass in order for the change to be merged into the repository.

What is Continuous Deployment?

A development strategy used by developers to release software automatically into production where any code commit must pass through an automated testing phase. Only when this is successful is the release considered production worthy. This eliminates any human interaction and should be implemented only after production-ready pipelines have been set with real-time monitoring and reporting of deployed assets. If any issues are detected in production it should be easy to rollback to previous working state.

For more info please read here

Can you describe an example of a CI (and/or CD) process starting the moment a developer submitted a change/PR to a repository?

There are many answers for such a question, as CI processes vary, depending on the technologies used and the type of the project to where the change was submitted.
Such processes can include one or more of the following stages:

Compile
Build
Install
Configure
Update
Test

An example of one possible answer:

A developer submitted a pull request to a project. The PR (pull request) triggered two jobs (or one combined job). One job for running lint test on the change and the second job for building a package which includes the submitted change, and running multiple api/scenario tests using that package. Once all tests passed and the change was approved by a maintainer/core, it's merged/pushed to the repository. If some of the tests failed, the change will not be allowed to merged/pushed to the repository.

A complete different answer or CI process, can describe how a developer pushes code to a repository, a workflow then triggered to build a container image and push it to the registry. Once in the registry, the k8s cluster is applied with the new changes.

What is Continuous Delivery?

A development strategy used to frequently deliver code to QA and Ops for testing. This entails having a staging area that has production like features where changes can only be accepted for production after a manual review. Because of this human entanglement there is usually a time lag between release and review making it slower and error prone as compared to continous deployment.

For more info please read here

What CI/CD best practices are you familiar with? Or what do you consider as CI/CD best practice?

Commit and test often.
Testing/Staging environment should be a clone of production environment.
Clean up your environments (e.g. your CI/CD pipelines may create a lot of resources. They should also take care of cleaning up everything they create)
The CI/CD pipelines should provide the same results when executed locally or remotely
Treat CI/CD as another application in your organization. Not as a glue code.
On demand environments instead of pre-allocated resources for CI/CD purposes
Stages/Steps/Tasks of pipelines should be shared between applications or microservices (don't re-invent common tasks like "cloning a project")

You are given a pipeline and a pool with 3 workers: virtual machine, baremetal and a container. How will you decide on which one of them to run the pipeline?
Where do you store CI/CD pipelines? Why?

There are multiple approaches as to where to store the CI/CD pipeline definitions:

App Repository - store them in the same repository of the application they are building or testing (perhaps the most popular one)
Central Repository - store all organization's/project's CI/CD pipelines in one separate repository (perhaps the best approach when multiple teams test the same set of projects and they end up having many pipelines)
CI repo for every app repo - you separate CI related code from app code but you don't put everything in one place (perhaps the worst option due to the maintenance)
The platform where the CI/CD pipelines are being executed (e.g. Kubernetes Cluster in case of Tekton/OpenShift Pipelines).

Would you prefer a "configuration->deployment" model or "deployment->configuration"? Why?

Both have advantages and disadvantages.
With "configuration->deployment" model for example, where you build one image to be used by multiple deployments, there is less chance of deployments being different from one another, so it has a clear advantage of a consistent environment.

Explain mutable vs. immutable infrastructure

In mutable infrastructure paradigm, changes are applied on top of the existing infrastructure and over time
the infrastructure builds up a history of changes. Ansible, Puppet and Chef are examples of tools which
follow mutable infrastructure paradigm.

In immutable infrastructure paradigm, every change is actually a new infrastructure. So a change
to a server will result in a new server instead of updating it. Terraform is an example of technology
which follows the immutable infrastructure paradigm.

Software Distribution

Explain "Software Distribution"

Read this fantastic article on the topic.

From the article: "Thus, software distribution is about the mechanism and the community that takes the burden and decisions to build an assemblage of coherent software that can be shipped."

Why are there multiple software distributions? What differences they can have?

Different distributions can focus on different things like: focus on different environments (server vs. mobile vs. desktop), support specific hardware, specialize in different domains (security, multimedia, ...), etc. Basically, different aspects of the software and what it supports, get different priority in each distribution.

What is a Software Repository?

Wikipedia: "A software repository, or “repo” for short, is a storage location for software packages. Often a table of contents is stored, as well as metadata."

Read more here

What ways are there to distribute software? What are the advantages and disadvantages of each method?

Source - Maintain build script within version control system so that user can build your app after cloning repository. Advantage: User can quickly checkout different versions of application. Disadvantage: requires build tools installed on users machine.
Archive - collect all your app files into one archive (e.g. tar) and deliver it to the user. Advantage: User gets everything he needs in one file. Disadvantage: Requires repeating the same procedure when updating, not good if there are a lot of dependencies.
Package - depends on the OS, you can use your OS package format (e.g. in RHEL/Fefodra it's RPM) to deliver your software with a way to install, uninstall and update it using the standard packager commands. Advantages: Package manager takes care of support for installation, uninstallation, updating and dependency management. Disadvantage: Requires managing package repository.
Images - Either VM or container images where your package is included with everything it needs in order to run successfully. Advantage: everything is preinstalled, it has high degree of environment isolation. Disadvantage: Requires knowledge of building and optimizing images.

Are you familiar with "The Cathedral and the Bazaar models"? Explain each of the models

Cathedral - source code released when software is released
Bazaar - source code is always available publicly (e.g. Linux Kernel)

What is caching? How does it works? Why is it important?

Caching is fast access to frequently used resources which are computationally expensive or IO intensive and do not change often. There can be several layers of cache that can start from CPU caches to distributed cache systems. Common ones are in memory caching and distributed caching.
Caches are typically data structures that contains some data, such as a hashtable or dictionary. However, any data structure can provide caching capabilities, like set, sorted set, sorted dictionary etc. While, caching is used in many applications, they can create subtle bugs if not implemented correctly or used correctly. For example,cache invalidation, expiration or updating is usually quite challenging and hard.

Explain stateless vs. stateful

Stateless applications don't store any data in the host which makes it ideal for horizontal scaling and microservices.
Stateful applications depend on the storage to save state and data, typically databases are stateful applications.

What is Reliability? How does it fit DevOps?

Reliability, when used in DevOps context, is the ability of a system to recover from infrastructure failure or disruption. Part of it is also being able to scale based on your organization or team demands.

What "Availability" means? What means are there to track Availability of a service?
Why 100% availability isn't a target? Why most companies or teams set it to be 99%.X?
Describe the workflow of setting up some type of web server (Apache, IIS, Tomcat, ...)
How a web server works?
Explain "Open Source"
Describe me the architecture of service/app/project/... you designed and/or implemented
What types of tests are you familiar with?

Styling, unit, functional, API, integration, smoke, scenario, ...

You should be able to explain those that you mention.

You need to install periodically a package (unless it's already exists) on different operating systems (Ubuntu, RHEL, ...). How would you do it?

There are multiple ways to answer this question (there is no right and wrong here):

Simple cron job
Pipeline with configuration management technology (such Puppet, Ansible, Chef, etc.)
...

What is Chaos Engineering?

Wikipedia: "Chaos engineering is the discipline of experimenting on a software system in production in order to build confidence in the system's capability to withstand turbulent and unexpected conditions"

Read about Chaos Engineering here

What is "infrastructure as code"? What implementation of IAC are you familiar with?

IAC (infrastructure as code) is a declerative approach of defining infrastructure or architecture of a system. Some implementations are ARM templates for Azure and Terraform that can work across multiple cloud providers.

What benefits infrastructure-as-code has?

fully automated process of provisioning, modifying and deleting your infrastructure
version control for your infrastructure which allows you to quickly rollback to previous versions
validate infrastructure quality and stability with automated tests and code reviews
makes infrastructure tasks less repetitive

How do you manage build artifacts?

Build artifacts are usually stored in a repository. They can be used in release pipelines for deployment purposes. Usually there is retention period on the build artifacts.

What Continuous Integration solution are you using/prefer and why?
What deployment strategies are you familiar with or have used?

There are several deployment strategies:
* Rolling
* Blue green deployment
* Canary releases
* Recreate strategy

You joined a team where everyone developing one project and the practice is to run tests locally on their workstation and push it to the repository if the tests passed. What is the problem with the process as it is now and how to improve it?
Explain test-driven development (TDD)
Explain agile software development
What do you think about the following sentence?: "implementing or practicing DevOps leads to more secure software"
Do you know what is a "post-mortem meeting"? What is your opinion on that?
How do you perform plan capacity for your CI/CD resources? (e.g. servers, storage, etc.)
How would you structure/implement CD for an application which depends on several other applications?
How do you measure your CI/CD quality? Are there any metrics or KPIs you are using for measuring the quality?
What is a configuration drift? What problems is it causing?

Configuration drift happens when in an environment of servers with the exact same configuration and software, a certain server
or servers are being applied with updates or configuration which other servers don't get and over time these servers become
slightly different than all others.

This situation might lead to bugs which hard to identify and reproduce.

How to deal with a configuration drift?
Configuration drift can be avoided with desired state configuration (DSC) implementation. Desired state configuration can be a declarative file that defined how a system should be. There are tools to enforce desired state such a terraform or azure dsc. There are incramental or complete strategies. Explain Declarative and Procedural styles. The technologies you are familiar with (or using) are using procedural or declarative style?

Declarative - You write code that specifies the desired end state
Procedural - You describe the steps to get to the desired end state

Declarative Tools - Terraform, Puppet, CloudFormation
Procedural Tools - Ansible, Chef

To better emphasize the difference, consider creating two virtual instances/servers.
In declarative style, you would specify two servers and the tool will figure out how to reach that state.
In procedural style, you need to specify the steps to reach the end state of two instances/servers - for example, create a loop and in each iteration of the loop create one instance (running the loop twice of course).

Do you have experience with testing cross-projects changes? (aka cross-dependency)

Note: cross-dependency is when you have two or more changes to separate projects and you would like to test them in mutual build instead of testing each change separately.

Have you contributed to an open source project? Tell me about this experience
What is Distributed Tracing?
What is GitOps?

GitLab: "GitOps is an operational framework that takes DevOps best practices used for application development such as version control, collaboration, compliance, and CI/CD tooling, and applies them to infrastructure automation".

Read more here

SRE

What are the differences between SRE and DevOps?

Google: "One could view DevOps as a generalization of several core SRE principles to a wider range of organizations, management structures, and personnel."

Read more about it here

What SRE team is responsible for?

Google: "the SRE team is responsible for availability, latency, performance, efficiency, change management, monitoring, emergency response, and capacity planning of their services"

Read more about it here

What is an error budget?

Atlassian: "An error budget is the maximum amount of time that a technical system can fail without contractual consequences."

Read more about it here

What do you think about the following statement: "100% is the only right availability target for a system"

Wrong. No system can guarantee 100% availability as no system is safe from experiencing zero downtime.
Many systems and services will fall somewhere between 99% and 100% uptime (or at least this is how most systems and services should be).

What are MTTF (mean time to failure) and MTTR (mean time to repair)? What these metrics help us to evaluate?

* MTTF (mean time to failure) other known as uptime, can be defined as how long the system runs before if fails.
* MTTR (mean time to recover) on the other hand, is the amount of time it takes to repair a broken system.
* MTBF (mean time between failures) is the amount of time between failures of the system.

What is the role of monitoring in SRE?

Google: "Monitoring is one of the primary means by which service owners keep track of a system’s health and availability"

Read more about it here

Jenkins

Jenkins Exercises

Name	Topic	Objective & Instructions	Solution
Jobs 101	Jobs	Exercise
Remove Jobs	Scripts - Jobs	Exercise	Solution
Remove Builds	Scripts - Builds	Exercise	Solution

Jenkins Self Assessment

What is Jenkins? What have you used it for?

Jenkins is an open source automation tool written in Java with plugins built for Continuous Integration purpose. Jenkins is used to build and test your software projects continuously making it easier for developers to integrate changes to the project, and making it easier for users to obtain a fresh build. It also allows you to continuously deliver your software by integrating with a large number of testing and deployment technologies.

Jenkins integrates development life-cycle processes of all kinds, including build, document, test, package, stage, deploy, static analysis and much more.

What are the advantages of Jenkins over its competitors? Can you compare it to one of the following systems?

Travis
Bamboo
Teamcity
CircleCI

What are the limitations or disadvantages of Jenkins?

This might be considered to be an opinionated answer:

Old fashioned dashboards with not many options to customize it
Containers readiness (this has improved with Jenkins X)
By itself, it doesn't have many features. On the other hand, there many plugins created by the community to expand its abilities
Managing Jenkins and its piplines as a code can be one hell of a nightmare

Explain the following:

Job
Build
Plugin
Node or Worker
Executor
Job is an automation definition = what and where to execute once the user clicks on "build"
Build is a running instance of a job. You can have one or more builds at any given point of time (unless limited by confiugration)
A worker is the machine/instance on which the build is running. When a build starts, it "acquires" a worker out of a pool to run on it.
An executor is variable of the worker, defining how many builds can run on that worker in parallel. An executor value of 3 means, that 3 builds can run at any point on that executor (not necessarily of the same job. Any builds)

What plugins have you used in Jenkins?
Have you used Jenkins for CI or CD processes? Can you describe them?
What type of jobs are there? Which types have you used?
How did you report build results to users? What ways are there to report the results?

You can report via:

Emails
Messaging apps
Dashboards

Each has its own disadvantages and advantages. Emails for example, if sent too often, can be eventually disregarded or ignored.

You need to run unit tests every time a change submitted to a given project. Describe in details how your pipeline would look like and what will be executed in each stage

The pipelines will have multiple stages:

Clone the project
Install test dependencies (for example, if I need tox package to run the tests, I will install it in this stage)
Run unit tests
(Optional) report results (For example an email to the users)
Archive the relevant logs/files

How to secure Jenkins?

Jenkins documentation provides some basic intro for securing your Jenkins server.

Describe how do you add new nodes (agents) to Jenkins

You can describe the UI way to add new nodes but better to explain how to do in a way that scales like a script or using dynamic source for nodes like one of the existing clouds.

How to acquire multiple nodes for one specific build?
Whenever a build fails, you would like to notify the team owning the job regarding the failure and provide failure reason. How would you do that?
There are four teams in your organization. How to prioritize the builds of each team? So the jobs of team x will always run before team y for example
If you are managing a dozen of jobs, you can probably use the Jenkins UI. But how do you manage the creation and deletion of hundreds of jobs every week/month?
What are some of Jenkins limitations?

Testing cross-dependencies (changes from multiple projects together)
Starting builds from any stage (although Cloudbees implemented something called checkpoints)

What is the different between a scripted pipeline to declarative pipeline? Which type are you using?
How would you implement an option of a starting a build from a certain stage and not from the beginning?

Jenkins Dev

Do you have experience with developing a Jenkins plugin? Can you describe this experience?
Have you written Jenkins scripts? If yes, what for and how they work?

Cloud

What is Cloud Computing? What is a Cloud Provider?

Cloud computing refers to the delivery of on-demand computing services
over the internet on a pay-as-you-go basis.

In simple words, Cloud computing is a service that lets you use any computing
service such as a server, storage, networking, databases, and intelligence,
right through your browser without owning anything. You can do anything you
can think of unless it doesn’t require you to stay close to your hardware.

Cloud service providers are companies that establish public clouds, manage private clouds, or offer on-demand cloud computing components (also known as cloud computing services) like Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS), and Software-as-a-Service(SaaS). Cloud services can reduce business process costs when compared to on-premise IT.

What are the advantages of cloud computing? Mention at least 3 advantages

Pay as you go (or consumption-based payment) - you are paying only for what you are using. No upfront payments and payment stops when resources are no longer used.
Scalable - resources are scaled down or up based on demand

What types of Cloud Computing services are there?

IAAS - Infrastructure as a Service
PAAS - Platform as a Service
SAAS - Software as a Service

Explain each of the following and give an example:

IAAS
PAAS
SAAS
IAAS - Users have control over complete Operating System and don't need to worry about the physical resources, which is managed by Cloud Service Provider.
PAAS - CLoud Service Provider takes care of Operating System, Middlewares and users only need to focus on our Data and Application.
SAAS - A cloud based method to provide software to users, software logics running on cloud, can be run on-premises or managed by Cloud Service Provider.

What types of clouds (or cloud deployments) are there?

Public
Hybrid
Private

Explain each of the following Cloud Computing Deployments:

Public
Private
Hybrid
Public - Cloud services sharing computing resources among multiple customers
Private - Cloud services having computing resources limited to specific customer or organization, managed by third party or organizations itself
Hybrid - Combination of public and private clouds

What are the differences between Cloud Providers and On-Premise solution?

In cloud providers, someone else owns and manages the hardware, hire the relevant infrastructure teams and pays for real-estate (for both hardware and people). You can focus on your business.

In On-Premise solution, it's quite the opposite. You need to take care of hardware, infrastructure teams and pay for everything which can be quite expensive. On the other hand it's tailored to your needs.

What is Serverless Computing?

The main idea behind serverless computing is that you don't need to manage the creation and configuration of server. All you need to focus on is splitting your app into multiple functions which will be triggered by some actions.

It's important to note that:

Serverless Computing is still using servers. So saying there are no servers in serverless computing is completely wrong
Serverless Computing allows you to have a different paying model. You basically pay only when your functions are running and not when the VM or containers are running as in other payment models

Can we replace any type of computing on servers with serverless?
Is there a difference between managed service to SaaS or is it the same thing?
What is auto scaling?

AWS definition: "AWS Auto Scaling monitors your applications and automatically adjusts capacity to maintain steady, predictable performance at the lowest possible cost"

Read more about auto scaling here

True or False? Auto Scaling is about adding resources (such as instances) and not about removing resource

False. Auto scaling adjusts capacity and this can mean removing some resources based on usage and performances.

AWS

AWS Exercises

AWS Lambda

Name	Topic	Objective & Instructions	Solution	Comments
Hello Function	Lambda	Exercise	Solution
URL Function	Lambda	Exercise	Solution

AWS Self Assessment

AWS - Global Infrastructure

Explain the following

Availability zone
Region
Edge location

AWS regions are data centers hosted across different geographical locations worldwide.

Within each region, there are multiple isolated locations known as Availability Zones. Multiple availability zones ensure high availability in case one of them goes down.

Edge locations are basically content delivery network which caches data and insures lower latency and faster delivery to the users in any location. They are located in major cities in the world.

True or False? Each AWS region is designed to be completely isolated from the other AWS regions

True.

Do you agree with the statement "AWS region should be chosen based on proximity alone"?

Note: opinionated answer.

No. There are a couple of factors to consider when choosing a region (order doesn't mean anything):

Cost - regions vary in cost and AWS Price List API can assist in calculating the difference in cost between the different regions.
Speed
Features

AWS IAM

What is IAM? What are some of its features?

Full explanation is here
In short: it's used for managing users, groups, access policies & roles

True or False? IAM configuration is defined globally and not per region

True

Given an example of IAM best practices?

Set up MFA
Delete root account access keys
Create IAM users instead of using root for daily management

What are Roles?

A way for allowing a service of AWS to use another service of AWS. You assign roles to AWS resources.
For example, you can make use of a role which allows EC2 service to acesses s3 buckets (read and write).

What are Policies?

Policies documents used to give permissions as to what a user, group or role are able to do. Their format is JSON.

A user is unable to access an s3 bucket. What might be the problem?

There can be several reasons for that. One of them is lack of policy. To solve that, the admin has to attach the user with a policy what allows him to access the s3 bucket.

What should you use to:

Grant access between two services/resources?
Grant user access to resources/services?
Role
Policy

What permissions does a new user have?

Only a login access.

AWS - Compute

What is EC2?

"a web service that provides secure, resizable compute capacity in the cloud".
Read more here

What is AMI?

Amazon Machine Images is "An Amazon Machine Image (AMI) provides the information required to launch an instance".
Read more here

What are the different source for AMIs?

Personal AMIs - AMIs you create
AWS Marketplace for AMIs - Paid AMIs usually with bundled with licensed software
Community AMIs - Free

What is instance type?

"the instance type that you specify determines the hardware of the host computer used for your instance"
Read more about instance types here

True or False? The following are instance types available for a user in AWS:

Compute optimizied
Network optimizied
Web optimized

False. From the above list only compute optimized is available.

What is EBS?

"provides block level storage volumes for use with EC2 instances. EBS volumes behave like raw, unformatted block devices."
More on EBS here

What EC2 pricing models are there?

On Demand - pay a fixed rate by the hour/second with no commitment. You can provision and terminate it at any given time.
Reserved - you get capacity reservation, basically purchase an instance for a fixed time of period. The longer, the cheaper.
Spot - Enables you to bid whatever price you want for instances or pay the spot price.
Dedicated Hosts - physical EC2 server dedicated for your use.

What are Security Groups?

"A security group acts as a virtual firewall that controls the traffic for one or more instances"
More on this subject here

How to migrate an instance to another availability zone?
What can you attach to an EC2 instance in order to store data?

EBS

What EC2 RI types are there?

Standard RI - most significant discount + suited for steady-state usage
Convertible RI - discount + change attribute of RI + suited for steady-state usage
Scheduled RI - launch within time windows you reserve

Learn more about EC2 RI here

You would like to invoke a function every time you enter a URL in the browser. Which service would you use for that?

AWS Lambda

AWS - Lambda

Explain what is AWS Lambda

AWS definition: "AWS Lambda lets you run code without provisioning or managing servers. You pay only for the compute time you consume."

Read more on it here

True or False? In AWS Lambda, you are charged as long as a function exists, regardless of whether it's running or not

False. Charges are being made when the code is executed.

Which of the following set of languages Lambda supports?

R, Swift, Rust, Kotlin
Python, Ruby, Go
Python, Ruby, PHP
Python, Ruby, Go

True or False? Basic lambda permissions allow you only to upload logs to Amazon CloudWatch Logs

True

AWS Containers

What is Amazon ECS?

Amazon definition: "Amazon Elastic Container Service (Amazon ECS) is a fully managed container orchestration service. Customers such as Duolingo, Samsung, GE, and Cook Pad use ECS to run their most sensitive and mission critical applications because of its security, reliability, and scalability."

Learn more here

What is Amazon ECR?

Amazon definition: "Amazon Elastic Container Registry (ECR) is a fully-managed Docker container registry that makes it easy for developers to store, manage, and deploy Docker container images."

Learn more here

What is AWS Fargate?

Amazon definition: "AWS Fargate is a serverless compute engine for containers that works with both Amazon Elastic Container Service (ECS) and Amazon Elastic Kubernetes Service (EKS)."

Learn more here

AWS Storage

Explain what is AWS S3?

S3 stands for 3 S, Simple Storage Service.
S3 is a object storage service which is fast, scalable and durable. S3 enables customers to upload, download or store any file or object that is up to 5 TB in size.

More on S3 here

What is a bucket?

An S3 bucket is a resource which is similar to folders in a file system and allows storing objects, which consist of data.

True or False? A bucket name must be globally unique

True

Explain folders and objects in regards to buckets

Folder - any sub folder in an s3 bucket
Object - The files which are stored in a bucket

Explain the following:

Object Lifecycles
Object Sharing
Object Versioning
Object Lifecycles - Transfer objects between storage classes based on defined rules of time periods
Object Sharing - Share objects via a URL link
Object Versioning - Manage multiple versions of an object

Explain Object Durability and Object Availability

Object Durability: The percent over a one-year time period that a file will not be lost
Object Availability: The percent over a one-year time period that a file will be accessible

What is a storage class? What storage classes are there?

Each object has a storage class assigned to, affecting its availability and durability. This also has effect on costs.
Storage classes offered today:

Standard:

Used for general, all-purpose storage (mostly storage that needs to be accessed frequently)
The most expensive storage class
11x9% durability
2x9% availability
Default storage class

Standard-IA (Infrequent Access)

Long lived, infrequently accessed data but must be available the moment it's being accessed
11x9% durability
99.90% availability

One Zone-IA (Infrequent Access):

Long-lived, infrequently accessed, non-critical data
Less expensive than Standard and Standard-IA storage classes
2x9% durability
99.50% availability

Intelligent-Tiering:

Long-lived data with changing or unknown access patterns. Basically, In this class the data automatically moves to the class most suitable for you based on usage patterns
Price depends on the used class
11x9% durability
99.90% availability

Glacier: Archive data with retrieval time ranging from minutes to hours

Glacier Deep Archive: Archive data that rarely, if ever, needs to be accessed with retrieval times in hours

Both Glacier and Glacier Deep Archive are:

The most cheap storage classes
have 9x9% durability

AWS Disaster Recovery

In regards to disaster recovery, what is RTO and RPO?

RTO - The maximum acceptable length of time that your application can be offline.

RPO - The maximum acceptable length of time during which data might be lost from your application due to an incident.

What types of disaster recovery techniques AWS supports?

The Cold Method - Periodically backups and sending the backups off-site
Pilot Light - Data is mirrored to an environment which is always running
Warm Standby - Running scaled down version of production environment
Multi-site - Duplicated environment that is always running

Which disaster recovery option has the highest downtime and which has the lowest?

Lowest - Multi-site
Highest - The cold method

AWS CloudFront

Explain what is CloudFront

AWS definition: "Amazon CloudFront is a fast content delivery network (CDN) service that securely delivers data, videos, applications, and APIs to customers globally with low latency, high transfer speeds, all within a developer-friendly environment."

More on CloudFront here

Explain the following

Origin
Edge location
Distribution

What delivery methods available for the user with CDN?
True or False?. Objects are cached for the life of TTL

True

What is AWS Snowball?

A transport solution which was designed for transferring large amounts of data (petabyte-scale) into and out the AWS cloud.

AWS ELB

What is ELB (Elastic Load Balancing)?

AWS definition: "Elastic Load Balancing automatically distributes incoming application traffic across multiple targets, such as Amazon EC2 instances, containers, IP addresses, and Lambda functions."

More on ELB here

What types of load balancers are supported in EC2 and what are they used for?

Application LB - layer 7 traffic
Network LB - ultra-high performances or static IP address
Classic LB - low costs, good for test or dev environments

AWS Security

What is the shared responsibility model? What AWS is responsible for and what the user is responsible for based on the shared responsibility model?

The shared responsibility model defines what the customer is responsible for and what AWS is responsible for.

More on the shared responsibility model here

True or False? Based on the shared responsibility model, Amazon is responsible for physical CPUs and security groups on instances

False. It is responsible for Hardware in its sites but not for security groups which created and managed by the users.

Explain "Shared Controls" in regards to the shared responsibility model

AWS definition: "apply to both the infrastructure layer and customer layers, but in completely separate contexts or perspectives. In a shared control, AWS provides the requirements for the infrastructure and the customer must provide their own control implementation within their use of AWS services"

Learn more about it here

What is the AWS compliance program?
What is AWS Artifact?

AWS definition: "AWS Artifact is your go-to, central resource for compliance-related information that matters to you. It provides on-demand access to AWS’ security and compliance reports and select online agreements."

Read more about it here

What is AWS Inspector?

AWS definition: "Amazon Inspector is an automated security assessment service that helps improve the security and compliance of applications deployed on AWS. Amazon Inspector automatically assesses applications for exposure, vulnerabilities, and deviations from best practices.""

Learn more here

What is AWS Guarduty?
What is AWS Shield?

AWS definition: "AWS Shield is a managed Distributed Denial of Service (DDoS) protection service that safeguards applications running on AWS."

What is AWS WAF? Give an example of how it can used and describe what resources or services you can use it with
What AWS VPN is used for?
What is the difference between Site-to-Site VPN and Client VPN?
What is AWS CloudHSM?

Amazon definition: "AWS CloudHSM is a cloud-based hardware security module (HSM) that enables you to easily generate and use your own encryption keys on the AWS Cloud."

Learn more here

True or False? AWS Inspector can perform both network and host assessments

True

What is AWS Key Management Service (KMS)?

AWS definition: "KMS makes it easy for you to create and manage cryptographic keys and control their use across a wide range of AWS services and in your applications."
More on KMS here

What is AWS Acceptable Use Policy?

It describes prohibited uses of the web services offered by AWS.
More on AWS Acceptable Use Policy here

True or False? A user is not allowed to perform penetration testing on any of the AWS services

False. On some services, like EC2, CloudFront and RDS, penetration testing is allowed.

True or False? DDoS attack is an example of allowed penetration testing activity

False.

True or False? AWS Access Key is a type of MFA device used for AWS resources protection

False. Security key is an example of an MFA device.

What is Amazon Cognito?

Amazon definition: "Amazon Cognito handles user authentication and authorization for your web and mobile apps."

Learn more here

What is AWS ACM?

Amazon definition: "AWS Certificate Manager is a service that lets you easily provision, manage, and deploy public and private Secure Sockets Layer/Transport Layer Security (SSL/TLS) certificates for use with AWS services and your internal connected resources."

Learn more here

AWS Databases

What is AWS RDS?
What is AWS DynamoDB?
Explain "Point-in-Time Recovery" feature in DynamoDB

Amazon definition: "You can create on-demand backups of your Amazon DynamoDB tables, or you can enable continuous backups using point-in-time recovery. For more information about on-demand backups, see On-Demand Backup and Restore for DynamoDB."

Learn more here

Explain "Global Tables" in DynamoDB

Amazon definition: "A global table is a collection of one or more replica tables, all owned by a single AWS account."

Learn more here

What is DynamoDB Accelerator?

Amazon definition: "Amazon DynamoDB Accelerator (DAX) is a fully managed, highly available, in-memory cache for DynamoDB that delivers up to a 10x performance improvement – from milliseconds to microseconds..."

Learn more here

What is AWS Redshift and how is it different than RDS?

cloud data warehouse

What do you if you suspect AWS Redshift performs slowly?

You can confirm your suspicion by going to AWS Redshift console and see running queries graph. This should tell you if there are any long-running queries.
If confirmed, you can query for running queries and cancel the irrelevant queries
Check for connection leaks (query for running connections and include their IP)
Check for table locks and kill irrelevant locking sessions

What is AWS ElastiCache? For what cases is it used?

Amazon Elasticache is a fully managed Redis or Memcached in-memory data store.
It's great for use cases like two-tier web applications where the most frequently accesses data is stored in ElastiCache so response time is optimal.

What is Amazon Aurora

A MySQL & Postgresql based relational database. Also, the default database proposed for the user when using RDS for creating a database.
Great for use cases like two-tier web applications that has a MySQL or Postgresql database layer and you need automated backups for your application.

What is Amazon DocumentDB?

Amazon definition: "Amazon DocumentDB (with MongoDB compatibility) is a fast, scalable, highly available, and fully managed document database service that supports MongoDB workloads. As a document database, Amazon DocumentDB makes it easy to store, query, and index JSON data."

Learn more here

What "AWS Database Migration Service" is used for?
What type of storage is used by Amazon RDS?

EBS

Explain Amazon RDS Read Replicas

AWS definition: "Amazon RDS Read Replicas provide enhanced performance and durability for RDS database (DB) instances. They make it easy to elastically scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads."
Read more about here

AWS Networking

What is VPC?

"A logically isolated section of the AWS cloud where you can launch AWS resources in a virtual network that you define"
Read more about it here.

True or False? VPC spans multiple regions

False

True or False? Subnets belong to the same VPC, can be in different availability zones

True. Just to clarify, a single subnet resides entirely in one AZ.

What is an Internet Gateway?

"component that allows communication between instances in your VPC and the internet" (AWS docs).
Read more about it here

True or False? NACL allow or deny traffic on the subnet level

True

What is VPC peering?

docs.aws: "A VPC peering connection is a networking connection between two VPCs that enables you to route traffic between them using private IPv4 addresses or IPv6 addresses."

True or False? Multiple Internet Gateways can be attached to one VPC

False. Only one internet gateway can be attached to a single VPC.

What is an Elastic IP address?
An Elastic IP address is a reserved public IP address that you can assign to any EC2 instance in a particular region, until you choose to release it. When you associate an Elastic IP address with an EC2 instance, it replaces the default public IP address. If an external hostname was allocated to the instance from your launch settings, it will also replace this hostname; otherwise, it will create one for the instance. The Elastic IP address remains in place through events that normally cause the address to change, such as stopping or restarting the instance. True or False? Route Tables used to allow or deny traffic from the internet to AWS instances

False.

Explain Security Groups and Network ACLs

NACL - security layer on the subnet level.
Security Group - security layer on the instance level.

Read more about it here and here

What is AWS Direct Connect?

Allows you to connect your corporate network to AWS network.

AWS - Identify the service or tool

What would you use for automating code/software deployments?

AWS CodeDeploy

What would you use for easily creating similar AWS environments/resources for different customers?

CloudFormation

Using which service, can you add user sign-up, sign-in and access control to mobile and web apps?

Cognito

Which service would you use for building a website or web application?

Lightsail

Which tool would you use for choosing between Reserved instances or On-Demand instances?

Cost Explorer

What would you use to check how many unassociated Elastic IP address you have?

Trusted Advisor

Which service allows you to transfer large amounts (Petabytes) of data in and out of the AWS cloud?

AWS Snowball

Which service would you use if you need a data warehouse?

AWS RedShift

Which service provides a virtual network dedicated to your AWS account?

VPC

What you would use for having automated backups for an application that has MySQL database layer?

Amazon Aurora

What would you use to migrate on-premise database to AWS?

AWS Database Migration Service (DMS)

What would you use to check why certain EC2 instances were terminated?

AWS CloudTrail

What would you use for SQL database?

AWS RDS

What would you use for NoSQL database?

AWS DynamoDB

What would you use for adding image and video analysis to your application?

AWS Rekognition

Which service would you use for debugging and improving performances issues with your applications?

AWS X-Ray

Which service is used for sending notifications?

SNS

What would you use for running SQL queries interactively on S3?

AWS Athena

What would you use for preparing and combining data for analytics or ML?

AWS Glue

Which service would you use for monitoring malicious activity and unauthorized behavior in regards to AWS accounts and workloads?

Amazon GuardDuty

Which service would you use for centrally manage billing, control access, compliance, and security across multiple AWS accounts?

AWS Organizations

Which service would you use for web application protection?

AWS WAF

You would like to monitor some of your resources in the different services. Which service would you use for that?

CloudWatch

Which service would you use for performing security assessment?

AWS Inspector

Which service would you use for creating DNS record?

Route 53

What would you use if you need a fully managed document database?

Amazon DocumentDB

Which service would you use to add access control (or sign-up, sign-in forms) to your web/mobile apps?

AWS Cognito

Which service would you use if you need messaging queue?

Simple Queue Service (SQS)

Which service would you use if you need managed DDOS protection?

AWS Shield

Which service would you use if you need store frequently used data for low latency access?

ElastiCache

What would you use to transfer files over long distances between a client and an S3 bucket?

Amazon S3 Transfer Acceleration

Which service would you use for distributing incoming requests across multiple?

Route 53

Which services are involved in getting a custom string (based on the input) when inserting a URL in the browser?

Lambda - to define a function that gets an input and returns a certain string

API Gateway - to define the URL trigger (= when you insert the URL, the function is invoked).

Which service would you use for data or events streaming?

Kinesis

AWS DNS

What is Route 53?

"Amazon Route 53 is a highly available and scalable cloud Domain Name System (DNS) web service..."
Some of Route 53 features:

Register domain
DNS service - domain name translations
Health checks - verify your app is available

More on Route 53 here

AWS Monitoring & Logging

What is AWS CloudWatch?

AWS definition: "Amazon CloudWatch is a monitoring and observability service..."

More on CloudWatch here

What is AWS CloudTrail?

AWS definition: "AWS CloudTrail is a service that enables governance, compliance, operational auditing, and risk auditing of your AWS account."

Read more on CloudTrail here

What is Simply Notification Service?

AWS definition: "a highly available, durable, secure, fully managed pub/sub messaging service that enables you to decouple microservices, distributed systems, and serverless applications."

Read more about it here

Explain the following in regards to SNS:

Topics
Subscribers
Publishers
Topics - used for grouping multiple endpoints
Subscribers - the endpoints where topics send messages to
Publishers - the provider of the message (event, person, ...)

AWS Billing & Support

What is AWS Organizations?

AWS definition: "AWS Organizations helps you centrally govern your environment as you grow and scale your workloads on AWS."
More on Organizations here

What are Service Control Policies and to what service they belong?

AWS organizations service and the definition by Amazon: "SCPs offer central control over the maximum available permissions for all accounts in your organization, allowing you to ensure your accounts stay within your organization’s access control guidelines."

Learn more here

Explain AWS pricing model

It mainly works on "pay-as-you-go" meaning you pay only for what are using and when you are using it.
In s3 you pay for 1. How much data you are storing 2. Making requests (PUT, POST, ...)
In EC2 it's based on the purchasing option (on-demand, spot, ...), instance type, AMI type and the region used.

More on AWS pricing model here

How one should estimate AWS costs when for example comparing to on-premise solutions?

TCO calculator
AWS simple calculator
Cost Explorer

What basic support in AWS includes?

24x7 customer service
Trusted Advisor
AWS personal Health Dashoard

How are EC2 instances billed?
What AWS Pricing Calculator is used for?
What is Amazon Connect?

Amazon definition: "Amazon Connect is an easy to use omnichannel cloud contact center that helps companies provide superior customer service at a lower cost."

Learn more here

What are "APN Consulting Partners"?

Amazon definition: "APN Consulting Partners are professional services firms that help customers of all types and sizes design, architect, build, migrate, and manage their workloads and applications on AWS, accelerating their journey to the cloud."

Learn more here

Which of the following are AWS accounts types (and are sorted by order)?

Basic, Developer, Business, Enterprise
Newbie, Intermediate, Pro, Enterprise
Developer, Basic, Business, Enterprise
Beginner, Pro, Intermediate Enterprise
Basic, Developer, Business, Enterprise

True or False? Region is a factor when it comes to EC2 costs/pricing

True. You pay differently based on the chosen region.

What is "AWS Infrastructure Event Management"?

AWS Definition: "AWS Infrastructure Event Management is a structured program available to Enterprise Support customers (and Business Support customers for an additional fee) that helps you plan for large-scale events such as product or application launches, infrastructure migrations, and marketing events."

AWS Automation

What is AWS CodeDeploy?

Amazon definition: "AWS CodeDeploy is a fully managed deployment service that automates software deployments to a variety of compute services such as Amazon EC2, AWS Fargate, AWS Lambda, and your on-premises servers."

Learn more here

Explain what is CloudFormation

AWS - Misc

Which AWS service you have experience with that you think is not very common?
What is AWS CloudSearch?
What is AWS Lightsail?

AWS definition: "Lightsail is an easy-to-use cloud platform that offers you everything needed to build an application or website, plus a cost-effective, monthly plan."

What is AWS Rekognition?

AWS definition: "Amazon Rekognition makes it easy to add image and video analysis to your applications using proven, highly scalable, deep learning technology that requires no machine learning expertise to use."

Learn more here

What AWS Resource Groups used for?

Amazon definition: "You can use resource groups to organize your AWS resources. Resource groups make it easier to manage and automate tasks on large numbers of resources at one time. "

Learn more here

What is AWS Global Accelerator?

Amazon definition: "AWS Global Accelerator is a service that improves the availability and performance of your applications with local or global users..."

Learn more here

What is AWS Config?

Amazon definition: "AWS Config is a service that enables you to assess, audit, and evaluate the configurations of your AWS resources."

Learn more here

What is AWS X-Ray?

AWS definition: "AWS X-Ray helps developers analyze and debug production, distributed applications, such as those built using a microservices architecture."
Learn more here

What is AWS OpsWorks?

Amazon definition: "AWS OpsWorks is a configuration management service that provides managed instances of Chef and Puppet."

Learn more about it here

What is AWS Athena?

"Amazon Athena is an interactive query service that makes it easy to analyze data in Amazon S3 using standard SQL."

Learn more about AWS Athena here

What is Amazon Cloud Directory?

Amazon definition: "Amazon Cloud Directory is a highly available multi-tenant directory-based store in AWS. These directories scale automatically to hundreds of millions of objects as needed for applications."

Learn more here

What is AWS Elastic Beanstalk?

AWS definition: "AWS Elastic Beanstalk is an easy-to-use service for deploying and scaling web applications and services...You can simply upload your code and Elastic Beanstalk automatically handles the deployment"

Learn more about it here

What is AWS SWF?

Amazon definition: "Amazon SWF helps developers build, run, and scale background jobs that have parallel or sequential steps. You can think of Amazon SWF as a fully-managed state tracker and task coordinator in the Cloud."

Learn more on Amazon Simple Workflow Service here

What is AWS EMR?

AWS definition: "big data platform for processing vast amounts of data using open source tools such as Apache Spark, Apache Hive, Apache HBase, Apache Flink, Apache Hudi, and Presto."

Learn more here

What is AWS Quick Starts?

AWS definition: "Quick Starts are built by AWS solutions architects and partners to help you deploy popular technologies on AWS, based on AWS best practices for security and high availability."

Read more here

What is the Trusted Advisor?
What is AWS Service Catalog?

Amazon definition: "AWS Service Catalog allows organizations to create and manage catalogs of IT services that are approved for use on AWS."

Learn more here

What is AWS CAF?

Amazon definition: "AWS Professional Services created the AWS Cloud Adoption Framework (AWS CAF) to help organizations design and travel an accelerated path to successful cloud adoption. "

Learn more here

What is AWS Cloud9?

AWS definition: "AWS Cloud9 is a cloud-based integrated development environment (IDE) that lets you write, run, and debug your code with just a browser"

What is AWS Application Discovery Service?

Amazon definition: "AWS Application Discovery Service helps enterprise customers plan migration projects by gathering information about their on-premises data centers."

Learn more here

What is the AWS well-architected framework and what pillars it's based on?

AWS definition: "The Well-Architected Framework has been developed to help cloud architects build secure, high-performing, resilient, and efficient infrastructure for their applications. Based on five pillars — operational excellence, security, reliability, performance efficiency, and cost optimization"

Learn more here

What AWS services are serverless (or have the option to be serverless)?

AWS Lambda
AWS Athena

What is Simple Queue Service (SQS)?

AWS definition: "Amazon Simple Queue Service (SQS) is a fully managed message queuing service that enables you to decouple and scale microservices, distributed systems, and serverless applications".

Learn more about it here

Network

What is Ethernet?

Ethernet simply refers to the most common type of Local Area Network (LAN) used today. A LAN—in contrast to a WAN (Wide Area Network), which spans a larger geographical area—is a connected network of computers in a small area, like your office, college campus, or even home.

What is TCP/IP?

A set of protocols that define how two or more devices can communicate with each other.
To learn more about TCP/IP, read here

What is a MAC address? What is it used for?

A MAC address is a unique identification number or code used to identify individual devices on the network.

Packets that are sent on the ethernet are always coming from a MAC address and sent to a MAC address. If a network adapter is receiving a packet, it is comparing the packet’s destination MAC address to the adapter’s own MAC address.

When is this MAC address used?: ff:ff:ff:ff:ff:ff

When a device sends a packet to the broadcast MAC address (FF:FF:FF:FF:FF:FF), it is delivered to all stations on the local network. It needs to be used in order for all devices to receive your packet at the datalink layer.

What is an IP address?

An Internet Protocol address (IP address) is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.An IP address serves two main functions: host or network interface identification and location addressing.

Explain subnet mask and given an example

A Subnet mask is a 32-bit number that masks an IP address, and divides the IP address into network address and host address. Subnet Mask is made by setting network bits to all "1"s and setting host bits to all "0"s. Within a given network, two host addresses are reserved for special purpose, and cannot be assigned to hosts. The "0" address is assigned a network address and "255" is assigned to a broadcast address, and they cannot be assigned to hosts.

For Example

| Address Class | No of Network Bits | No of Host Bits | Subnet mask     | CIDR notation |
| ------------- | ------------------ | --------------- | --------------- | ------------- |
| A             | 8                  | 24              | 255.0.0.0       | /8            |
| A             | 9                  | 23              | 255.128.0.0     | /9            |
| A             | 12                 | 20              | 255.240.0.0     | /12           |
| A             | 14                 | 18              | 255.252.0.0     | /14           |
| B             | 16                 | 16              | 255.255.0.0     | /16           |
| B             | 17                 | 15              | 255.255.128.0   | /17           |
| B             | 20                 | 12              | 255.255.240.0   | /20           |
| B             | 22                 | 10              | 255.255.252.0   | /22           |
| C             | 24                 | 8               | 255.255.255.0   | /24           |
| C             | 25                 | 7               | 255.255.255.128 | /25           |
| C             | 28                 | 4               | 255.255.255.240 | /28           |
| C             | 30                 | 2               | 255.255.255.252 | /30           |

What is a private IP address? In which scenarios/system designs, one should use it?
What is a public IP address? In which scenarios/system designs, one should use it?
Explain the OSI model. What layers there are? What each layer is responsible for?

Application: user end (HTTP is here)
Presentation: establishes context between application-layer entities (Encryption is here)
Session: establishes, manages and terminates the connections
Transport: transfers variable-length data sequences from a source to a destination host (TCP & UDP are here)
Network: transfers datagrams from one network to another (IP is here)
Data link: provides a link between two directly connected nodes (MAC is here)
Physical: the electrical and physical spec the data connection (Bits are here)

You can read more about the OSI model in penguintutor.com

For each of the following determine to which OSI layer it belongs:

Error correction
Packets routing
Cables and electrical signals
MAC address
IP address
Terminate connections
3 way handshake
Error correction
Packets routing - Network
Cables and electrical signals - Physical
MAC address - Data link
IP address - Network
Terminate connections - Session
3 way handshake - Transport

What delivery schemes are you familiar with?

Unitcast: One to one communication where there is one sender and one receiver.

Broadcast: Sending a message to everyone in the network. The address ff:ff:ff:ff:ff:ff is used for broadcasting.
Two common protocols which use broadcast are ARP and DHCP.

Multicast: Sending a message to a group of subscribers. It can be one-to-many or many-to-many.

What is CSMA/CD? Is it used in modern ethernet networks?

CSMA/CD stands for Carrier Sense Multiple Access / Collision Detection.
Its primarily focus it to manage access to shared medium/bus where only one host can transmit at a given point of time.

CSMA/CD algorithm:

Before sending a frame, it checks whether another host already transmitting a frame.
If no one transmitting, it starts transmitting the frame.
If two hosts transmitted at the same time, we have a collision.
Both hosts stop sending the frame and they send to everyone a 'jam signal' notifying everyone that a collision occurred
They are waiting for a random time before sending again
Once each host waited for a random time, they try to send the frame again and so the

Describe the following network devices and the difference between them:

router
switch
hub

How does a router works?

A router is a physical or virtual appliance that passes information between two or more packet-switched computer networks. A router inspects a given data packet's destination Internet Protocol address (IP address), calculates the best way for it to reach its destination and then forwards it accordingly.

What is NAT?

Network Address Translation (NAT) is a process in which one or more local IP address is translated into one or more Global IP address and vice versa in order to provide Internet access to the local hosts.

What is a proxy? How does it works? What do we need it for?

A proxy server acts as a gateway between you and the internet. It’s an intermediary server separating end users from the websites they browse.

If you’re using a proxy server, internet traffic flows through the proxy server on its way to the address you requested. The request then comes back through that same proxy server (there are exceptions to this rule), and then the proxy server forwards the data received from the website to you.

roxy servers provide varying levels of functionality, security, and privacy depending on your use case, needs, or company policy.

What is TCP? How does it works? What is the 3 way handshake?

TCP 3-way handshake or three-way handshake is a process which is used in a TCP/IP network to make a connection between server and client.

A three-way handshake is primarily used to create a TCP socket connection. It works when:

A client node sends a SYN data packet over an IP network to a server on the same or an external network. The objective of this packet is to ask/infer if the server is open for new connections.
The target server must have open ports that can accept and initiate new connections. When the server receives the SYN packet from the client node, it responds and returns a confirmation receipt – the ACK packet or SYN/ACK packet.
The client node receives the SYN/ACK from the server and responds with an ACK packet.

What is round-trip delay or round-trip time?

From wikipedia: "the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgement of that signal to be received"

Bonus question: what is the RTT of LAN?

How does SSL handshake work?
What is the difference between TCP and UDP?

TCP establishes a connection between the client and the server to guarantee the order of the packages, on the other hand, UDP does not establish a connection between client and server and doesn't handle package order. This makes UDP more lightweight than TCP and a perfect candidate for services like streaming.

Penguintutor.com provides a good explanation.

What TCP/IP protocols are you familiar with?
Explain "default gateway"

A default gateway serves as an access point or IP router that a networked computer uses to send information to a computer in another network or the internet.

What is ARP? How does it works?

ARP stands for Address Resolution Protocol. When you try to ping an IP address on your local network, say 192.168.1.1, your system has to turn the IP address 192.168.1.1 into a MAC address. This involves using ARP to resolve the address, hence its name.

Systems keep an ARP look-up table where they store information about what IP addresses are associated with what MAC addresses. When trying to send a packet to an IP address, the system will first consult this table to see if it already knows the MAC address. If there is a value cached, ARP is not used.

What is TTL? What does it helps to prevent?
What is DHCP? How does it works?

It stands for Dynamic Host Configuration Protocol, and allocates IP addresses, subnet masks and gateways to hosts. This is how it works:

A host upon entering a network, broadcasts a message in search of a DHCP server (DHCP DISCOVER)
An offer message is sent back by the DHCP server as a packet containing lease time, subnet mask, IP addresses, etc (DHCP OFFER)
Depending on which offer accepted, the client sends back a reply broadcast letting all DHCP servers know (DHCP REQUEST)
Server sends an acknowledgment (DHCP ACK)

Read more here

Can you have two DHCP servers in the same network? How it works?
What is SSL tunneling? How does it works?
What is a socket? Where can you see the list of sockets in your system?
What is IPv6? Why should we consider using it if we have IPv4?
What is VLAN?
What is MTU?
What happens if you send a packet that is bigger than the MTU?
True or False?. Ping is using UDP because it doesn't care about reliable connection
What is SDN?
What is ICMP? What is it used for?
What is NAT? How does it work?

NAT stands for network address translation. It’s a way to map multiple local private addresses to a public one before transferring the information. Organizations that want multiple devices to employ a single IP address use NAT, as do most home routers.
For example, your computer's private IP could be 192.168.1.100, but your router maps the traffic to it's public IP (e.g. 1.1.1.1). Any device on the internet would see the traffic coming from your public IP (1.1.1.1) instead of your private IP (192.168.1.100).

Which factors affect network performances
What the terms "Data Plane" and "Control Plane" refer?

The exact meaning is usually depends on the context but overall data plane refers to all the functions that forward packets and/or frames from one interface to another while control plane refers to all the functions that make use of routing protocols.

There is also "Management Plane" which refers to monitoring and management functions.

Explain Spanning Tree Protocol (STP)
What is link aggregation? Why is it used?
What is Asymmetric Routing? How do deal with it?
What overlay (tunnel) protocols are you familiar with?
What is GRE? How does it works?
What is VXLAN? How does it works?
What is SNAT?
Explain OSPF
What is latency?
What is bandwidth?
What is throughput?
When performing a search query, what is more important, latency or throughput? And how to assure that what managing global infrastructure?

Latency. To have a good latency, a search query should be forwarded to the closest datacenter.

When uploading a video, what is more important, latency or throughput? And how to assure that?

Throughput. To have a good throughput, the upload stream should be routed to an underutilized link.

What other considerations (except latency and throughput) are there when forwarding requests?

Keep caches updated (which means the request could be forwarded not to the closest datacenter)

Explain Spine & Leaf
What is Network Congestion? What can cause it?
What can you tell me about UDP packet format? What about TCP packet format? How is it different?
What is the exponential backoff algorithm? Where is it used?
Using Hamming code, what would be the code word for the following data word 100111010001101?

00110011110100011101

Give examples of protocols found in the application layer

Hypertext Transfer Protocol (HTTP) - used for the webpages on the internet
Simple Mail Transfer Protocol (SMTP) - email transmission
Telecommunications Network - (TELNET) - terminal emulation to allow client access to telnet server
File Transfer Protocol (FTP) - facilitates transfer of files between any two machines
Domain Name System (DNS) - domain name translation
Dynamic Host Configuration Protocol (DHCP) - allocates IP addresses, subnet masks and gateways to hosts
Simple Network Management Protocol (SNMP) - gathers data of devices on the network

Give examples of protocols found in the network Layer

Internet Protocol (IP) - assists in routing packets from one machine to another
Internet Control Message Protocol (ICMP) - lets one know what is going such as error messages and debugging information

What is HSTS?
HTTP Strict Transport Security is a web server directive that informs user agents and web browsers how to handle its connection through a response header sent at the very beginning and back to the browser. This forces connections over HTTPS encryption, disregarding any script's call to load any resource in that domain over HTTP.

What is the difference if any between SSL and TLS?

Network - Misc

What is the Internet? Is it the same as the World Wide Web?

The internet refers to network of networks, transferring huge amounts of data around the globe.

The World Wide Web is an application running on millions of server, on top of the internet, accessed through what is know as the web browser

What is the ISP?

ISP (Internet Service Provider) is the local internet company provider.

Linux

Linux Master Application

A completely free application for testing your knowledge on Linux

Linux Self Assessment

What is your experience with Linux?

Only you know :)

For example:

Administration
Troubleshooting & Debugging
Storage
Networking
Development
Deployments

Explain what each of the following commands does and give an example on how to use it:

touch
ls
rm
cat
cp
mkdir
pwd
cd
touch - update file's timestamp. More commonly used for creating files
ls - listing files and directories
rm - remove files and directories
cat - create, view and concatenate files
cp - copy files and directories
mkdir - create directories
pwd - print current working directory (= at what path the user currently located)
cd - change directory

What each of the following commands does?

cd /
cd ~
cd
cd ..
cd .
cd -
cd / -> change to the root directory
cd ~ -> change to your home directory
cd -> change to your home directory
cd .. -> change to the directory above your current i.e parent directory
cd . -> change to the directory you currently in
cd - -> change to the last visited path

Some of the commands in the previous question can be run with the -r/-R flag. What does it do? Give an example to when you would use it

The -r (or -R in some commands) flag allows the user to run a certain command recursively. For example, listing all the files under the following tree is possible when done recursively (ls -R):

/dir1/
dir2/
file1
file2
dir3/
file3

To list all the files, one can run ls -R /dir1

Explain each field in the output of `ls -l` command
It shows a detailed list of files in a long format. From the left:

file permissions, number of links, owner name, owner group, file size, timestamp of last modification and directory/file name

What are hidden files/directories? How to list them?

These are files directly not displayed after performing a standard ls direct listing. An example of these files are .bashrc which are used to execute some scripts. Some also store configuration about services on your host like .KUBECONFIG. The command used to list them is, ls -a

What do > and < do in terms of input and output for programs?
They take in input (<) and output for a given file (>) using stdin and stdout.

myProgram < input.txt > executionOutput.txt

Explain what each of the following commands does and give an example on how to use it:

sed
grep
cut
awk
sed: a stream editor. Can be used for various purposes like replacing a word in a file: sed -i s/salad/burger/g

How to rename the name of a file or a directory?

Using the mv command.

Specify which command would you use (and how) for each of the following scenarios

Remove a directory with files
Display the content of a file
Provides access to the file /tmp/x for everyone
Change working directory to user home directory
Replace every occurrence of the word "good" with "great" in the file /tmp/y
rm -rf dir
cat or less
chmod 777 /tmp/x
cd ~
sed -i s/good/great/g /tmp/y

How can you check what is the path of a certain command?

whereis
which

Explain redirection
What is the difference between these two commands? Will it result in the same output?

echo hello world
echo "hello world"

The echo command receives two separate arguments in the first execution and in the second execution it gets one argument which is the string "hello world". The output will be the same.

Explain piping. How do you perform piping?

Using a pipe in Linux, allows you to send the output of one command to the input of another command. For example: cat /etc/services | wc -l

Fix the following commands:

sed "s/1/2/g' /tmp/myFile
find . -iname *.yaml -exec sed -i "s/1/2/g" {} ;

sed 's/1/2/g' /tmp/myFile  # sed "s/1/2/g" is also fine
find . -iname "*.yaml" -exec sed -i "s/1/2/g" {} \;

How to check which commands you executed in the past?

history command or .bash_history file

Running the command df you get "command not found". What could be wrong and how to fix it?

Most likely the default/generated $PATH was somehow modified or overridden thus not containing /bin/ where df would normally go. This issue could also happen if bash_profile or any configuration file of your interpreter was wrongly modified, causing erratics behaviours. You would solve this by fixing your $PATH variable:

As to fix it there are several options:

Manually adding what you need to your $PATH PATH="$PATH":/user/bin:/..etc
You have your weird env variables backed up.
You would look for your distro default $PATH variable, copy paste using method #1

Note: There are many ways of getting errors like this: if bash_profile or any configuration file of your interpreter was wrongly modified; causing erratics behaviours,
permissions issues, bad compiled software (if you compiled it by yourself)... there is no answer that will be true 100% of the time.

How do you schedule tasks periodically?

You can use the commands cron and at.
With cron, tasks are scheduled using the following format:

*/30 * * * * bash myscript.sh Executes the script every 30 minutes.

The tasks are stored in a cron file, you can write in it using crontab -e

Alternatively if you are using a distro with systemd it's recommended to use systemd timers.

Linux FHS

In Linux FHS (Filesystem Hierarchy Standard) what is the /?

The root of the filesystem. The beginning of the tree.

What is stored in each of the following paths?

/bin, /sbin, /usr/bin and /usr/sbin
/etc
/home
/var
/tmp
binaries
configuration files
home directories of the different users
files that tend to change and be modified like logs
temporary files

What is special about the /tmp directory when compared to other directories?

/tmp folder is cleaned automatically, usually upon reboot.

What kind of information one can find in /proc?
Can you create files in /proc?
In which path can you find the system devices (e.g. block storage)?

Linux - Permissions

How to change the permissions of a file?

Using the chmod command.

What does the following permissions mean?:

777
644
750
777 - You give the owner, group and other: Execute (1), Write (2) and Read (4); 4+2+1 = 7. 644 - Owner has Read (4), Write (2), 4+2 = 6; Group and Other have Read (4). 750 - Owner has x+r+w, Group has Read (4) and Execute (1); 4+1 = 5. Other have no permissions.

What this command does? chmod +x some_file
It adds execute permissions to all sets i.e user, group and others Explain what is setgid and setuid

setuid is a linux file permission that permits a user to run a file or program with the permissions of the owner of that file. This is possible by elevation of current user privileges.
setgid is a process when executed will run as the group that owns the file.

What is the purpose of sticky bit?
Its a bit that only allows the owner or the root user to delete or modify the file. What the following commands do?

chmod
chown
chgrp
chmod - changes access permissions to files system objects
chown - changes the owner of file system files and directories
chgrp - changes the group associated with a file system object

What is sudo? How do you set it up?
True or False? In order to install packages on the system one must be the root user or use the sudo command

True

Explain what are ACLs. For what use cases would you recommend to use them?
You try to create a file but it fails. Name at least three different reason as to why it could happen

No more disk space
No more inodes
No permissions

A user accidentally executed the following chmod -x $(which chmod). How to fix it?

Linux - Shell Scripting

What this line in scripts mean?: #!/bin/bash

#!/bin/bash is She-bang

/bin/bash is the most common shell used as default shell for user login of the linux system. The shell’s name is an acronym for Bourne-again shell. Bash can execute the vast majority of scripts and thus is widely used because it has more features, is well developed and better syntax.

True or False?: when a certain command/line fails, the script, by default, will exit and will no keep running

Depends on the language and settings used.
When a script written in Bash fails to run a certain command it will keep running and will execute all other commands mentioned after the command which failed.
Most of the time we would actually want the opposite to happen. In order to make Bash exist when a specific command fails, use 'set -e' in your script.

Explain what would be the result of each command:

echo $0
echo $?
echo $$
echo $@
echo $#

How do you debug shell scripts?

Answer depends on the language you are using for writing your scripts. If Bash is used for example then:

Adding -x to the script I'm running in Bash
Old good way of adding echo statements

If Python, then using pdb is very useful.

How do you get input from the user in shell scripts?

Using the keyword read so for example read x will wait for user input and will store it in the variable x.

Explain continue and break. When do you use them if at all?
Running the following bash script, we don't get 2 as a result, why?

x = 2
echo $x

Should be x=2

How to store the output of a command in a variable?

OUTPUT=$(ls -1)
echo "${OUTPUT}"

Source

How do you check variable length?
Generate 8 digit random number

shuf -i 9999999-99999999 -n 1

Explain the following code:

:(){ :|:& };:

Can you give an example to some Bash best practices?
What is the ternary operator? How do you use it in bash?

A short way of using if/else. An example:

[[ $a = 1 ]] && b="yes, equal" || b="nope"

What does the following code do and when would you use it?

diff <(ls /tmp) <(ls /var/tmp)
It is called 'process substitution'. It provides a way to pass the output of a command to another command when using a pipe | is not possible. It can be used when a command does not support STDIN or you need the output of multiple commands. https://superuser.com/a/1060002/167769

Linux - systemd

What is systemd?
Systemd is a daemon (System 'd', d stands from daemon).

A daemon is a program that runs in the background without direct control of the user, although the user can at any time
talk to the daemon.

systemd has many features such as user processes control/tracking, snapshot support, inhibitor locks..

If we visualize the unix/linux system in layers, systemd would fall directly after the linux kernel.

Hardware -> Kernel -> Daemons, System Libraries, Server Display.

On a system which uses systemd, how would you display the logs?

journalctl

Describe how to make a certain process/app a service

Linux - Troubleshooting & Debugging

Where system logs are located?

/var/log

How to follow file's content as it being appended without opening the file every time?

tail -f <file_name>

What are you using for troubleshooting and debugging network issues?

dstat -t is great for identifying network and disk issues.
netstat -tnlaup can be used to see which processes are running on which ports.
lsof -i -P can be used for the same purpose as netstat.
ngrep -d any metafilter for matching regex against payloads of packets.
tcpdump for capturing packets
wireshark same concept as tcpdump but with GUI (optional).

What are you using for troubleshooting and debugging disk & file system issues?

dstat -t is great for identifying network and disk issues.
opensnoop can be used to see which files are being opened on the system (in real time).

What are you using for troubleshooting and debugging process issues?

strace is great for understanding what your program does. It prints every system call your program executed.

What are you using for debugging CPU related issues?

top will show you how much CPU percentage each process consumes
perf is a great choice for sampling profiler and in general, figuring out what your CPU cycles are "wasted" on
flamegraphs is great for CPU consumption visualization (http://www.brendangregg.com/flamegraphs.html)

You get a call from someone claiming "my system is SLOW". What do you do?

Check with top for anything unusual
Run dstat -t to check if it's related to disk or network.
Check if it's network related with sar
Check I/O stats with iostat

Explain iostat output
How to debug binaries?
What is the difference between CPU load and utilization?
How you measure time execution of a program?

Linux Kernel

What is a kernel, and what does it do?

The kernel is part of the operating system and is responsible for tasks like:

Allocating memory
Schedule processes
Control CPU

How do you find out which Kernel version your system is using?

uname -a command

What is a Linux kernel module and how do you load a new module?
Explain user space vs. kernel space

The operating system executes the kernel in protected memory to prevent anyone from changing (and risking it crashing). This is what is known as "Kernel space".
"User space" is where users executes their commands or applications. It's important to create this separation since we can't rely on user applications to not tamper with the kernel, causing it to crash.

Applications can access system resources and indirectly the kernel space by making what is called "system calls".

Linux - SSH

What is SSH? How to check if a Linux server is running SSH?

Wikipedia Definition: "SSH or Secure Shell is a cryptographic network protocol for operating network services securely over an unsecured network."

Hostinger.com Definition: "SSH, or Secure Shell, is a remote administration protocol that allows users to control and modify their remote servers over the Internet."

An SSH server will have SSH daemon running. Depends on the distribution, you should be able to check whether the service is running (e.g. systemctl status sshd).

Why SSH is considered better than telnet?

Telnet also allows you to connect to a remote host but as opposed to SSH where the communication is encrypted, in telnet, the data is sent in clear text, so it doesn't considered to be secured because anyone on the network can see what exactly is sent, including passwords.

What is stored in ~/.ssh/known_hosts?
You try to ssh to a server and you get "Host key verification failed". What does it mean?

It means that the key of the remote host was changed and doesn't match the one that stored on the machine (in ~/.ssh/known_hosts).

What is the difference between SSH and SSL?
What ssh-keygen is used for?
What is SSH port forwarding?

Linux - Globbing, Wildcards

What is Globbing?
What are wildcards? Can you give an example of how to use them?
Explain what will ls [XYZ] match
Explain what will ls [^XYZ] match
Explain what will ls [0-5] match
What each of the following matches

?
*
The ? matches any single character
The * matches zero or more characters

What do we grep for in each of the following commands?:

grep '[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}' some_file
grep -E "error|failure" some_file
grep '[0-9]$' some_file

An IP address
The word "error" or "failure"
Lines which end with a number

Which line numbers will be printed when running `grep '\baaa\b'` on the following content:

aaa
bbb
ccc.aaa
aaaaaa

lines 1 and 3.

What is the difference single and double quotes?
What is escaping? What escape character is used for escaping?
What is an exit code? What exit codes are you familiar with?

An exit code (or return code) represents the code returned by a child process to its
parent process.

0 is an exit code which represents success while anything higher than 1 represents error.
Each number has different meaning, based on how the application was developed.

I consider this as a good blog post to read more about it: https://shapeshed.com/unix-exit-codes

Linux Boot Process

Tell me everything you know about the Linux boot process

Another way to ask this: what happens from the moment you turned on the server until you get a prompt

What is GRUB2?
What is Secure Boot?
What can you find in /boot?

Linux Disk & Filesystem

What's an inode?

For each file (and directory) in Linux there is an inode, a data structure which stores meta data
related to the file like its size, owner, permissions, etc.

Which of the following is not included in inode:

Link count
File size
File name
File timestamp

File name (it's part of the directory file)

How to check which disks are currently mounted?

Run mount

You run the mount command but you get no output. How would you check what mounts you have on your system?

cat /proc/mounts

What is the difference between a soft link and hard link?

Hard link is the same file, using the same inode.
Soft link is a shortcut to another file, using a different inode.

True or False? You can create an hard link for a directory

False

True or False? You can create a soft link between different filesystems

True

True or False? Directories always have by minimum 2 links

True.

What happens when you delete the original file in case of soft link and hard link?
Can you check what type of filesystem is used in /home?

There are many answers for this question. One way is running df -T

What is a swap partition? What is it used for?
How to create a

new empty file
a file with text (without using text editor)
a file with given size

You are trying to create a new file but you get "File system is full". You check with df for free space and you see you used only 20% of the space. What could be the problem?
How would you check what is the size of a certain directory?

du -sh

What is LVM?
Explain the following in regards to LVM:

What is NFS? What is it used for?
What RAID is used for? Can you explain the differences between RAID 0, 1, 5 and 10?
Describe the process of extending a filesystem disk space
What is lazy umount?
What is tmpfs?
What is stored in each of the following logs?

/var/log/messages
/var/log/boot.log

True or False? both /tmp and /var/tmp cleared upon system boot

False. /tmp is cleared upon system boot while /var/tmp is cleared every a couple of days or not cleared at all (depends on distro).

Linux Performance Analysis

How to check what is the current load average?

One can use uptime or top

You know how to see the load average, great. but what each part of it means? for example 1.43, 2.34, 2.78

This article summarizes the load average topic in a great way

How to check process usage?

pidstat

How to check disk I/O?

iostat -xz 1

How to check how much free memory a system has? How to check memory consumption by each process?

You can use the commands top and free

How to check TCP stats?

sar -n TCP,ETCP 1

Linux Processes

how to list all the processes running in your system?

ps -ef

How to run a process in the background and why to do that in the first place?

You can achieve that by specifying & at the end of the command.
As to why, since some commands/processes can take a lot of time to finish
execution or run forever, you may want to run them in the background instead of waiting for them to finish before gaining control again in current session.

How can you find how much memory a specific process consumes?
mem() { ps -eo rss,pid,euser,args:100 --sort %mem | grep -v grep | grep -i $@ | awk '{printf $1/1024 "MB"; $1=""; print }' } [Source](https://stackoverflow.com/questions/3853655/in-linux-how-to-tell-how-much-memory-processes-are-using) What signal is used by default when you run 'kill *process id*'?
The default signal is SIGTERM (15). This signal kills process gracefully which means it allows it to save current state configuration. What signals are you familiar with?

SIGTERM - default signal for terminating a process
SIGHUP - common usage is for reloading configuration
SIGKILL - a signal which cannot caught or ignored

To view all available signals run kill -l

What kill 0 does?
What kill -0 does?
What is a trap?
Every couple of days, a certain process stops running. How can you look into why it's happening?
What happens when you press ctrl + c?
What is a Daemon in Linux?

A background process. Most of these processes are waiting for requests or set of conditions to be met before actually running anything.
Some examples: sshd, crond, rpcbind.

What are the possible states of a process in Linux?
Running (R) Uninterruptible Sleep (D) - The process is waiting for I/O Interruptible Sleep (S) Stopped (T) Dead (x) Zombie (z) How do you kill a process in D state?
What is a zombie process?

A process which has finished to run but has not exited.

One reason it happens is when a parent process is programmed incorrectly. Every parent process should execute wait() to get the exit code from the child process which finished to run. But when the parent isn't checking for the child exit code, the child process can still exists although it finished to run.

How to get rid of zombie processes?

You can't kill a zombie process the regular way with kill -9 for example as it's already dead.

One way to kill zombie process is by sending SIGCHLD to the parent process telling it to terminate its child processes. This might not work if the parent process wasn't programmed properly. The invocation is kill -s SIGCHLD [parent_pid]

You can also try closing/terminating the parent process. This will make the zombie process a child of init (1) which does periodic cleanups and will at some point clean up the zombie process.

How to find all the

Processes executed/owned by a certain user
Process which are Java processes
Zombie Processes

If you mention at any point ps command with arugments, be familiar with what these arguments does exactly.

What is the init process?
It is the first process executed by the kernel during the booting of a system. It is a daemon process which runs till the system is shutdown. That is why, it is the parent of all the processes Can you describe how processes are being created?
How to change the priority of a process? Why would you want to do that?
Can you explain how network process/connection is established and how it's terminated?>
What strace does? What about ltrace?
Find all the files which end with '.yml' and replace the number 1 in 2 in each file

find /some_dir -iname *.yml -print0 | xargs -0 -r sed -i "s/1/2/g"

You run ls and you get "/lib/ld-linux-armhf.so.3 no such file or directory". What is the problem?

The ls executable is built for an incompatible architecture.

How would you split a 50 lines file into 2 files of 25 lines each?

You can use the split command this way: split -l 25 some_file

What is a file descriptor? What file descriptors are you familiar with?
Kerberos File descriptor, also known as file handler, is a unique number which identifies an open file in the operating system.

In Linux (and Unix) the first three file descriptors are:

0 - the default data stream for input
1 - the default data stream for output
2 - the default data stream for output related to errors

This is a great article on the topic: https://www.computerhope.com/jargon/f/file-descriptor.htm

What is NTP? What is it used for?
Explain Kernel OOM

Linux Security

What is chroot? In what scenarios would you consider using it?
What is SELiunx?
What is Kerberos?
What is nftables?
What firewalld daemon is responsible for?
Do you have experience with hardening servers? Can you describe the process?

Linux - Networking

How to list all the interfaces?

ip link show

What is the loopback (lo) interface?

The loopback interface is a special, virtual network interface that your computer uses to communicate with itself. It is used mainly for diagnostics and troubleshooting, and to connect to servers running on the local machine.

What the following commands are used for?

ip addr
ip route
ip link
ping
netstat
traceroute

What is a network namespace? What is it used for?
How to check if a certain port is being used?

One of the following would work:

netstat -tnlp | grep <port_number>
lsof -i -n -P | grep <port_number>

How can you turn your Linux server into a router?
What is a virtual IP? In what situation would you use it?
True or False? The MAC address of an interface is assigned/set by the OS

False

Can you have more than one default gateway in a given system?

Technically, yes.

Which port is used in each of the following protocols?:

SSH
SMTP
HTTP
DNS
HTTPS
SSH - 22
SMTP - 25
HTTP - 80
DNS - 53
HTTPS - 443

What is telnet and why is it a bad idea to use it in production? (or at all)

Telnet is a type of client-server protocol that can be used to open a command line on a remote computer, typically a server.
By default, all the data sent and received via telnet is transmitted in clear/plain text, therefore it should not be used as it does not encrypt any data between the client and the server.

What is the routing table? How do you view it?
How can you send an HTTP request from your shell?

Using nc is one way
What are packet sniffers? Have you used one in the past? If yes, which packet sniffers have you used and for what purpose?
It is a network utility that analyses and may inject tasks into the data-stream travelling over the targeted network. How to list active connections?
How to trigger neighbor discovery in IPv6?

One way would be ping6 ff02::1

What is network interface bonding and do you know how it's performed in Linux?
What network bonding modes are there?

There a couple of modes:

balance-rr: round robing bonding
active-backup: a fault tolerance mode where only one is active
balance-tlb: Adaptive transmit load balancing
balance-alb: Adaptive load balancing

What is a bridge? How it's added in Linux OS?

Linux DNS

How to check what is the hostname of the system?

cat /etc/hostname

You can also run hostnamectl or hostname but that might print only a temporary hostname. The one in the file is the permanent one.

What the file /etc/resolv.conf is used for? What does it include?
What commands are you using for performing DNS queries (or troubleshoot DNS related issues)?

You can specify one or more of the following:

dig
host
nslookup

Linux - Packaging

Do you have experience with packaging? (as in building packages) Can you explain how does it works?
How packages installation/removal is performed on the distribution you are using?

The answer depends on the distribution being used.

In Fedora/CentOS/RHEL/Rocky it can be done with rpm or dnf commands.
In Ubuntu it can be done with the apt command.

RPM: explain the spec format (what it should and can include)
How do you list the content of a package without actually installing it?
How to know to which package a file on the system belongs to? Is it a problem if it doesn't belongs to any package?
Where repositories are stored? (based on the distribution you are using)
What is an archive? How do you create one in Linux?
How to extract the content of an archive?
Why do we need package managers? Why not simply creating archives and publish them?

Package managers allow you to manage packages lifecycle as in installing, removing and updating the packages.

In addition, you can specify in a spec how a certain package will be installed - where to copy the files, which commands to run prior to the installation, post the installation, etc.

Linux DNF

How to look for a package that provides the command /usr/bin/git? (the package isn't necessarily installed)

dnf provides /usr/bin/git

Linux Applications and Services

What can you find in /etc/services?
How to make sure a Service starts automatically after a reboot or crash?

Depends on the init system.

Systemd: systemctl enable [service_name]
System V: update-rc.d [service_name] and add this line id:5678:respawn:/bin/sh /path/to/app to /etc/inittab
Upstart: add Upstart init script at /etc/init/service.conf

You run ssh 127.0.0.1 but it fails with "connection refused". What could be the problem?

SSH server is not installed
SSH server is not running

How to print the shared libraries required by a certain program? What is it useful for?
What is CUPS?
What types of web servers are you familiar with?

Nginx, Apache httpd.

Linux Users and Groups

What is a "superuser" (or root user)? How is it different from regular users?
How do you create users? Where user information is stored?
Which file stores information about groups?
How do you change/set the password of a user?
Which file stores users passwords? Is it visible for everyone?
Do you know how to create a new user without using adduser/useradd command?
What information is stored in /etc/passwd? explain each field
How to add a new user to the system without providing him the ability to log-in into the system?

adduser user_name --shell=/bin/false --no-create-home
You can also add a user and then edit /etc/passwd.

How to switch to another user? How to switch to the root user?

su command.
Use su - to switch to root

What is the UID the root user? What about a regular user?
What can you do if you lost/forogt the root password?

Re-install the OS IS NOT the right answer :)

What is /etc/skel?
How to see a list of who logged-in to the system?

Using the last command.

Explain what each of the following commands does:

useradd
usermod
whoami
id

Linux Hardware

Where can you find information on the processor?

/proc/cpuinfo

How can you print information on the BIOS, motherboard, processor and RAM?

dmidecoode

How can you print all the information on connected block devices in your system?

lsblk

True or False? In user space, applications don't have full access to hardware resources

True. Only in kernel space they have full access to hardware resources.

Linux - Security

How do you create a private key for a CA (certificate authority)?

One way is using openssl this way:

openssl genrsa -aes256 -out ca-private-key.pem 4096

How do you create a public key for a CA (certificate authority)?

openssl req -new -x509 -days 730 -key [private key file name] -sha256 -out ca.pem

If using the private key from the previous question then the command would be:

openssl req -new -x509 -days 730 -key ca-private-key.pem -sha256 -out ca.pem

Linux - Namespaces

What types of namespaces are there in Linux?

Process ID namespaces: these namespaces include independent set of process IDs
Mount namespaces: Isolation and control of mountpoints
Network namespaces: Isolates system networking resources such as routing table, interfaces, ARP table, etc.
UTS namespaces: Isolate host and domains
IPC namespaces: Isolates interprocess communications
User namespaces: Isolate user and group IDs
Time namespaces: Isolates time machine

True or False? In every PID (Process ID) namespace the first process assigned with the process id number 1

True. Inside the namespace it's PID 1 while to the parent namespace the PID is a different one.

True or False? In a child PID namespace all processes are aware of parent PID namespace and processes and the parent PID namespace has no visibility of child PID namespace processes

False. The opposite is true. Parent PID namespace is aware and has visibility of processes in child PID namespace and child PID namespace has no visibility as to what is going on in the parent PID namespace.

True or False? By default, when creating two separate network namespaces, a ping from one namespace to another will work fine

False. Network namespace has its own interfaces and routing table. There is no way (without creating a bridge for example) for one network namespace to reach another.

True or False? With UTS namespaces, processes may appear as if they run on different hosts and domains while running on the same host

True

True or False? It's not possible to have a root user with ID 0 in child user namespaces

False. In every child user namespace, it's possible to have a separate root user with uid of 0.

What time namespaces are used for?

In time namespaces processes can use different system time.

Linux - Virtualization

What virtualization solutions are available for Linux?
What is KVM?

Linux - AWK

What the awk command does? Have you used it? What for?

From Wikipedia: "AWK is domain-specific language designed for text processing and typically used as a data extraction and reporting tool"

How to print the 4th column in a file?

awk '{print $4}' file

How to print every line that is longer than 79 characters?

awk 'length($0) > 79' file

What the lsof command does? Have you used it? What for?
What is the difference between find and locate?
How a user process performs a privileged operation, such as reading from the disk?

Using system calls

Linux - System Calls

What is a system call? What system calls are you familiar with?
How a program executes a system call?

A program executes a trap instruction. The instruction jump into the kernel while raising the privileged level to kernel space.
Once in kernel space, it can perform any privileged operation
Once it's finished, it calls a "return-from-trap" instruction which returns to user space while reducing back the privilege level to user space.

Explain the fork() system call

fork() is used for creating a new process. It does so by cloning the calling process but the child process has its own PID and any memory locks, I/O operations and semaphores are not inherited.

What is the return value of fork()?

On success, the PID of the child process in parent and 0 in child process
On error, -1 in the parent

Name one reason for fork() to fail

Not enough memory to create a new process

Why do we need the wait() system call?

wait() is used by a parent process to wait for the child process to finish execution.
If wait is not used by a parent process then a child process might become a zombie process.

How the kernel notifies the parent process about child process termination?

The kernel notifies the parent by sending the SIGCHLD to the parent.

How the waitpid() is different from wait()?

The waitpid() is a non-blocking version of the wait() function.

It also supports using library routine (e.g. system()) to wait a child process without messing up with other children processes for which the process has not waited.

True or False? The wait() system call won't return until the child process has run and exited

True in most cases though there are cases where wait() returns before the child exits.

Explain the exec() system call

It transforms the current running program into another program.

Given the name of an executable and some arguments, it loads the code and static data from the specified executable and overwrites its current code segment and current static code data. After initializing its memory space (like stack and heap) the OS runs the program passing any arguments as the argv of that process.

True or False? A successful call to exec() never returns

True

Since a succesful exec replace the current process, it can't return anything to the process that made the call.

What system call is used for listing files?
What system calls are used for creating a new process?

fork(), exec() and the wait() system call is also included in this workflow.

What execve() does?

Executes a program. The program is passed as a filename (or path) and must be a binary executable or a script.

What is the return value of malloc?
Explain the pipe() system call. What does it used for?

Unix pipe implementation

"Pipes provide a unidirectional interprocess communication channel. A pipe has a read end and a write end. Data written to the write end of a pipe can be read from the read end of the pipe.
A pipe is created using pipe(2), which returns two file descriptors, one referring to the read end of the pipe, the other referring to the write end."

What happens when you execute ls -l?

Shell reads the input using getline() which reads the input file stream and stores into a buffer as a string

The buffer is broken down into tokens and stored in an array this way: {"ls", "-l", "NULL"}

Shell checks if an expansion is required (in case of ls *.c)

Once the program in memory, its execution starts. First by calling readdir()

Notes:

getline() originates in GNU C library and used to read lines from input stream and stores those lines in the buffer

What happens when you execute ls -l *.log?
What readdir() system call does?
What exactly the command alias x=y does?
Why running a new program is done using the fork() and exec() system calls? why a different API wasn't developed where there is one call to run a new program?

This way provides a lot of flexibility. It allows the shell for example, to run code after the call to fork() but before the call to exec(). Such code can be used to alter the environment of the program it about to run.

Describe shortly what happens when you execute a command in the shell

The shell figures out, using the PATH variable, where the executable of the command resides in the filesystem. It then calls fork() to create a new child process for running the command. Once the fork was executed successfully, it calls a variant of exec() to execute the command and finally, waits the command to finish using wait(). When the child completes, the shell returns from wait() and prints out the prompt again.

Linux Filesystem & Files

How to create a file of a certain size?

There are a couple of ways to do that:

dd if=/dev/urandom of=new_file.txt bs=2MB count=1
truncate -s 2M new_file.txt
fallocate -l 2097152 new_file.txt

What does the following block do?:

open("/my/file") = 5
read(5, "file content")

These system calls are reading the file /my/file and 5 is the file descriptor number.

Describe three different ways to remove a file (or its content)
What is the difference between a process and a thread?
What is context switch?

From wikipedia: a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point

You found there is a server with high CPU load but you didn't find a process with high CPU. How is that possible?

Linux Advanced - Networking

When you run ip a you see there is a device called 'lo'. What is it and why do we need it?
What the traceroute command does? How does it works?

Another common way to task this questions is "what part of the tcp header does traceroute modify?"

What is network bonding? What types are you familiar with?
How to link two separate network namespaces so you can ping an interface on one namespace from the second one?
What are cgroups?
Explain Process Descriptor and Task Structure
What are the differences between threads and processes?
Explain Kernel Threads
What happens when socket system call is used?

This is a good article about the topic: https://ops.tips/blog/how-linux-creates-sockets

You executed a script and while still running, it got accidentally removed. Is it possible to restore the script while it's still running?

Linux Memory

What is the difference between MemFree and MemAvailable in /proc/meminfo?

MemFree - The amount of unused physical RAM in your system
MemAvailable - The amount of available memory for new workloads (without pushing system to use swap) based on MemFree, Active(file), Inactive(file), and SReclaimable.

What is the difference between paging and swapping?
Explain what is OOM killer

Distribution

What is a Linux distribution?
What Linux distributions are you familiar with?
What are the components of a Linux distribution?

Kernel
Utilities
Services
Software/Packages Management

Linux - Misc

Name 5 commands which are two letters long

ls, wc, dd, df, du, ps, ip, cp, cd ...

What ways are there for creating a new empty file?

touch new_file
echo "" > new_file

How `cd -` works? How does it knows the previous location?

$OLDPWD

List three ways to print all the files in the current directory

ls
find .
echo *

How to count the number of lines in a file? What about words?
You define x=2 in /etc/bashrc and x=6 ~/.bashrc you then login to the system. What would be the value of x?
What is the difference between man and info?

A good answer can be found here

Explain "environment variables". How do you list all environment variables?
What is a TTY device?
How to create your own environment variables?

X=2 for example. But this will persist to new shells. To have it in new shells as well, use export X=2

What a double dash (--) mean?

It's used in commands to mark the end of commands options. One common example is when used with git to discard local changes: git checkout -- some_file

Wildcards are implemented on user or kernel space?
If I plug a new device into a Linux machine, where on the system, a new device entry/file will be created?

/dev

Why there are different sections in man? What is the difference between the sections?
What is User-mode Linux?
Under which license Linux is distributed?

GPL v2

Operating System

Operating System Exercises

Name	Topic	Objective & Instructions	Solution	Comments
Fork 101	Fork	Link	Link
Fork 102	Fork	Link	Link

Operating System - Self Assessment

What is an operating system?

From the book "Operating Systems: Three Easy Pieces":

"responsible for making it easy to run programs (even allowing you to seemingly run many at the same time), allowing programs to share memory, enabling programs to interact with devices, and other fun stuff like that".

Operating System - Process

Can you explain what is a process?

A process is a running program. A program is one or more instructions and the program (or process) is executed by the operating system.

If you had to design an API for processes in an operating system, what would this API look like?

It would support the following:

Create - allow to create new processes
Delete - allow to remove/destroy processes
State - allow to check the state of the process, whether it's running, stopped, waiting, etc.
Stop - allow to stop a running process

How a process is created?

The OS is reading program's code and any additional relevant data
Program's code is loaded into the memory or more specifically, into the address space of the process.
Memory is allocated for program's stack (aka run-time stack). The stack also initialized by the OS with data like argv, argc and parameters to main()
Memory is allocated for program's heap which is required for dynamically allocated data like the data structures linked lists and hash tables
I/O initialization tasks are performed, like in Unix/Linux based systems where each process has 3 file descriptors (input, output and error)
OS is running the program, starting from main()

True or False? The loading of the program into the memory is done eagerly (all at once)

False. It was true in the past but today's operating systems perform lazy loading which means only the relevant pieces required for the process to run are loaded first.

What are different states of a process?

Running - it's executing instructions
Ready - it's ready to run but for different reasons it's on hold
Blocked - it's waiting for some operation to complete. For example I/O disk request

What are some reasons for a process to become blocked?

I/O operations (e.g. Reading from a disk)
Waiting for a packet from a network

What is Inter Process Communication (IPC)?
What is "time sharing"?

Even when using a system with one physical CPU, it's possible to allow multiple users to work on it and run programs. This is possible with time sharing where computing resources are shared in a way it seems to the user the system has multiple CPUs but in fact it's simply one CPU shared by applying multiprogramming and multi-tasking.

What is "space sharing"?

Somewhat the opposite of time sharing. While in time sharing a resource is used for a while by one entity and then the same resource can be used by another resource, in space sharing the space is shared by multiple entities but in a way where it's not being transferred between them.

It's used by one entity until this entity decides to get rid of it. Take for example storage. In storage, a file is yours until you decide to delete it.

What component determines which process runs at a given moment in time?

CPU scheduler

Operating System - Memory

What is "virtual memory" and what purpose it serves?
What is demand paging?
What is copy-on-write or shadowing?
What is a kernel, and what does it do?

The kernel is part of the operating system and is responsible for tasks like:

Allocating memory
Schedule processes
Control CPU

True or False? Some pieces of the code in the kernel are loaded into protected areas of the memory so applications can't overwritten them

True

What is POSIX?
Explain what is Semaphore and what its role in operating systems
What is cache? What is buffer?

Buffer: Reserved place in RAM which is used to hold data for temporary purposes
Cache: Cache is usually used when processes reading and writing to the disk to make the process faster by making similar data used by different programs easily accessible.

Virtualization

What is Virtualization?

Virtualization uses software to create an abstraction layer over computer hardware that allows the hardware elements of a single computer—processors, memory, storage and more - to be divided into multiple virtual computers, commonly called virtual machines (VMs).

What is a hypervisor?

Red Hat: "A hypervisor is software that creates and runs virtual machines (VMs). A hypervisor, sometimes called a virtual machine monitor (VMM), isolates the hypervisor operating system and resources from the virtual machines and enables the creation and management of those VMs."

Read more here

What types of hypervisors are there?

Hosted hypervisors and bare-metal hypervisors.

What are the advantages and disadvantges of bare-metal hypervisor over a hosted hypervisor?

Due to having its own drivers and a direct access to hardware components, a baremetal hypervisor will often have better performances along with stability and scalability.

On the other hand, there will probably be some limitation regarding loading (any) drivers so a hosted hypervisor will usually benefit from having a better hardware compatibility.

What types of virtualization are there?

Operating system virtualization
Network functions virtualization
Desktop virtualization

Is containerization is a type of Virtualization?

Yes, it's a operating-system-level virtualization, where the kernel is shared and allows to use multiple isolated user-spaces instances.

How the introduction of virtual machines changed the industry and the way applications were deployed?

The introduction of virtual machines allowed companies to deploy multiple business applications on the same hardware while each application is separated from each other in secured way, where each is running on its own separate operating system.

Ansible

Ansible Exercises

Name	Topic	Objective & Instructions	Solution
My First Task	Tasks	Exercise	Solution
Upgrade and Update Task	Tasks	Exercise	Solution
My First Playbook	Playbooks	Exercise	Solution

Ansible Self Assesment

Describe each of the following components in Ansible, including the relationship between them:

Task
Module
Play
Playbook
Role

Task – a call to a specific Ansible module
Module – the actual unit of code executed by Ansible on your own host or a remote host. Modules are indexed by category (database, file, network, …) and also referred to as task plugins.

Play – One or more tasks executed on a given host(s)

Playbook – One or more plays. Each play can be executed on the same or different hosts

Role – Ansible roles allows you to group resources based on certain functionality/service such that they can be easily reused. In a role, you have directories for variables, defaults, files, templates, handlers, tasks, and metadata. You can then use the role by simply specifying it in your playbook.

How Ansible is different from other automation tools? (e.g. Chef, Puppet, etc.)

Ansible is:

Agentless
Minimal run requirements (Python & SSH) and simple to use
Default mode is "push" (it supports also pull)
Focus on simpleness and ease-of-use

True or False? Ansible follows the mutable infrastructure paradigm

True. In immutable infrastructure approach, you'll replace infrastructure instead of modifying it.

Ansible rather follows the mutable infrastructure paradigm where it allows you to change the configuration of different components, but this approach is not perfect and has its own disadvantges like "configuration drift" where different components may reach different state for different reasons.

True or False? Ansible uses declarative style to describe the expected end state

False. It uses a procedural style.

What kind of automation you wouldn't do with Ansible and why?

While it's possible to provision resources with Ansible, some prefer to use tools that follow immutable infrastructure paradigm.
Ansible doesn't saves state by default. So a task that creates 5 instances for example, when executed again will create additional 5 instances (unless
additional check is implemented or explicit names are provided) while other tools might check if 5 instances exist. If only 4 exist (by checking the state file for example), one additional instance will be created to reach the end goal of 5 instances.

How do you list all modules and how can you see details on a specific module?

Ansible online docs
ansible-doc -l for list of modules and ansible-doc [module_name] for detailed information on a specific module

Ansible - Inventory

What is an inventory file and how do you define one?

An inventory file defines hosts and/or groups of hosts on which Ansible tasks executed upon.

An example of inventory file:

192.168.1.2
192.168.1.3
192.168.1.4

[web_servers]
190.40.2.20
190.40.2.21
190.40.2.22

What is a dynamic inventory file? When you would use one?

A dynamic inventory file tracks hosts from one or more sources like cloud providers and CMDB systems.

You should use one when using external sources and especially when the hosts in your environment are being automatically

spun up and shut down, without you tracking every change in these sources.

Ansible - Variables

Modify the following task to use a variable instead of the value "zlib" and have "zlib" as the default in case the variable is not defined

- name: Install a package
  package:
    name: "zlib"
    state: present

- name: Install a package
  package:
    name: "{{ package_name|default('zlib') }}"
    state: present

How to make the variable "use_var" optional?

- name: Install a package
  package:
    name: "zlib"
    state: present
    use: "{{ use_var }}"

With "default(omit)"

- name: Install a package
  package:
    name: "zlib"
    state: present
    use: "{{ use_var|default(omit) }}"

What would be the result of the following play?

---
- name: Print information about my host
  hosts: localhost
  gather_facts: 'no'
  tasks:
      - name: Print hostname
        debug:
            msg: "It's me, {{ ansible_hostname }}"

When given a written code, always inspect it thoroughly. If your answer is “this will fail” then you are right. We are using a fact (ansible_hostname), which is a gathered piece of information from the host we are running on. But in this case, we disabled facts gathering (gather_facts: no) so the variable would be undefined which will result in failure.

When the value '2017'' will be used in this case: `{{ lookup('env', 'BEST_YEAR') | default('2017', true) }}`?

when the environment variable 'BEST_YEAR' is empty or false.

If the value of certain variable is 1, you would like to use the value "one", otherwise, use "two". How would you do it?

{{ (certain_variable == 1) | ternary("one", "two") }}

The value of a certain variable you use is the string "True". You would like the value to be a boolean. How would you cast it?

{{ some_string_var | bool }}

You want to run Ansible playbook only on specific minor version of your OS, how would you achieve that?
What the "become" directive used for in Ansible?
What are facts? How to see all the facts of a certain host?
What would be the result of running the following task? How to fix it?

- hosts: localhost
  tasks:
      - name: Install zlib
        package:
          name: zlib
          state: present

Which Ansible best practices are you familiar with?. Name at least three
Explain the directory layout of an Ansible role
What 'blocks' are used for in Ansible?
How do you handle errors in Ansible?
You would like to run a certain command if a task fails. How would you achieve that?
Write a playbook to install ‘zlib’ and ‘vim’ on all hosts if the file ‘/tmp/mario’ exists on the system.

---
- hosts: all
  vars:
      mario_file: /tmp/mario
      package_list:
          - 'zlib'
          - 'vim'
  tasks:
      - name: Check for mario file
        stat:
            path: "{{ mario_file }}"
        register: mario_f

      - name: Install zlib and vim if mario file exists
        become: "yes"
        package:
            name: "{{ item }}"
            state: present
        with_items: "{{ package_list }}"
        when: mario_f.stat.exists

Write a single task that verifies all the files in files_list variable exist on the host

- name: Ensure all files exist
  assert:
    that:
      - item.stat.exists
  loop: "{{ files_list }}"

Write a playbook to deploy the file ‘/tmp/system_info’ on all hosts except for controllers group, with the following content

I'm <HOSTNAME> and my operating system is <OS>

Replace and with the actual data for the specific host you are running on

The playbook to deploy the system_info file

---
- name: Deploy /tmp/system_info file
  hosts: all:!controllers
  tasks:
      - name: Deploy /tmp/system_info
        template:
            src: system_info.j2
            dest: /tmp/system_info

The content of the system_info.j2 template

# {{ ansible_managed }}
I'm {{ ansible_hostname }} and my operating system is {{ ansible_distribution }

The variable 'whoami' defined in the following places:

role defaults -> whoami: mario
extra vars (variables you pass to Ansible CLI with -e) -> whoami: toad
host facts -> whoami: luigi
inventory variables (doesn’t matter which type) -> whoami: browser

According to variable precedence, which one will be used?

The right answer is ‘toad’.

Variable precedence is about how variables override each other when they set in different locations. If you didn’t experience it so far I’m sure at some point you will, which makes it a useful topic to be aware of.

In the context of our question, the order will be extra vars (always override any other variable) -> host facts -> inventory variables -> role defaults (the weakest).

Here is the order of precedence from least to greatest (the last listed variables winning prioritization):

command line values (eg “-u user”)
role defaults [1]
inventory file or script group vars [2]
inventory group_vars/all [3]
playbook group_vars/all [3]
inventory group_vars/* [3]
playbook group_vars/* [3]
inventory file or script host vars [2]
inventory host_vars/* [3]
playbook host_vars/* [3]
host facts / cached set_facts [4]
play vars
play vars_prompt
play vars_files
role vars (defined in role/vars/main.yml)
block vars (only for tasks in block)
task vars (only for the task)
include_vars
set_facts / registered vars
role (and include_role) params
include params
extra vars (always win precedence)

A full list can be found at PlayBook Variables . Also, note there is a significant difference between Ansible 1.x and 2.x.

For each of the following statements determine if it's true or false:

A module is a collection of tasks
It’s better to use shell or command instead of a specific module
Host facts override play variables
A role might include the following: vars, meta, and handlers
Dynamic inventory is generated by extracting information from external sources
It’s a best practice to use indention of 2 spaces instead of 4
‘notify’ used to trigger handlers
This “hosts: all:!controllers” means ‘run only on controllers group hosts

Explain the Diffrence between Forks and Serial & Throttle.

Serial is like running the playbook for each host in turn, waiting for completion of the complete playbook before moving on to the next host. forks=1 means run the first task in a play on one host before running the same task on the next host, so the first task will be run for each host before the next task is touched. Default fork is 5 in ansible.

[defaults]
forks = 30

- hosts: webservers
  serial: 1
  tasks:
    - name: ...

Ansible also supports throttle This keyword limits the number of workers up to the maximum set via the forks setting or serial. This can be useful in restricting tasks that may be CPU-intensive or interact with a rate-limiting API

tasks:
- command: /path/to/cpu_intensive_command
  throttle: 1

What is ansible-pull? How is it different from how ansible-playbook works?
What is Ansible Vault?
Demonstrate each of the following with Ansible:

Conditionals
Loops

What are filters? Do you have experience with writing filters?
Write a filter to capitalize a string

def cap(self, string):
    return string.capitalize()

You would like to run a task only if previous task changed anything. How would you achieve that?
What are callback plugins? What can you achieve by using callback plugins?
What is Ansible Collections?
What is the difference between `include_task` and `import_task`?
File '/tmp/exercise' includes the following content

Goku = 9001
Vegeta = 5200
Trunks = 6000
Gotenks = 32

With one task, switch the content to:

Goku = 9001
Vegeta = 250
Trunks = 40
Gotenks = 32

- name: Change saiyans levels
  lineinfile:
    dest: /tmp/exercise
    regexp: "{{ item.regexp }}"
    line: "{{ item.line }}"
  with_items:
    - { regexp: '^Vegeta', line: 'Vegeta = 250' }
    - { regexp: '^Trunks', line: 'Trunks = 40' }
    ...

Ansible - Execution and Strategy

True or False? By default, Ansible will execute all the tasks in play on a single host before proceeding to the next host

False. Ansible will execute a single task on all hosts before moving to the next task in a play. As for today, it uses 5 forks by default.

This behaviour is described as "strategy" in Ansible and it's configurable.

What is a "strategy" in Ansible? What is the default strategy?

A strategy in Ansible describes how Ansible will execute the different tasks on the hosts. By default Ansible is using the "Linear strategy" which defines that each task will run on all hosts before proceeding to the next task.

What strategies are you familiar with in Ansible?

Linear: the default strategy in Ansible. Run each task on all hosts before proceeding.
Free: For each host, run all the tasks until the end of the play as soon as possible
Debug: Run tasks in an interactive way

What the serial keyword is used for?

It's used to specify the number (or percentage) of hosts to run the full play on, before moving to next number of hosts in the group.

For example:

- name: Some play
  hosts: databases
  serial: 4

If your group has 8 hosts. It will run the whole play on 4 hosts and then the same play on another 4 hosts.

Ansible Testing

How do you test your Ansible based projects?
What is Molecule? How does it works?
You run Ansibe tests and you get "idempotence test failed". What does it mean? Why idempotence is important?

Ansible - Debugging

How to find out the data type of a certain variable in one of the playbooks?

"{{ some_var | type_debug }}"

Ansible - Collections

What are collections in Ansible?

Terraform

Explain what Terraform is and how does it works

Terraform.io: "Terraform is an infrastructure as code (IaC) tool that allows you to build, change, and version infrastructure safely and efficiently."

Why one would prefer using Terraform and not other technologies? (e.g. Ansible, Puppet, CloufFormation)

A common wrong answer is to say that Ansible and Puppet are configuration management tools
and Terraform is a provisioning tool. While technically true, it doesn't mean Ansible and Puppet can't
be used for provisioning infrastructure. Also, it doesn't explain why Terraform should be used over
CloudFormation if at all.

The benefits of Terraform over the other tools:

It follows the immutable infrastructure approach which has benefits like avoiding a configuration drift over time
Ansible and Puppet are more procedural (you mention what to execute in each step) and Terraform is declarative since you describe the overall desired state and not per resource or task. You can give the example of going from 1 to 2 servers in each tool. In Terraform you specify 2, in Ansible and puppet you have to only provision 1 additional server so you need to explicitly make sure you provision only another one server.

How do you structure your Terraform projects?

terraform_directory
providers.tf -> List providers (source, version, etc.)
variables.tf -> any variable used in other files such as main.tf
main.tf -> Lists the resources

True or False? Terraform follows the mutable infrastructure paradigm

False. Terraform follows immutable infrastructure paradigm.

True or False? Terraform uses declarative style to describe the expected end state
True What is HCL?
HCL stands for Hashicorp Configuration Language. It is the language Hashicorp made to use as the configuration language for a number of its tools, including terraform. Explain what is "Terraform configuration"

A configuration is a root module along with a tree of child modules that are called as dependencies from the root module.

Explain what the following commands do:

terraform init
terraform plan
terraform validate
terraform apply

terraform init scans your code to figure which providers are you using and download them.
terraform plan will let you see what terraform is about to do before actually doing it.
terraform validate checks if configuration is syntactically valid and internally consistent within a directory.
terraform apply will provision the resources specified in the .tf files.

Terraform - Resources

What is a "resource"?

HashiCorp: "Terraform uses resource blocks to manage infrastructure, such as virtual networks, compute instances, or higher-level components such as DNS records. Resource blocks represent one or more infrastructure objects in your Terraform configuration."

Explain each part of the following line: `resource "aws_instance" "web_server" {...}`

resource: keyword for defining a resource
"aws_instance": the type of the resource
"web_server": the name of the resource

What is the ID of the following resource: `resource "aws_instance" "web_server" {...}`

aws_instance.web_server

True or False? Resource ID must be unique within a workspace

True

Explain each of the following in regards to resources

Arguments
Attributes
Meta-arguments
Arguments: resource specific configurations
Attributes: values exposed by the resource in a form of resource_type.resource_name.attribute_name. They are set by the provider or API usually.
Meta-arguments: Functions of Terraform to change resource's behaviour

Terraform - Providers

Explain what is a "provider"

terraform.io: "Terraform relies on plugins called "providers" to interact with cloud providers, SaaS providers, and other APIs...Each provider adds a set of resource types and/or data sources that Terraform can manage. Every resource type is implemented by a provider; without providers, Terraform can't manage any kind of infrastructure."

What is the name of the provider in this case: `resource "libvirt_domain" "instance" {...}`

libvirt

Terraform - Variables

What are Input Variables in Terraform? Why one should use them?

Input variables serve as parameters to the module in Terraform. They allow you for example to define once the value of a variable and use that variable in different places in the module so next time you would want to change the value, you will change it in one place instead of changing the value in different places in the module.

How to define variables?

variable "app_id" {
  type = string
  description = "The id of application"
  default = "some_value"
}

Usually they are defined in their own file (vars.tf for example).

How variables are used in modules?

They are referenced with var.VARIABLE_NAME

vars.tf:

variable "memory" {
  type = string
  default "8192"
}

variable "cpu" {
  type = string
  default = "4"
}

main.tf:

resource "libvirt_domain" "vm1" {
   name = "vm1"
   memory = var.memory
   cpu = var.cpu
}

How would you enforce users that use your variables to provide values with certain constraints? For example, a number greater than 1

Using validation block

variable "some_var" {
  type = number
  
  validation {
    condition = var.some_var > 1
    error_message = "you have to specify a number greater than 1"
  }

}

What is the effect of setting variable as "sensitive"?

It doesn't show its value when you run terraform apply or terraform plan but eventually it's still recorded in the state file.

True or Fales? If an expression's result depends on a sensitive variable, it will be treated as sensitive as well

True

The same variable is defined in the following places:

The file terraform.tfvars
Environment variable
Using -var or -var-file

According to varaible precedence, which source will be used first?

The order is:

Environment variable
The file terraform.tfvars
Using -var or -var-file

What other way is there to define lots of variables in more "simplified" way?

Using .tfvars file which contains variable consists of simple variable names assignments this way:

x = 2
y = "mario"
z = "luigi"

Terraform - State

What terraform.tfstate file is used for?

It keeps track of the IDs of created resources so that Terraform knows what it's managing.

How do you rename an existing resource?

terraform state mv

Why does it matter where you store the tfstate file? Where would you store it?

tfstate contains credentials in plain text. You don't want to put it in publicly shared location
tfstate shouldn't be modified concurrently so putting it in a shared location available for everyone with "write" permissions might lead to issues. (Terraform remote state doesn't has this problem).
tfstate is in important file. As such, it might be better to put it in a location that has regular backups.

As such, tfstate shouldn't be stored in git repositories. secured storage such as secured buckets, is a better option.

Which command is responsible for creating state file?

terraform apply file.terraform
Above command will create tfstate file in the working folder.

By default where does the state get stored?

The state is stored by default in a local file named terraform.tfstate.

Can we store tfstate file at remote location? If yes, then in which condition you will do this?

Yes, It can also be stored remotely, which works better in a team environment. Given condition that remote location is not publicly accessible since tfstate file contain sensitive information as well. Access to this remote location must be only shared with team members.

Mention some best practices related to tfstate

Don't edit it manually. tfstate was designed to be manipulated by terraform and not by users directly.
Store it in secured location (since it can include credentials and sensitive data in general)
Backup it regularly so you can roll-back easily when needed
Store it in remote shared storage. This is especially needed when working in a team and the state can be updated by any of the team members
Enabled versioning if the storage where you store the state file, supports it. Versioning is great for backups and roll-backs in case of an issue.

How and why concurrent edits of the state file should be avoided?

If there are two users or processes concurrently editing the state file it can result in invalid state file that doesn't actually represents the state of resources.

To avoid that, Terraform can apply state locking if the backend supports that. For example, AWS s3 supports state locking and consistency via DynamoDB. Often, if the backend support it, Terraform will make use of state locking automatically so nothing is required from the user to activate it.

Describe how you manage state file(s) when you have multiple environments (e.g. development, staging and production)

There is no right or wrong here, but it seems that the overall preferred way is to have a dedicated state file per environment.

How to write down a variable which changes by an external source or during terraform apply?

You use it this way: variable “my_var” {}

You've deployed a virtual machine with Terraform and you would like to pass data to it (or execute some commands). Which concept of Terraform would you use?

Provisioners

Terraform - Provisioners

What are "Provisioners"? What they are used for?

Provisioners used to execute actions on local or remote machine. It's extremely useful in case you provisioned an instance and you want to make a couple of changes in the machine you've created without manually ssh into it after Terraform finished to run and manually run them.

What is local-exec and remote-exec in the context of provisioners?
What is a "tainted resource"?

It's a resource which was successfully created but failed during provisioning. Terraform will fail and mark this resource as "tainted".

What terraform taint does?
terraform taint resource.id manually marks the resource as tainted in the state file. So when you run terraform apply the next time, the resource will be destroyed and recreated. What types of variables are supported in Terraform?

string
number
bool
list()
set()
map()
object({<ATTR_NAME> = , ... })
tuple([, ...])

What is a data source? In what scenarios for example would need to use it?
Data sources lookup or compute values that can be used elsewhere in terraform configuration.

There are quite a few cases you might need to use them:

you want to reference resources not managed through terraform
you want to reference resources managed by a different terraform module
you want to cleanly compute a value with typechecking, such as with aws_iam_policy_document

What are output variables and what terraform output does?
Output variables are named values that are sourced from the attributes of a module. They are stored in terraform state, and can be used by other modules through remote_stateExplain Modules

A Terraform module is a set of Terraform configuration files in a single directory. Modules are small, reusable Terraform configurations that let you manage a group of related resources as if they were a single resource. Even a simple configuration consisting of a single directory with one or more .tf files is a module. When you run Terraform commands directly from such a directory, it is considered the root module. So in this sense, every Terraform configuration is part of a module.

What is the Terraform Registry?

The Terraform Registry provides a centralized location for official and community-managed providers and modules.

Explain remote-exec and local-exec
Explain "Remote State". When would you use it and how?
Terraform generates a `terraform.tfstate` json file that describes components/service provisioned on the specified provider. Remote State stores this file in a remote storage media to enable collaboration amongst team. Explain "State Locking"
State locking is a mechanism that blocks an operations against a specific state file from multiple callers so as to avoid conflicting operations from different team members. Once the first caller's operation's lock is released the other team member may go ahead to carryout his own operation. Nevertheless Terraform will first check the state file to see if the desired resource already exist and if not it goes ahead to create it. What is the "Random" provider? What is it used for
The random provider aids in generating numeric or alphabetic characters to use as a prefix or suffix for a desired named identifier. How do you test a terraform module?
Many examples are acceptable, but the most common answer would likely to be using the tool terratest, and to test that a module can be initialized, can create resources, and can destroy those resources cleanly. Aside from .tfvars files or CLI arguments, how can you inject dependencies from other modules?
The built-in terraform way would be to use remote-state to lookup the outputs from other modules. It is also common in the community to use a tool called terragrunt to explicitly inject variables between modules. What is Terraform import?

Terraform import is used to import existing infrastucture. It allows you to bring resources created by some other means (eg. manually launched cloud resources) and bring it under Terraform management.

How do you import existing resource using Terraform import?

Identify which resource you want to import.
Write terraform code matching configuration of that resource.
Run terraform command terraform import RESOURCE ID

eg. Let's say you want to import an aws instance. Then you'll perform following:

Identify that aws instance in console
Refer to it's configuration and write Terraform code which will look something like:

resource "aws_instance" "tf_aws_instance" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"

  tags = {
    Name = "import-me"
  }
}

Run terraform command terraform import aws_instance.tf_aws_instance i-12345678

Containers

Containers Exercises

Name	Topic	Objective & Instructions	Solution
Running Containers	Intro	Exercise	Solution
Working with Images	Image	Exercise	Solution
My First Dockerfile	Dockerfile	Exercise	Solution

Containers Self Assesment

What is a Container?

This can be tricky to answer since there are many ways to create a containers:

Docker
systemd-nspawn
LXC

If to focus on OCI (Open Container Initiative) based containers, it offers the following definition: "An environment for executing processes with configurable isolation and resource limitations. For example, namespaces, resource limits, and mounts are all part of the container environment."

Why containers are needed? What is their goal?

OCI provides a good explanation: "Define a unit of software delivery called a Standard Container. The goal of a Standard Container is to encapsulate a software component and all its dependencies in a format that is self-describing and portable, so that any compliant runtime can run it without extra dependencies, regardless of the underlying machine and the contents of the container."

How are containers different from virtual machines (VMs)?

The primary difference between containers and VMs is that containers allow you to virtualize
multiple workloads on a single operating system while in the case of VMs, the hardware is being virtualized to run multiple machines each with its own guest OS.
You can also think about it as containers are for OS-level virtualization while VMs are for hardware virtualization.

Containers don't require an entire guest operating system as VMs. Containers share the system's kernel as opposed to VMs. They isolate themselves via the use of kernel's features such as namespaces and cgroups
It usually takes a few seconds to set up a container as opposed to VMs which can take minutes or at least more time than containers as there is an entire OS to boot and initialize as opposed to containers which has share of the underlying OS
Virtual machines considered to be more secured than containers
VMs portability considered to be limited when compared to containers

Do we need virtual machines in the edge of containers? Are they still relevant?
In which scenarios would you use containers and in which you would prefer to use VMs?

You should choose VMs when:

You need run an application which requires all the resources and functionalities of an OS
You need full isolation and security

You should choose containers when:

You need a lightweight solution
Running multiple versions or instances of a single application

Containers - OCI

What is the OCI?

OCI (Open Container Initiative) is an open governance established in 2015 to standardize container creation - mostly image format and runtime. At that time there were a number of parties involved and the most prominent one was Docker.

Specifications published by OCI:

Which operations OCI based containers must support?

Create, Kill, Delete, Start and Query State.

Containers - Basic Commands

How to list all the containers on a given host?

podman container ls or docker container ls (depends on which containers technology you are using)

How to run a container?

Depends on which containers technology you are using.

docker container run ubuntu or podman container run ubuntu for example.

Why after running podman container run ubuntu the output of podman container ls is empty?

Because the container immediately exits after running the ubuntu image. This is completely normal and expected as containers designed to run a service or a app and exit when they are done running it.

If you want the container to keep running, you can run a command like sleep 100 which will run for 100 seconds or you can attach to terminal of the container with a command similar: podman container run -it ubuntu /bin/bash

How to attach your shell to a terminal of a running container?

podman container exec -it [container id/name] bash

This can be done in advance while running the container: podman container run -it [image:tag] /bin/bash

True or False? You can remove a running container if it doesn't running anything

False. You have to stop the container before removing it.

How to stop and remove a container?

podman container stop [contatiner id/name] && podman container rm [container id/name]

Containers - Images

What is a container image?

An image of a container contains the application, its dependencies and the operating system where the application is executed.
It's a collection of read-only layers. These layers are loosely coupled
Each layer is assembled out of one or more files

Why container images are relatively small?

Most of the images don't contain Kernel. They share and access the one used by the host on which they are running
Containers intended to run specific application in most cases. This means they hold only what the application needs in order to run

How to list the container images on certain host?

podman image ls

docker image ls

Depends on which containers engine you use.

How the centralized location, where images are stored, is called?

Registry

A registry contains one or more ____ which in turn contain one or more ____

A registry contains one or more repositories which in turn contain one or more images.

How to find out which registry do you use by default from your environment?

Depends on the containers technology you are using. For example, in case of Docker, it can be done with docker info

> docker info
Registry: https://index.docker.io/v1

How to retrieve the latest ubuntu image?

docker image pull ubuntu:latest

True or False? It's not possible to remove an image if a certain container is using it

True. You should stop and remove the container before trying to remove the image it uses.

True or False? If a tag isn't specified when pulling an image, the 'latest' tag is being used

True

Using the 'latest' tag when pulling an image means, you are pulling the most recently published image

False. While this might be true in some cases, it's not guaranteed that you'll pull the latest published image when using the 'latest' tag.

For example, in some images, 'edge' tag is used for the most recently published images.

Where pulled images are stored?

Depends on the container technology being used. For example, in case of Docker, images are stored in /var/lib/docker/

Explain container image layers

The layers of an image is where all the content is stored - code, files, etc.
Each layer is independent
Each layer has an ID that is an hash based on its content
The layers (as the image) are immutable which means a change to one of the layers can be easily identified

True or False? Changing the content of any of the image layers will cause the hash content of the image to change

True. These hashes are content based and since images (and their layers) are immutable, any change will cause the hashes to change.

How to list the layers of an image?

In case of Docker, you can use docker image inspect <name>

True or False? In most cases, container images contain their own kernel

False. They share and access the one used by the host on which they are running.

True or False? A single container image can have multiple tags

True. When listing images, you might be able to see two images with the same ID but different tags.

What is a dangling image?

It's an image without tags attached to it.
One way to reach this situation is by building an image with exact same name and tag as another already existing image. It can be still referenced by using its full SHA.

How to see changes done to a given image over time?

In the case of Docker, you could use docker history <name>

True or False? Multiple images can share layers

True.

One evidence for that can be found in pulling images. Sometimes when you pull an image, you'll see a line similar to the following:

fa20momervif17: already exists

This is because it recognizes such layer already exists on the host, so there is no need to pull the same layer twice.

What is the digest of an image? What problem does it solves?

Tags are mutable. This is mean that we can have two different images with the same name and the same tag. It can be very confusing to see two images with the same name and the same tag in your environment. How would you know if they are truly the same or are they different?

This is where "digests` come handy. A digest is a content-addressable identifier. It isn't mutable as tags. Its value is predictable and this is how you can tell if two images are the same content wise and not merely by looking at the name and the tag of the images.

True or False? A single image can support multiple architectures (Linux x64, Windows x64, ...)

True.

What is a distribution hash in regards to layers?

Layers are compressed when pushed or pulled
distribution hash is the hash of the compressed layer
the distribution hash used when pulling or pushing images for verification (making sure no one tempered with image or layers)
It's also used for avoiding ID collisions (a case where two images have exactly the same generated ID)

How multi-architecture images work? Explain by describing what happens when an image is pulled

A client makes a call to the registry to use a specific image (using an image name and optionally a tag)
A manifest list is parsed (assuming it exists) to check if the architecture of the client is supported and available as a manifest
If it is supported (a manifest for the architecture is available) the relevant manifest is parsed to obtain the IDs of the layers
Each layer is then pulled using the obtained IDs from the previous step

How to check which architectures a certain container image supports?

docker manifest inspect <name>

Containers - Volume

How to create a new volume?

docker volume create some_volume

Containers - Dockerfile

What is Dockerfile

Docker can build images automatically by reading the instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image.

What is the difference between ADD and COPY in Dockerfile?

COPY takes in a src and destination. It only lets you copy in a local file or directory from your host (the machine building the Docker image) into the Docker image itself.
ADD lets you do that too, but it also supports 2 other sources. First, you can use a URL instead of a local file / directory. Secondly, you can extract a tar file from the source directly into the destination.
Although ADD and COPY are functionally similar, generally speaking, COPY is preferred. That’s because it’s more transparent than ADD. COPY only supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support) that are not immediately obvious.

What is the difference between CMD and RUN in Dockerfile?

RUN lets you execute commands inside of your Docker image. These commands get executed once at build time and get written into your Docker image as a new layer.
CMD is the command the container executes by default when you launch the built image. A Dockerfile can only have one CMD.
You could say that CMD is a Docker run-time operation, meaning it’s not something that gets executed at build time. It happens when you run an image. A running image is called a container.

How to create a new image using a Dockerfile?

The following command is executed from within the directory where Dockefile resides:

podman image build -t some_app:latest .

Do you perform any checks or testing related to your Dockerfile?

One option is to use hadolint project which is a linter based on Dockerfile best practices.

Containers - Architecture

How container achieve isolation from the rest of the system?

Through the use of namespaces and cgroups. Linux kernel has several types of namespaces:

Process ID namespaces: these namespaces include independent set of process IDs
Mount namespaces: Isolation and control of mountpoints
Network namespaces: Isolates system networking resources such as routing table, interfaces, ARP table, etc.
UTS namespaces: Isolate host and domains
IPC namespaces: Isolates interprocess communications
User namespaces: Isolate user and group IDs
Time namespaces: Isolates time machine

Describe in detail what happens when you run `podman/docker run hello-world`?

Docker/Podman CLI passes your request to Docker daemon.
Docker/Podman daemon downloads the image from Docker Hub
Docker/Podman daemon creates a new container by using the image it downloaded
Docker/Podman daemon redirects output from container to Docker CLI which redirects it to the standard output

Describe difference between cgroups and namespaces
cgroup: Control Groups provide a mechanism for aggregating/partitioning sets of tasks, and all their future children, into hierarchical groups with specialized behaviour. namespace: wraps a global system resource in an abstraction that makes it appear to the processes within the namespace that they have their own isolated instance of the global resource.

In short:

Cgroups = limits how much you can use;
namespaces = limits what you can see (and therefore use)

Cgroups involve resource metering and limiting:
memory
CPU
block I/O
network

Namespaces provide processes with their own view of the system

Multiple namespaces: pid,net, mnt, uts, ipc, user

Containers - Docker Architecture

Which components/layers compose the Docker technology?

Runtime - responsible for starting and stopping containers
Daemon - implements the Docker API and takes care of managing images (including builds), authentication, security, networking, etc.
Orchestrator

What components are part of the Docker engine?

Docker daemon
containerd
runc

What is the low-level runtime?

The low level runtime is called runc
It manages every container running on Docker host
Its purpose is to interact with the underlying OS to start and stop containers
Its reference implementation is of the OCI (Open Containers Initiative) container-runtime-spec
It's a small CLI wrapper for libcontainer

What is the high-level runtime?

The high level runtime is called containerd
It was developed by Docker Inc and at some point donated to CNCF
It manages the whole lifecycle of a container - start, stop, remove and pause
It take care of setting up network interfaces, volume, pushing and pulling images, ...
It manages the lower level runtime (runc) instances
It's used both by Docker and Kubernetes as a container runtime
It sits between Docker daemon and runc at the OCI layer

Note: running ps -ef | grep -i containerd on a system with Docker installed and running, you should see a process of containerd

True or False? The docker daemon (dockerd) performs lower-level tasks compared to containerd

False. The Docker daemon performs higher-level tasks compared to containerd.

It's responsible for managing networks, volumes, images, ...

Describe in detail what happens when you run `docker pull image:tag`?
Docker CLI passes your request to Docker daemon. Dockerd Logs shows the process

docker.io/library/busybox:latest resolved to a manifestList object with 9 entries; looking for a unknown/amd64 match

found match for linux/amd64 with media type application/vnd.docker.distribution.manifest.v2+json, digest sha256:400ee2ed939df769d4681023810d2e4fb9479b8401d97003c710d0e20f7c49c6

pulling blob "sha256:61c5ed1cbdf8e801f3b73d906c61261ad916b2532d6756e7c4fbcacb975299fb Downloaded 61c5ed1cbdf8 to tempfile /var/lib/docker/tmp/GetImageBlob909736690

Applying tar in /var/lib/docker/overlay2/507df36fe373108f19df4b22a07d10de7800f33c9613acb139827ba2645444f7/diff" storage-driver=overlay2

Applied tar sha256:514c3a3e64d4ebf15f482c9e8909d130bcd53bcc452f0225b0a04744de7b8c43 to 507df36fe373108f19df4b22a07d10de7800f33c9613acb139827ba2645444f7, size: 1223534

Describe in detail what happens when you run a container

The Docker client converts the run command into an API payload
It then POST the payload to the API endpoint exposed by the Docker daemon
When the daemon receives the command to create a new container, it makes a call to containerd via gRPC
containerd converts the required image into an OCI bundle and tells runc to use that bundle for creating the container
runc interfaces with the OS kernel to pull together the different constructs (namespace, cgroups, etc.) used for creating the container
Container process is started as a child-process of runc
Once it starts, runc exists

True or False? Killing the Docker daemon will kill all the running containers

False. While this was true at some point, today the container runtime isn't part of the daemon (it's part of containerd and runc) so stopping or killing the daemon will not affect running containers.

True or False? containerd forks a new instance runc for every container it creates

True

True or False? Running a dozen of containers will result in having a dozen of runc processes

False. Once a container is created, the parent runc process exists.

What is shim in regards to Docker?

shim is the process that becomes the container's parent when runc process exists. It's responsible for:

Reporting exit code back to the Docker daemon
Making sure the container doesn't terminate if the daemon is being restarted. It does so by keeping the stdout and stdin open

What `podman commit` does?. When will you use it?

Create a new image from a container’s changes

How would you transfer data from one container into another?
What happens to data of the container when a container exists?
Explain what each of the following commands do:

docker run
docker rm
docker ps
docker pull
docker build
docker commit

How do you remove old, non running, containers?

To remove one or more Docker images use the docker container rm command followed by the ID of the containers you want to remove.
The docker system prune command will remove all stopped containers, all dangling images, and all unused networks
docker rm $(docker ps -a -q) - This command will delete all stopped containers. The command docker ps -a -q will return all existing container IDs and pass them to the rm command which will delete them. Any running containers will not be deleted.

How the Docker client communicates with the daemon?

Via the local socket at /var/run/docker.sock

Explain what is Docker compose and what is it used for

Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.

For example, you can use it to set up ELK stack where the services are: elasticsearch, logstash and kibana. Each running in its own container.

Describe the process of using Docker Compose

Define the services you would like to run together in a docker-compose.yml file
Run docker-compose up to run the services

Explain Docker interlock
What is Docker Repository?
Explain image layers

A Docker image is built up from a series of layers. Each layer represents an instruction in the image’s Dockerfile. Each layer except the very last one is read-only.
Each layer is only a set of differences from the layer before it. The layers are stacked on top of each other. When you create a new container, you add a new writable layer on top of the underlying layers. This layer is often called the “container layer”. All changes made to the running container, such as writing new files, modifying existing files, and deleting files, are written to this thin writable container layer.
The major difference between a container and an image is the top writable layer. All writes to the container that add new or modify existing data are stored in this writable layer. When the container is deleted, the writable layer is also deleted. The underlying image remains unchanged.
Because each container has its own writable container layer, and all changes are stored in this container layer, multiple containers can share access to the same underlying image and yet have their own data state.

What best practices are you familiar related to working with containers?
How do you manage persistent storage in Docker?
How can you connect from the inside of your container to the localhost of your host, where the container runs?
How do you copy files from Docker container to the host and vice versa?

Containers - Docker Images

What is Docker Hub?

One of the most common registries for retrieving images.

How to push an image to Docker Hub?

docker image push [username]/[image name]:[tag]

For example:

docker image mario/web_app:latest

What is the difference between Docker Hub and Docker cloud?

Docker Hub is a native Docker registry service which allows you to run pull
and push commands to install and deploy Docker images from the Docker Hub.

Docker Cloud is built on top of the Docker Hub so Docker Cloud provides
you with more options/features compared to Docker Hub. One example is
Swarm management which means you can create new swarms in Docker Cloud.

Containers - Docker in Production

True or False? It's recommended for production environments that Docker client and server will communicate over network using HTTP socket

False. Communication between client and server shouldn't be done over HTTP since it's insecure. It's better to enforce the daemon to only accept network connection that are secured with TLS.

Basically, the Docker daemon will only accept secured connections with certificates from trusted CA.

Kubernetes

Kubernetes Exercises

Developer & "Regular" User Path

Name	Topic	Objective & Instructions	Solution
My First Pod	Pods	Exercise	Solution
"Killing" Containers	Pods	Exercise	Solution
Creating a Service	Service	Exercise	Solution
Creating a ReplicaSet	ReplicaSet	Exercise	Solution
Operating ReplicaSets	ReplicaSet	Exercise	Solution
ReplicaSets Selectors	ReplicaSet	Exercise	Solution

Kubernetes Self Assesment

What is Kubernetes? Why organizations are using it?

Kubernetes is an open-source system for automating deployment, scaling, and management of containerized applications.

To understand what Kubernetes is good for, let's look at some examples:

You would like to run a certain application in a container on multiple different locations. Sure, if it's 2-3 servers/locations, you can do it by yourself but it can be challenging to scale it up to additional multiple location.
Performing updates and changes across hundreds of containers
Handle cases where the current load requires to scale up (or down)

What is a Kubernetes Cluster?

Red Hat Definition: "A Kubernetes cluster is a set of node machines for running containerized applications. If you’re running Kubernetes, you’re running a cluster.
At a minimum, a cluster contains a worker node and a master node."

Read more here

Do you have experience with deploying a Kubernetes cluster? If so, can you describe the process in high-level?

Create multiple instances you will use as Kubernetes nodes/workers. Create also an instance to act as the Master. The instances can be provisioned in a cloud or they can be virtual machines on bare metal hosts.
Provision a certificate authority that will be used to generate TLS certificates for the different components of a Kubernetes cluster (kubelet, etcd, ...)
Generate a certificate and private key for the different components
Generate kubeconfigs so the different clients of Kubernetes can locate the API servers and authenticate.
Generate encryption key that will be used for encrypting the cluster data
Create an etcd cluster

When or why NOT to use Kubernetes?

If you are big team of engineers (e.g. 200) deploying applications using containers and you need to manage scaling, rolling out updates, etc. You probably want to use Kubernetes

If you manage low level infrastructure or baremetals, Kubernetes is probably not what you need or want

If you are a small team (e.g. 20-50 engineers) Kubernetes might be an overkill (even if you need scale, rolling out updates, etc.)

What Kubernetes objects are there?

Pod
Service
ReplicationController
ReplicaSet
DaemonSet
Namespace
ConfigMap
...

What fields are mandatory with any Kubernetes object?

metadata, kind and apiVersion

What actions or operations you consider as best practices when it comes to Kuberentes?

Always make sure Kubernetes YAML files are valid. Applying automated checks and pipelines is recommended.
Always specify requests and limits to prevent situation where containers are using the entire cluster memory which may lead to OOM issue

What is kubectl?

Kubectl is the Kubernetes command line tool that allows you to run commands against Kubernetes clusters. For example, you can use kubectl to deploy applications, inspect and manage cluster resources, and view logs.

Kubernetes - Cluster

What is a Node?

A node is a virtual or a physical machine that serves as a worker for running the applications.

It's recommended to have at least 3 nodes in a production environment.

What the master node is responsible for?

The master coordinates all the workflows in the cluster:

Scheduling applications
Managing desired state
Rolling out new updates

Which command will list the nodes of the cluster?

kubectl get nodes

True or False? Every cluster must have 0 or more master nodes and at least 1 worker

False. A Kubernetes cluster consists of at least 1 master and can have 0 workers (although that wouldn't be very useful...)

What are the components of the master node?

API Server - the Kubernetes API. All cluster components communicate through it
Scheduler - assigns an application with a worker node it can run on
Controller Manager - cluster maintenance (replications, node failures, etc.)
etcd - stores cluster configuration

What are the components of a worker node?

Kubelet - an agent responsible for node communication with the master.
Kube-proxy - load balancing traffic between app components
Container runtime - the engine runs the containers (Podman, Docker, ...)

Place the components on the right side of the image in the right place in the drawing

You are managing multiple Kubernetes clusters. How do you quickly change between the clusters using kubectl?

kubectl config use-context

How do you prevent high memory usage in your Kubernetes cluster and possibly issues like memory leak and OOM?

Apply requests and limits, especially on third party applications (where the uncertainty is even bigger)

Kubernetes - Pods

Explain what is a Pod

A Pod is a group of one or more containers, with shared storage and network resources, and a specification for how to run the containers.

Pods are the smallest deployable units of computing that you can create and manage in Kubernetes.

Deploy a pod called "my-pod" using the nginx:alpine image

kubectl run my-pod --image=nginx:alpine --restart=Never

If you are a Kubernetes beginner you should know that this is not a common way to run Pods. The common way is to run a Deployment which in turn runs Pod(s).

In addition, Pods and/or Deployments are usually defined in files rather than executed directly using only the CLI arguments.

What are your thoughts on "Pods are not meant to be created directly"?

Pods are usually indeed not created directly. You'll notice that Pods are usually created as part of another entities such as Deployments or ReplicaSets.

If a Pod dies, Kubernetes will not bring it back. This is why it's more useful for example to define ReplicaSets that will make sure that a given number of Pods will always run, even after a certain Pod dies.

How many containers can a pod contain?

A pod can include multiple containers but in most cases it would probably be one container per pod.

What use cases exist for running multiple containers in a single pod?

A web application with separate (= in their own containers) logging and monitoring components/adapters is one examples.

A CI/CD pipeline (using Tekton for example) can run multiple containers in one Pod if a Task contains multiple commands.

What are the possible Pod phases?

Running - The Pod bound to a node and at least one container is running
Failed - At least one container in the Pod terminated with a failure
Succeeded - Every container in the Pod terminated with success
Unknown - Pod's state could not be obtained
Pending - Containers are not yet running (Perhaps images are still being downloaded or the pod wasn't scheduled yet)

True or False? By default, pods are isolated. This means they are unable to receive traffic from any source

False. By default, pods are non-isolated = pods accept traffic from any source.

True or False? The "Pending" phase means the Pod was not yet accepted by the Kubernetes cluster so the scheduler can't run it unless it's accepted

False. "Pending" is after the Pod was accepted by the cluster, but the container can't run for different reasons like images not yet downloaded.

How to list the pods in the current namespace?

kubectl get po

How view all the pods running in all the namespaces?

kubectl get pods --all-namespaces

True or False? A single Pod can be split across multiple nodes

False. A single Pod can run on a single node.

How to delete a pod?

kubectl delete pod pod_name

How to find out on which node a certain pod is running?

kubectl get po -o wide

What are "Static Pods"?

Managed directly by Kubelet on specific node
API server is not observing static Pods
For each static Pod there is a mirror Pod on kubernetes API server but it can't be managed from there

Read more about it here

True or False? A volume defined in Pod can be accessed by all the containers of that Pod

True.

What happens when you run a Pod?

Kubectl sends a request to the API server to create the Pod
The Scheduler detects that there is an unassigned Pod (by monitoring the API server)
The Scheduler chooses a node to assign the Pod to
The Scheduler updates the API server about which node it chose
Kubelet (which also monitors the API server) notices there is a Pod assigned to the same node on which it runs and that Pod isn't running
Kubelet sends request to the container engine (e.g. Docker) to create and run the containers
An update is sent by Kubelet to the API server (notifying it that the Pod is running)

How to confirm a container is running after running the command kubectl run web --image nginxinc/nginx-unprivileged

When you run kubectl describe pods <POD_NAME> it will tell whether the container is running:
Status: Running
Run a command inside the container: kubectl exec web -- ls

After running kubectl run database --image mongo you see the status is "CrashLoopBackOff". What could possibly went wrong and what do you do to confirm?

"CrashLoopBackOff" means the Pod is starting, crashing, starting...and so it repeats itself.

There are many different reasons to get this error - lack of permissions, init-container misconfiguration, persistent volume connection issue, etc.

One of the ways to check why it happened it to run kubectl describe po <POD_NAME> and having a look at the exit code

 Last State:     Terminated
   Reason:       Error
   Exit Code:    100

Another way to check what's going on, is to run kubectl logs <POD_NAME>. This will provide us with the logs from the containers running in that Pod.

Explain the purpose of the following lines

livenessProbe:
  exec:
    command:
    - cat
    - /appStatus
  initialDelaySeconds: 10
  periodSeconds: 5

These lines make use of liveness probe. It's used to restart a container when it reaches a non-desired state.

In this case, if the command cat /appStatus fails, Kubernetes will kill the container and will apply the restart policy. The initialDelaySeconds: 10 means that Kubelet will wait 10 seconds before running the command/probe for the first time. From that point on, it will run it every 5 seconds, as defined with periodSeconds

Explain the purpose of the following lines

readinessProbe:
      tcpSocket:
        port: 2017
      initialDelaySeconds: 15
      periodSeconds: 20

They define a readiness probe where the Pod will not be marked as "Ready" before it will be possible to connect to port 2017 of the container. The first check/probe will start after 15 seconds from the moment the container started to run and will continue to run the check/probe every 20 seconds until it will manage to connect to the defined port.

What does the "ErrImagePull" status of a Pod means?

It wasn't able to pull the image specified for running the container(s). This can happen if the client didn't authenticated for example.

More details can be obtained with kubectl describe po <POD_NAME>.

What happens when you delete a Pod?

The TERM signal is sent to kill the main processes inside the containers of the given Pod
Each container is given a period of 30 seconds to shut down the processes gracefully
If the grace period expires, the KILL signal is used to kill the processes forcefully and the containers as well

Explain liveness probes

Liveness probes is a useful mechanism used for restarting the container when a certain check/probe, the user has defined, fails.

For example, the user can define that the command cat /app/status will run every X seconds and the moment this command fails, the container will be restarted.

You can read more about it in kubernetes.io

Explain readiness probes

readiness probes used by Kubelet to know when a container is ready to start running, accepting traffic.

For example, a readiness probe can be to connect port 8080 on a container. Once Kubelet manages to connect it, the Pod is marked as ready

You can read more about it in kubernetes.io

How readiness probe status affect Services when they are combined?

Only containers whose state set to Success will be able to receive requests sent to the Service.

Why it's usually considered better to include one container per Pod?

One reason is that it makes it harder to scale, when you need to scale only one of the containers in a given Pod.

Kubernetes - Deployments

What is a "Deployment" in Kubernetes?

A Kubernetes Deployment is used to tell Kubernetes how to create or modify instances of the pods that hold a containerized application.
Deployments can scale the number of replica pods, enable rollout of updated code in a controlled manner, or roll back to an earlier deployment version if necessary.

A Deployment is a declarative statement for the desired state for Pods and Replica Sets.

How to create a deployment?

cat << EOF | kubectl create -f -
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: nginx
    image: nginx
EOF

How to edit a deployment?

kubectl edit deployment some-deployment

What happens after you edit a deployment and change the image?

The pod will terminate and another, new pod, will be created.

Also, when looking at the replicaset, you'll see the old replica doesn't have any pods and a new replicaset is created.

How to delete a deployment?

One way is by specifying the deployment name: kubectl delete deployment [deployment_name]
Another way is using the deployment configuration file: kubectl delete -f deployment.yaml

What happens when you delete a deployment?

The pod related to the deployment will terminate and the replicaset will be removed.

How make an app accessible on private or external network?

Using a Service.

Kubernetes - Services

What is a Service in Kubernetes?

"An abstract way to expose an application running on a set of Pods as a network service." - read more here

In simpler words, it allows you to add an internal or external connectivity to a certain application running in a container.

True or False? The lifecycle of Pods and Services isn't connected so when a Pod dies, the Service still stays

True

What Service types are there?

ClusterIP
NodePort
LoadBalancer
ExternalName

More on this topic here

What is the default service type in Kubernetes and what is it used for?

The default is ClusterIP and it's used for exposing a port internally. It's useful when you want to enable internal communication between Pods and prevent any external access.

How to get information on a certain service?

kubctl describe service <SERVICE_NAME>

It's more common to use kubectl describe svc ...

What the following command does?

kubectl expose rs some-replicaset --name=replicaset-svc --target-port=2017 --type=NodePort

It exposes a ReplicaSet by creating a service called 'replicaset-svc'. The exposed port is 2017 (this is the port used by the application) and the service type is NodePort which means it will be reachable externally.

True or False? the target port, in the case of running the following command, will be exposed only on one of the Kubernetes cluster nodes but it will routed to all the pods

kubectl expose rs some-replicaset --name=replicaset-svc --target-port=2017 --type=NodePort

False. It will be exposed on every node of the cluster and will be routed to one of the Pods (which belong to the ReplicaSet)

How to verify that a certain service configured to forward the requests to a given pod

Run kubectl describe service and see if the IPs from "Endpoints" match any IPs from the output of kubectl get pod -o wide

Explain what will happen when running apply on the following block

apiVersion: v1
kind: Service
metadata:
  name: some-app
spec:
  type: NodePort
  ports:
  - port: 8080
    nodePort: 2017
    protocol: TCP
  selector:
    type: backend
    service: some-app

It creates a new Service of the type "NodePort" which means it can be used for internal and external communication with the app.

The port of the application is 8080 and the requests will forwarded to this port. The exposed port is 2017. As a note, this is not a common practice, to specify the nodePort.

The port used TCP (instead of UDP) and this is also the default so you don't have to specify it.

The selector used by the Service to know to which Pods to forward the requests. In this case, Pods with the label "type: backend" and "service: some-app".

How to turn the following service into an external one?

spec:
  selector:
    app: some-app
  ports:
    - protocol: TCP
      port: 8081
      targetPort: 8081

Adding type: LoadBalancer and nodePort

spec:
  selector:
    app: some-app
  type: LoadBalancer
  ports:
    - protocol: TCP
      port: 8081
      targetPort: 8081
      nodePort: 32412

What would you use to route traffic from outside the Kubernetes cluster to services within a cluster?

Ingress

True or False? When "NodePort" is used, "ClusterIP" will be created automatically?

True

When would you use the "LoadBalancer" type

Mostly when you would like to combine it with cloud provider's load balancer

How would you map a service to an external address?

Using the 'ExternalName' directive.

Describe in detail what happens when you create a service

Kubectl sends a request to the API server to create a Service
The controller detects there is a new Service
Endpoint objects created with the same name as the service, by the controller
The controller is using the Service selector to identify the endpoints
kube-proxy detects there is a new endpoint object + new service and adds iptables rules to capture traffic to the Service port and redirect it to endpoints
kube-dns detects there is a new Service and adds the container record to the dns server

How to list the endpoints of a certain app?

kubectl get ep <name>

How can you find out information on a Service related to a certain Pod if all you can use is kubectl exec --

You can run kubectl exec <POD_NAME> -- env which will give you a couple environment variables related to the Service.

Variables such as [SERVICE_NAME]_SERVICE_HOST, [SERVICE_NAME]_SERVICE_PORT, ...

Describe what happens when a container tries to connect with its corresponding Service for the first time. Explain who added each of the components you include in your description

The container looks at the nameserver defined in /etc/resolv.conf
The container queries the nameserver so the address is resolved to the Service IP
Requests sent to the Service IP are forwarded with iptables rules (or other chosen software) to the endpoint(s).

Explanation as to who added them:

The nameserver in the container is added by kubelet during the scheduling of the Pod, by using kube-dns
The DNS record of the service is added by kube-dns during the Service creation
iptables rules are added by kube-proxy during Endpoint and Service creation

Kubernetes - Ingress

What is Ingress?

From Kubernetes docs: "Ingress exposes HTTP and HTTPS routes from outside the cluster to services within the cluster. Traffic routing is controlled by rules defined on the Ingress resource."

Read more here

Complete the following configuration file to make it Ingress

metadata:
  name: someapp-ingress
spec:

There are several ways to answer this question.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: someapp-ingress
spec:
  rules:
  - host: my.host
    http:
      paths:
      - backend:
          serviceName: someapp-internal-service
          servicePort: 8080

Explain the meaning of "http", "host" and "backend" directives

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: someapp-ingress
spec:
  rules:
  - host: my.host
    http:
      paths:
      - backend:
          serviceName: someapp-internal-service
          servicePort: 8080

host is the entry point of the cluster so basically a valid domain address that maps to cluster's node IP address

the http line used for specifying that incoming requests will be forwarded to the internal service using http.

backend is referencing the internal service (serviceName is the name under metadata and servicePort is the port under the ports section).

Why using a wildcard in ingress host may lead to issues?

The reason you should not wildcard value in a host (like - host: *) is because you basically tell your Kubernetes cluster to forward all the traffic to the container where you used this ingress. This may cause the entire cluster to go down.

What is Ingress Controller?

An implementation for Ingress. It's basically another pod (or set of pods) that does evaluates and processes Ingress rules and this it manages all the redirections.

There are multiple Ingress Controller implementations (the one from Kubernetes is Kubernetes Nginx Ingress Controller).

What are some use cases for using Ingress?

Multiple sub-domains (multiple host entries, each with its own service)
One domain with multiple services (multiple paths where each one is mapped to a different service/application)

How to list Ingress in your namespace?

kubectl get ingress

What is Ingress Default Backend?

It specifies what do with an incoming request to the Kubernetes cluster that isn't mapped to any backend (= no rule to for mapping the request to a service). If the default backend service isn't defined, it's recommended to define so users still see some kind of message instead of nothing or unclear error.

How to configure a default backend?

Create Service resource that specifies the name of the default backend as reflected in kubectl desrcibe ingress ... and the port under the ports section.

How to configure TLS with Ingress?

Add tls and secretName entries.

spec:
  tls:
  - hosts:
    - some_app.com
    secretName: someapp-secret-tls

True or False? When configuring Ingress with TLS, the Secret component must be in the same namespace as the Ingress component

True

Which Kubernetes concept would you use to control traffic flow at the IP address or port level?

Network Policies

What the following block of lines does?

spec:
  replicas: 2
  selector:
    matchLabels:
      type: backend
  template:
    metadata:
      labels:
        type: backend
    spec:
      containers:
      - name: httpd-yup
        image: httpd

It defines a replicaset for Pods whose type is set to "backend" so at any given point of time there will be 2 concurrent Pods running.

Kubernetes - ReplicaSets

What is the purpose of ReplicaSet?

kubernetes.io: "A ReplicaSet's purpose is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods."

In simpler words, a ReplicaSet will ensure the specified number of Pods replicas is running for a selected Pod. If there are more Pods than defined in the ReplicaSet, some will be removed. If there are less than what is defined in the ReplicaSet then, then more replicas will be added.

What will happen when a Pod, created by ReplicaSet, is deleted directly with kubectl delete po ...?

The ReplicaSet will create a new Pod in order to reach the desired number of replicas.

True or False? If a ReplicaSet defines 2 replicas but there 3 Pods running matching the ReplicaSet selector, it will do nothing

False. It will terminate one of the Pods to reach the desired state of 2 replicas.

Describe the sequence of events in case of creating a ReplicaSet

The client (e.g. kubectl) sends a request to the API server to create a ReplicaSet
The Controller detects there is a new event requesting for a ReplicaSet
The controller creates new Pod definitions (the exact number depends on what is defined in the ReplicaSet definition)
The scheduler detects unassigned Pods and decides to which nodes to assign the Pods. This information sent to the API server
Kubelet detects that two Pods were assigned to the node it's running on (as it constantly watching the API server)
Kubelet sends requests to the container engine, to create the containers that are part of the Pod
Kubelet sends a request to the API server to notify it the Pods were created

How to list ReplicaSets in the current namespace?

kubectl get rs

Is it possible to delete ReplicaSet without deleting the Pods it created?

Yes, with --cascase=false.

kubectl delete -f rs.yaml --cascade=false

What is the default number of replicas if not explicitly specified?

What the following output of kubectl get rs means?

NAME DESIRED CURRENT READY AGE
web 2 2 0 2m23s

The replicaset web has 2 replicas. It seems that the containers inside the Pod(s) are not yet running since the value of READY is 0. It might be normal since it takes time for some containers to start running and it might be due to an error. Running kubectl describe po POD_NAME or kubectl logs POD_NAME can give us more information.

You run kubectl get rs and while DESIRED is set to 2, you see that READY is set to 0. What are some possible reasons for it to be 0?

Images are still being pulled
There is an error and the containers can't reach the state "Running"

True or False? Pods specified by the selector field of ReplicaSet must be created by the ReplicaSet itself

False. The Pods can be already running and initially they can be created by any object. It doesn't matter for the ReplicaSet and not a requirement for it to acquire and monitor them.

True or False? In case of a ReplicaSet, if Pods specified in the selector field don't exists, the ReplicaSet will wait for them to run before doing anything

False. It will take care of running the missing Pods.

In case of a ReplicaSet, Which field is mandatory in the spec section?

The field template in spec section is mandatory. It's used by the ReplicaSet to create new Pods when needed.

You've created a ReplicaSet, how to check whether the ReplicaSet found matching Pods or it created new Pods?

kubectl describe rs <ReplicaSet Name>

It will be visible under Events (the very last lines)

True or False? Deleting a ReplicaSet will delete the Pods it created

True (and not only the Pods but anything else it created).

True or False? Removing the label from a Pod that is used by ReplicaSet to match Pods, will cause the ReplicaSet to create a new Pod

True. When the label, used by a ReplicaSet in the selector field, removed from a Pod, that Pod no longer controlled by the ReplicaSet and the ReplicaSet will create a new Pod to compensate for the one it "lost".

How to scale a deployment to 8 replicas?

kubectl scale deploy <DEPLOYMENT_NAME> --replicas=8

ReplicaSets are running the moment the user executed the command to create them (like kubectl create -f rs.yaml)

False. It can take some time, depends on what exactly you are running. To see if they are up and running, run kubectl get rs and watch the 'READY' column.

How to expose a ReplicaSet as a new service?

kubectl expose rs <ReplicaSet Name> --name=<Service Name> --target-port=<Port to expose> --type=NodePort

Few notes:

the target port depends on which port the app is using in the container
type can be different and doesn't has to be specifically "NodePort"

Kubernetes - Storage

What is a volume in regards to Kubernetes?

A directory accessible by the containers inside a certain Pod. The mechanism responsible for creating the directory and managing it, ... is mainly depends on the volume type.

Which problems volumes in Kubernetes solve?

Sharing files between containers running in the same Pod
Storage in containers is ephemeral - it usually doesn't last for long. For example, when a container crashes, you lose all on-disk data.

Explain ephemeral volume types vs. persistent volumes in regards to Pods

Ephemeral volume types have the lifetime of a pod as opposed to persistent volumes which exist beyond the lifetime of a Pod.

Kubernetes - Network Policies

Explain Network Policies

kubernetes.io: "NetworkPolicies are an application-centric construct which allow you to specify how a pod is allowed to communicate with various network "entities"..."

In simpler words, Network Policies specify how pods are allowed/disallowed to communicate with each other and/or other network endpoints.

What are some use cases for using Network Policies?

Security: You want to prevent from everyone to communicate with a certain pod for security reasons
Controling network traffic: You would like to deny network flow between two specific nodes

True or False? If no network policies are applied to a pod, then no connections to or from it are allowed

False. By default pods are non-isolated.

In case of two pods, if there is an egress policy on the source denining traffic and ingress policy on the destination that allows traffic then, traffic will be allowed or denied?

Denied. Both source and destination policies has to allow traffic for it to be allowed.

Kubernetes - Configuration File

Which parts a configuration file has?

It has three main parts:

Metadata
Specification
Status (this automatically generated and added by Kubernetes)

What is the format of a configuration file?

YAML

How to get latest configuration of a deployment?

kubectl get deployment [deployment_name] -o yaml

Where Kubernetes cluster stores the cluster state?

etcd

Kubernetes - etcd

What is etcd?
True or False? Etcd holds the current status of any kubernetes component

True

True or False? The API server is the only component which communicates directly with etcd

True

True or False? application data is not stored in etcd

True

Why etcd? Why not some SQL or NoSQL database?

Kubernetes - Namespaces

What are namespaces?

Namespaces allow you split your cluster into virtual clusters where you can group your applications in a way that makes sense and is completely separated from the other groups (so you can for example create an app with the same name in two different namespaces)

Why to use namespaces? What is the problem with using one default namespace?

When using the default namespace alone, it becomes hard over time to get an overview of all the applications you manage in your cluster. Namespaces make it easier to organize the applications into groups that makes sense, like a namespace of all the monitoring applications and a namespace for all the security applications, etc.

Namespaces can also be useful for managing Blue/Green environments where each namespace can include a different version of an app and also share resources that are in other namespaces (namespaces like logging, monitoring, etc.).

Another use case for namespaces is one cluster, multiple teams. When multiple teams use the same cluster, they might end up stepping on each others toes. For example if they end up creating an app with the same name it means one of the teams overriden the app of the other team because there can't be too apps in Kubernetes with the same name (in the same namespace).

True or False? When a namespace is deleted all resources in that namespace are not deleted but moved to another default namespace

False. When a namespace is deleted, the resources in that namespace are deleted as well.

What special namespaces are there by default when creating a Kubernetes cluster?

default
kube-system
kube-public
kube-node-lease

What can you find in kube-system namespace?

Master and Kubectl processes
System processes

How to list all namespaces?

kubectl get namespaces

What kube-public contains?

A configmap, which contains cluster information
Publicely accessible data

How to get the name of the current namespace?

kubectl config view | grep namespace

What kube-node-lease contains?

It holds information on hearbeats of nodes. Each node gets an object which holds information about its availability.

How to create a namespace?

One way is by running kubectl create namespace [NAMESPACE_NAME]

Another way is by using namespace configuration file:

apiVersion: v1
kind: ConfigMap
metadata:
  name: some-cofngimap
  namespace: some-namespace

What default namespace contains?

Any resource you create while using Kubernetes.

True or False? With namespaces you can limit the resources consumed by the users/teams

True. With namespaces you can limit CPU, RAM and storage usage.

How to switch to another namespace? In other words how to change active namespace?

kubectl config set-context --current --namespace=some-namespace and validate with kubectl config view --minify | grep namespace:

kubens some-namespace

What is Resource Quota?
How to create a Resource Quota?

kubectl create quota some-quota --hard-cpu=2,pods=2

Which resources are accessible from different namespaces?

Service.

Let's say you have three namespaces: x, y and z. In x namespace you have a ConfigMap referencing service in z namespace. Can you reference the ConfigMap in x namespace from y namespace?

No, you would have to create separate namespace in y namespace.

Which service and in which namespace the following file is referencing?

apiVersion: v1
kind: ConfigMap
metadata:
  name: some-configmap
data:
  some_url: samurai.jack

It's referencing the service "samurai" in the namespace called "jack".

Which components can't be created within a namespace?

Volume and Node.

How to list all the components that bound to a namespace?

kubectl api-resources --namespaced=true

How to create components in a namespace?

One way is by specifying --namespace like this: kubectl apply -f my_component.yaml --namespace=some-namespace
Another way is by specifying it in the YAML itself:

apiVersion: v1
kind: ConfigMap
metadata:
  name: some-configmap
  namespace: some-namespace

and you can verify with: kubectl get configmap -n some-namespace

How to execute the command "ls" in an existing pod?

kubectl exec some-pod -it -- ls

How to create a service that exposes a deployment?

kubectl expose deploy some-deployment --port=80 --target-port=8080

How to create a pod and a service with one command?

kubectl run nginx --image=nginx --restart=Never --port 80 --expose

Describe in detail what the following command does kubectl create deployment kubernetes-httpd --image=httpd
Why to create kind deployment, if pods can be launched with replicaset?
How to get list of resources which are not in a namespace?

kubectl api-resources --namespaced=false

How to delete all pods whose status is not "Running"?

kubectl delete pods --field-selector=status.phase!='Running'

What kubectl logs [pod-name] command does?
What kubectl describe pod [pod name] does? command does?
How to display the resources usages of pods?

kubectl top pod

What kubectl get componentstatus does?

Outputs the status of each of the control plane components.

What is Minikube?

Minikube is a lightweight Kubernetes implementation. It create a local virtual machine and deploys a simple (single node) cluster.

How do you monitor your Kubernetes?
You suspect one of the pods is having issues, what do you do?

Start by inspecting the pods status. we can use the command kubectl get pods (--all-namespaces for pods in system namespace)

If we see "Error" status, we can keep debugging by running the command kubectl describe pod [name]. In case we still don't see anything useful we can try stern for log tailing.

In case we find out there was a temporary issue with the pod or the system, we can try restarting the pod with the following kubectl scale deployment [name] --replicas=0

Setting the replicas to 0 will shut down the process. Now start it with kubectl scale deployment [name] --replicas=1

What the Kubernetes Scheduler does?
What happens to running pods if if you stop Kubelet on the worker nodes?
What happens what pods are using too much memory? (more than its limit)

They become candidates to for termination.

Describe how roll-back works
True or False? Memory is a compressible resource, meaning that when a container reach the memory limit, it will keep running

False. CPU is a compressible resource while memory is a non compressible resource - once a container reached the memory limit, it will be terminated.

What is the control loop? How it works?

Explained here

Kubernetes - Operators

What is an Operator?

Explained here

"Operators are software extensions to Kubernetes that make use of custom resources to manage applications and their components. Operators follow Kubernetes principles, notably the control loop."

Why do we need Operators?

The process of managing stateful applications in Kubernetes isn't as straightforward as managing stateless applications where reaching the desired status and upgrades are both handled the same way for every replica. In stateful applications, upgrading each replica might require different handling due to the stateful nature of the app, each replica might be in a different status. As a result, we often need a human operator to manage stateful applications. Kubernetes Operator is suppose to assist with this.

This also help with automating a standard process on multiple Kubernetes clusters

What components the Operator consists of?

CRD (custom resource definition)
Controller - Custom control loop which runs against the CRD

How Operator works?

It uses the control loop used by Kubernetes in general. It watches for changes in the application state. The difference is that is uses a custom control loop.
In additions.

In addition, it also makes use of CRD's (Custom Resources Definitions) so basically it extends Kubernetes API.

True or False? Kubernetes Operator used for stateful applications

True

What is the Operator Framework?

open source toolkit used to manage k8s native applications, called operators, in an automated and efficient way.

What components the Operator Framework consists of?

Operator SDK - allows developers to build operators
Operator Lifecycle Manager - helps to install, update and generally manage the lifecycle of all operators
Operator Metering - Enables usage reporting for operators that provide specialized services

Describe in detail what is the Operator Lifecycle Manager

It's part of the Operator Framework, used for managing the lifecycle of operators. It basically extends Kubernetes so a user can use a declarative way to manage operators (installation, upgrade, ...).

What openshift-operator-lifecycle-manager namespace includes?

It includes:

catalog-operator - Resolving and installing ClusterServiceVersions the resource they specify.
olm-operator - Deploys applications defined by ClusterServiceVersion resource

What is kubconfig? What do you use it for?
Can you use a Deployment for stateful applications?
Explain StatefulSet

Kubernetes - Secrets

Explain Kubernetes Secrets

Secrets let you store and manage sensitive information (passwords, ssh keys, etc.)

How to create a Secret from a key and value?

kubectl create secret generic some-secret --from-literal=password='donttellmypassword'

How to create a Secret from a file?

kubectl create secret generic some-secret --from-file=/some/file.txt

What type: Opaque in a secret file means? What other types are there?

Opaque is the default type used for key-value pairs.

True or False? storing data in a Secret component makes it automatically secured

False. Some known security mechanisms like "encryption" aren't enabled by default.

What is the problem with the following Secret file:

apiVersion: v1   
kind: Secret
metadata:
    name: some-secret
type: Opaque
data:
    password: mySecretPassword

Password isn't encrypted. You should run something like this: `echo -n 'mySecretPassword' | base64` and paste the result to the file instead of using plain-text. How to create a Secret from a configuration file?

kubectl apply -f some-secret.yaml

What the following in Deployment configuration file means?

spec:
  containers:
    - name: USER_PASSWORD
      valueFrom:
        secretKeyRef:
          name: some-secret
          key: password

USER_PASSWORD environment variable will store the value from password key in the secret called "some-secret" In other words, you reference a value from a Kubernetes Secret.

Kubernetes - Volumes

True or False? Kubernetes provides data persistence out of the box, so when you restart a pod, data is saved

False

Explain "Persistent Volumes". Why do we need it?

Persistent Volumes allow us to save data so basically they provide storage that doesn't depend on the pod lifecycle.

True or False? Persistent Volume must be available to all nodes because the pod can restart on any of them

True

What types of persistent volumes are there?

NFS
iSCSI
CephFS
...

What is PersistentVolumeClaim?
Explain Volume Snapshots
True or False? Kubernetes manages data persistence

False

Explain Storage Classes
Explain "Dynamic Provisioning" and "Static Provisioning"
Explain Access Modes
What is CSI Volume Cloning?
Explain "Ephemeral Volumes"
What types of ephemeral volumes Kubernetes supports?
What is Reclaim Policy?
What reclaim policies are there?

Retain
Recycle
Delete

Kubernetes - Access Control

What is RBAC?
Explain the Role and RoleBinding" objects
What is the difference between Role and ClusterRole objects?
Explain what are "Service Accounts" and in which scenario would use create/use one

Kubernetes.io: "A service account provides an identity for processes that run in a Pod."

An example of when to use one:
You define a pipeline that needs to build and push an image. In order to have sufficient permissions to build an push an image, that pipeline would require a service account with sufficient permissions.

What happens you create a pod and you DON'T specify a service account?

The pod is automatically assigned with the default service account (in the namespace where the pod is running).

Explain how Service Accounts are different from User Accounts

User accounts are global while Service accounts unique per namespace
User accounts are meant for humans or client processes while Service accounts are for processes which run in pods

How to list Service Accounts?

kubectl get serviceaccounts

Explain "Security Context"

kubernetes.io: "A security context defines privilege and access control settings for a Pod or Container."

Kubernetes - Patterns

Explain the sidecar container pattern

Kubernetes - CronJob

Explain what is CronJob and what is it used for
What possible issue can arise from using the following spec and how to fix it?

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: some-cron-job
spec:
  schedule: '*/1 * * * *'
  startingDeadlineSeconds: 10
  concurrencyPolicy: Allow

If the cron job fails, the next job will not replace the previous one due to the "concurrencyPolicy" value which is "Allow". It will keep spawning new jobs and so eventually the system will be filled with failed cron jobs.
To avoid such problem, the "concurrencyPolicy" value should be either "Replace" or "Forbid".

What issue might arise from using the following CronJob and how to fix it?

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: "some-cron-job"
spec:
  schedule: '*/1 * * * *'
jobTemplate:
  spec:
    template:
      spec:
      restartPolicy: Never
      concurrencyPolicy: Forbid
      successfulJobsHistoryLimit: 1
      failedJobsHistoryLimit: 1

The following lines placed under the template:

concurrencyPolicy: Forbid
successfulJobsHistoryLimit: 1
failedJobsHistoryLimit: 1

As a result this configuration isn't part of the cron job spec hence the cron job has no limits which can cause issues like OOM and potentially lead to API server being down.

To fix it, these lines should placed in the spec of the cron job, above or under the "schedule" directive in the above example.

Kubernetes - Misc

Explain Imperative Management vs. Declarative Management
Explain what Kubernetes Service Discovery means
You have one Kubernetes cluster and multiple teams that would like to use it. You would like to limit the resources each team consumes in the cluster. Which Kubernetes concept would you use for that?

Namespaces will allow to limit resources and also make sure there are no collisions between teams when working in the cluster (like creating an app with the same name).

What Kube Proxy does?
What "Resources Quotas" are used for and how?
Explain ConfigMap

Separate configuration from pods.
It's good for cases where you might need to change configuration at some point but you don't want to restart the application or rebuild the image so you create a ConfigMap and connect it to a pod but externally to the pod.

Overall it's good for:

Sharing the same configuration between different pods
Storing external to the pod configuration

How to use ConfigMaps?

Create it (from key&value, a file or an env file)
Attach it. Mount a configmap as a volume

True or False? Sensitive data, like credentials, should be stored in a ConfigMap

False. Use secret.

Explain "Horizontal Pod Autoscaler"

Scale the number of pods automatically on observed CPU utilization.

When you delete a pod, is it deleted instantly? (a moment after running the command)
What does being cloud-native mean?
Explain the pet and cattle approach of infrastructure with respect to kubernetes
Describe how you one proceeds to run a containerised web app in K8s, which should be reachable from a public URL.
How would you troubleshoot your cluster if some applications are not reachable any more?
Describe what CustomResourceDefinitions there are in the Kubernetes world? What they can be used for?
How does scheduling work in kubernetes?

The control plane component kube-scheduler asks the following questions,

What to schedule? It tries to understand the pod-definition specifications
Which node to schedule? It tries to determine the best node with available resources to spin a pod
Binds the Pod to a given node

View more here

How are labels and selectors used?
What QoS classes are there?

Guaranteed
Burstable
BestEffort

Explain Labels. What are they and why would one use them?
Explain Selectors
What is Kubeconfig?

Kubernetes - Gatekeeper

What is Gatekeeper?

Gatekeeper docs: "Gatekeeper is a validating (mutating TBA) webhook that enforces CRD-based policies executed by Open Policy Agent"

Explain how Gatekeeper works

On every request sent to the Kubernetes cluster, Gatekeeper sends the policies and the resources to OPA (Open Policy Agent) to check if it violates any policy. If it does, Gatekeeper will return the policy error message back. If it isn't violates any policy, the request will reach the cluster.

Kubernetes - Policy Testing

What is Conftest?

Conftest allows you to write tests against structured files. You can think of it as tests library for Kubernetes resources.

It is mostly used in testing environments such as CI pipelines or local hooks.

What is Datree? How is it different from Conftest?

Same as Conftest, it is used for policy testing and enforcement. The difference is that it comes with built-in policies.

Kubernetes - Helm

What is Helm?

Package manager for Kubernetes. Basically the ability to package YAML files and distribute them to other users and apply them in different clusters.

Why do we need Helm? What would be the use case for using it?

Sometimes when you would like to deploy a certain application to your cluster, you need to create multiple YAML files/components like: Secret, Service, ConfigMap, etc. This can be tedious task. So it would make sense to ease the process by introducing something that will allow us to share these bundle of YAMLs every time we would like to add an application to our cluster. This something is called Helm.

A common scenario is having multiple Kubernetes clusters (prod, dev, staging). Instead of individually applying different YAMLs in each cluster, it makes more sense to create one Chart and install it in every cluster.

Explain "Helm Charts"

Helm Charts is a bundle of YAML files. A bundle that you can consume from repositories or create your own and publish it to the repositories.

It is said that Helm is also Templating Engine. What does it mean?

It is useful for scenarios where you have multiple applications and all are similar, so there are minor differences in their configuration files and most values are the same. With Helm you can define a common blueprint for all of them and the values that are not fixed and change can be placeholders. This is called a template file and it looks similar to the following

apiVersion: v1
kind: Pod
metadata:
  name: {[ .Values.name ]}
spec:
  containers:
  - name: {{ .Values.container.name }}
  image: {{ .Values.container.image }}
  port: {{ .Values.container.port }}

The values themselves will in separate file:

name: some-app
container:
  name: some-app-container
  image: some-app-image
  port: 1991

What are some use cases for using Helm template file?

Deploy the same application across multiple different environments
CI/CD

Explain the Helm Chart Directory Structure

someChart/ -> the name of the chart
Chart.yaml -> meta information on the chart
values.yaml -> values for template files
charts/ -> chart dependencies
templates/ -> templates files :)

How do you search for charts?

helm search hub [some_keyword]

Is it possible to override values in values.yaml file when installing a chart?
Yes. You can pass another values file: `helm install --values=override-values.yaml [CHART_NAME]`

Or directly on the command line: helm install --set some_key=some_value

How Helm supports release management?

Helm allows you to upgrade, remove and rollback to previous versions of charts. In version 2 of Helm it was with what is known as "Tiller". In version 3, it was removed due to security concerns.

Submariner

Explain what is Submariner and what is it used for

"Submariner enables direct networking between pods and services in different Kubernetes clusters, either on premise or in the cloud."

You can learn more here

What each of the following components does?:

Lighthouse
Broker
Gateway Engine
Route Agent

Istio

What is Istio? What is it used for?

Programming

What programming language do you prefer to use for DevOps related tasks? Why specifically this one?
What are static typed (or simply typed) languages?

In static typed languages the variable type is known at compile-time instead of at run-time.
Such languages are: C, C++ and Java

Explain expressions and statements

An expression is anything that results in a value (even if the value is None). Basically, any sequence of literals so, you can say that a string, integer, list, ... are all expressions.

Statements are instructions executed by the interpreter like variable assignments, for loops and conditionals (if-else).

What is Object Oriented Programming? Why is it important?
Explain Composition
What is a compiler?
What is an interpreter?
Are you familiar with SOLID design principles?

SOLID design principles are about:

Make it easier to extend the functionality of the system
Make the code more readable and easier to maintain

SOLID is:

Single Responsibility - A class should only have a single responsibility
Open-Closed - An entity should be open for extension, but closed for modification. What this practically means is that you should extend functionality by adding a new code and not by modifying it. Your system should be separated into components so it can be easily extended without breaking everything.
Liskov Substitution - Any derived class should be able to substitute the its parent without altering its corrections. Practically, every part of the code will get the expected result no matter which part is using it
Interface segregation - A client should never depend on anything it doesn't uses
Dependency Inversion - High level modules should depend on abstractions, not low level modules

What is YAGNI? What is your opinion on it?
What is DRY? What is your opinion on it?
What are the four pillars of object oriented programming?
Explain recursion
Explain Inversion of Control
Explain Dependency Injection
True or False? In Dynamically typed languages the variable type is known at run-time instead of at compile-time

True

Explain what are design patterns and describe three of them in detail
Explain big O notation
What is "Duck Typing"?
Explain string interpolation

Common algorithms

Binary search:

How does it works?
Can you implement it? (in any language you prefer)
What is the average performance of the algorithm you wrote?

It's a search algorithm used with sorted arrays/lists to find a target value by dividing the array each iteration and comparing the middle value to the target value. If the middle value is smaller than target value, then the target value is searched in the right part of the divided array, else in the left side. This continues until the value is found (or the array divided max times)

python implementation

The average performance of the above algorithm is O(log n). Best performance can be O(1) and worst O(log n).

Code Review

What are your code-review best practices?
Do you agree/disagree with each of the following statements and why?:

The commit message is not important. When reviewing a change/patch one should focus on the actual change
You shouldn't test your code before submitting it. This is what CI/CD exists for.

Strings

In any language you want, write a function to determine if a given string is a palindrome
In any language you want, write a function to determine if two strings are Anagrams

Integers

In any language you would like, print the numbers from 1 to a given integer. For example for input: 5, the output is: 12345

Time Complexity

Describe what would be the time complexity of the operations access, search insert and remove for the following data structures:

Stack
Queue
Linked List
Binary Search Tree

What is the complexity for the best, worst and average cases of each of the following algorithms?:

Quick sort
Merge sort
Bucket Sort
Radix Sort

Data Structures & Types

Implement Stack in any language you would like
Tell me everything you know about Linked Lists

A linked list is a data structure
It consists of a collection of nodes. Together these nodes represent a sequence
Useful for use cases where you need to insert or remove an element from any position of the linked list
Some programming languages don't have linked lists as a built-in data type (like Python for example) but it can be easily implemented

Describe (no need to implement) how to detect a loop in a Linked List

There are multiple ways to detect a loop in a linked list. I'll mention three here:

Worst solution:

Two pointers where one points to the head and one points to the last node. Each time you advance the last pointer by one and check whether the distance between head pointer to the moved pointer is bigger than the last time you measured the same distance (if not, you have a loop).

The reason it's probably the worst solution, is because time complexity here is O(n^2)

Decent solution:

Create an hash table and start traversing the linked list. Every time you move, check whether the node you moved to is in the hash table. If it isn't, insert it to the hash table. If you do find at any point the node in the hash table, it means you have a loop. When you reach None/Null, it's the end and you can return "no loop" value.
This one is very easy to implement (just create a hash table, update it and check whether the node is in the hash table every time you move to the next node) but since the auxiliary space is O(n) because you create a hash table then, it's not the best solution

Good solution:

Instead of creating a hash table to document which nodes in the linked list you have visited, as in the previous solution, you can modify the Linked List (or the Node to be precise) to have a "visited" attribute. Every time you visit a node, you set "visited" to True.

Time compleixty is O(n) and Auxiliary space is O(1), so it's a good solution but the only problem, is that you have to modify the Linked List.

Best solution:

You set two pointers to traverse the linked list from the beginning. You move one pointer by one each time and the other pointer by two. If at any point they meet, you have a loop. This solution is also called "Floyd's Cycle-Finding"

Time complexity is O(n) and auxiliary space is O(1). Perfect :)

Implement Hash table in any language you would like
What is Integer Overflow? How is it handled?
Name 3 design patterns. Do you know how to implement (= provide an example) these design pattern in any language you'll choose?
Given an array/list of integers, find 3 integers which are adding up to 0 (in any language you would like)

def find_triplets_sum_to_zero(li):
    li = sorted(li)
    for i, val in enumerate(li):
        low, up = 0, len(li)-1
        while low < i < up:
            tmp = var + li[low] + li[up]
            if tmp > 0:
                up -= 1
            elif tmp < 0:
                low += 1
            else:
                yield li[low], val, li[up]
                low += 1
                up -= 1

Python

Python Exercises

Name	Topic	Objective & Instructions	Solution
Identify the data type	Data Types	Exercise	Solution
Identify the data type - Advanced	Data Types	Exercise	Solution
Reverse String	Strings	Exercise	Solution
Compress String	Strings	Exercise	Solution

Python Self Assessment

What are some characteristics of the Python programming language?

1. It is a high level general purpose programming language created in 1991 by Guido Van Rosum.
2. The language is interpreted, being the CPython (Written in C) the most used/maintained implementation.
3. It is strongly typed. The typing discipline is duck typing and gradual.
4. Python focuses on readability and makes use of whitespaces/identation instead of brackets { }
5. The python package manager is called PIP "pip installs packages", having more than 200.000 available packages.
6. Python comes with pip installed and a big standard library that offers the programmer many precooked solutions.
7. In python **Everything** is an object.

What built-in types Python has?

List
Dictionary
Set
Numbers (int, float, ...)
String
Bool
Tuple
Frozenset

What is mutability? Which of the built-in types in Python are mutable?

Mutability determines whether you can modify an object of specific type.

The mutable data types are:

List
Dictionary
Set

The immutable data types are:

Numbers (int, float, ...)
String
Bool
Tuple
Frozenset

Python - Booleans

What is the result of each of the following?

1 > 2
'b' > 'a'
1 == 'one'
2 > 'one'
False
True
False
TypeError

What is the result of `bool("")`? What about `bool(" ")`? Explain

bool("") -> evaluates to False

bool(" ") -> evaluates to True

What is the result of running [] is not []? explain the result

It evaluates to True.

The reason is that the two created empty list are different objects. x is y only evaluates to true when x and y are the same object.

What is the result of running True-True?

Python - Strings

True or False? String is an immutable data type in Python

True

What is the result of of each of the following?

"abc"*3
"abc"*2.5
"abc"*2.0
"abc"*True
"abc"*False
abcabcabc
TypeError
TypeError
"abc"
""

Improve the following code:

char = input("Insert a character: ")
if char == "a" or char == "o" or char == "e" or char =="u" or char == "i":
    print("It's a vowel!")

char = input("Insert a character: ") # For readablity
if lower(char[0]) in "aieou": # Takes care of multiple characters and separate cases
    print("It's a vowel!")

if lower(input("Insert a character: ")[0]) in "aieou": # Takes care of multiple characters and small/Capital cases
    print("It's a vowel!")

Python - Functions

How to define a function with Python?
Using the `def` keyword. For Examples:

def sum(a, b):
    return (a + b)

In Python, functions are first-class objects. What does it mean?

In general, first class objects in programming languages are objects which can be assigned to variable, used as a return value and can be used as arguments or parameters.

In python you can treat functions this way. Let's say we have the following function

def my_function():
    return 5

You can then assign a function to a variables like this x = my_function or you can return functions as return values like this return my_function

Python - OOP

Explain inheritance and how to use it in Python

By definition inheritance is the mechanism where an object acts as a base of another object, retaining all its
properties.

So if Class B inherits from Class A, every characteristics from class A will be also available in class B.
Class A would be the 'Base class' and B class would be the 'derived class'.

This comes handy when you have several classes that share the same functionalities.

The basic syntax is:

class Base: pass

class Derived(Base): pass

A more forged example:

class Animal:
    def __init__(self):
        print("and I'm alive!")

    def eat(self, food):
        print("ñom ñom ñom", food)

class Human(Animal):
    def __init__(self, name):
        print('My name is ', name)
        super().__init__()

    def write_poem(self):
        print('Foo bar bar foo foo bar!')

class Dog(Animal):
    def __init__(self, name):
        print('My name is', name)
        super().__init__()

    def bark(self):
        print('woof woof')


michael = Human('Michael')
michael.eat('Spam')
michael.write_poem()

bruno = Dog('Bruno')
bruno.eat('bone')
bruno.bark()

>>> My name is  Michael
>>> and I'm alive!
>>> ñom ñom ñom Spam
>>> Foo bar bar foo foo bar!
>>> My name is Bruno
>>> and I'm alive!
>>> ñom ñom ñom bone
>>> woof woof

Calling super() calls the Base method, thus, calling super().__init__() we called the Animal __init__.

There is a more advanced python feature called MetaClasses that aid the programmer to directly control class creation.

Explain and demonstrate class attributes & instance attributes

In the following block of code x is a class attribute while self.y is a instance attribute

class MyClass(object):
    x = 1

    def __init__(self, y):
        self.y = y

Python - Exceptions

What is an error? What is an exception? What types of exceptions are you familiar with?

#  Note that you generally don't need to know the compiling process but knowing where everything comes from
#  and giving complete answers shows that you truly know what you are talking about.

Generally, every compiling process have a two steps.
    - Analysis
    - Code Generation.

    Analysis can be broken into:
        1. Lexical analysis   (Tokenizes source code)
        2. Syntactic analysis (Check whether the tokens are legal or not, tldr, if syntax is correct)

               for i in 'foo'
                          ^
             SyntaxError: invalid syntax

        We missed ':'


        3. Semantic analysis  (Contextual analysis, legal syntax can still trigger errors, did you try to divide by 0,
          hash a mutable object or use an undeclared function?)

                 1/0
                ZeroDivisionError: division by zero

    These three analysis steps are the responsible for error handlings.

    The second step would be responsible for errors, mostly syntax errors, the most common error.
    The third step would be responsible for Exceptions.

    As we have seen, Exceptions are semantic errors, there are many builtin Exceptions:

        ImportError
        ValueError
        KeyError
        FileNotFoundError
        IndentationError
        IndexError
        ...

    You can also have user defined Exceptions that have to inherit from the `Exception` class, directly or indirectly.

    Basic example:

    class DividedBy2Error(Exception):
        def __init__(self, message):
            self.message = message


    def division(dividend,divisor):
        if divisor == 2:
            raise DividedBy2Error('I dont want you to divide by 2!')
        return dividend / divisor

    division(100, 2)

    >>> __main__.DividedBy2Error: I dont want you to divide by 2!

Explain Exception Handling and how to use it in Python

Exceptions: Errors detected during execution are called Exceptions.

Handling Exception: When an error occurs, or exception as we call it, Python will normally stop and generate an error message.
Exceptions can be handled using try and except statement in python.

Example: Following example asks the user for input until a valid integer has been entered.
If user enter a non-integer value it will raise exception and using except it will catch that exception and ask the user to enter valid integer again.

while True:
    try:
        a = int(input("please enter an integer value: "))
        break
    except ValueError:
        print("Ops! Please enter a valid integer value.")

For more details about errors and exceptions follow this https://docs.python.org/3/tutorial/errors.html

What is the result of running the following function?

def true_or_false():
    try:
        return True
    finally:
        return False

False

Python Built-in functions

Explain the following built-in functions (their purpose + use case example):

repr
any
all

What is the difference between repr function and str?
What is the __call__ method?
Do classes has the __call__ method as well? What for?
What _ is used for in Python?

Translation lookup in i18n
Hold the result of the last executed expression or statement in the interactive interpreter.
As a general purpose "throwaway" variable name. For example: x, y, _ = get_data() (x and y are used but since we don't care about third variable, we "threw it away").

Explain what is GIL
What is Lambda? How is it used?

A lambda expression is an 'anonymous' function, the difference from a normal defined function using the keyword `def`` is the syntax and usage.

The syntax is:

lambda[parameters]: [expresion]

Examples:

A lambda function add 10 with any argument passed.

x = lambda a: a + 10
print(x(10))

An addition function

addition = lambda x, y: x + y
print(addition(10, 20))

Squaring function

square = lambda x : x ** 2
print(square(5))

Generally it is considered a bad practice under PEP 8 to assign a lambda expresion, they are meant to be used as parameters and inside of other defined functions.

Properties

Are there private variables in Python? How would you make an attribute of a class, private?
Explain the following:

getter
setter
deleter

Explain what is @property
How do you swap values between two variables?

x, y = y, x

Explain the following object's magic variables:

dict

Write a function to return the sum of one or more numbers. The user will decide how many numbers to use

First you ask the user for the amount of numbers that will be use. Use a while loop that runs until amount_of_numbers becomes 0 through subtracting amount_of_numbers by one each loop. In the while loop you want ask the user for a number which will be added a variable each time the loop runs.

def return_sum():
	amount_of_numbers = int(input("How many numbers? "))
	total_sum = 0
	while amount_of_numbers != 0:
		num = int(input("Input a number. "))
		total_sum += num
		amount_of_numbers -= 1
	return total_sum

Print the average of [2, 5, 6]. It should be rounded to 3 decimal places

li = [2, 5, 6]
print("{0:.3f}".format(sum(li)/len(li)))

Python - Lists

What is a tuple in Python? What is it used for?

A tuple is a built-in data type in Python. It's used for storing multiple items in a single variable.

List, like a tuple, is also used for storing multiple items. What is then, the difference between a tuple and a list?

List, as opposed to a tuple, is a mutable data type. It means we can modify it and at items to it.

How to add the number 2 to the list x = [1, 2, 3]

x.append(2)

How to check how many items a list contains?

len(sone_list)

How to get the last element of a list?

some_list[-1]

How to add the items of [1, 2, 3] to the list [4, 5, 6]?
x = [4, 5, 6] x.extend([1, 2, 3])

Don't use append unless you would like the list as one item.

How to remove the first 3 items from a list?

my_list[0:3] = []

How do you get the maximum and minimum values from a list?

Maximum: max(some_list)
Minimum: min(some_list)

How to get the top/biggest 3 items from a list?

sorted(some_list, reverse=True)[:3]

some_list.sort(reverse=True)
some_list[:3]

How to insert an item to the beginning of a list? What about two items?
How to sort list by the length of items?

sorted_li = sorted(li, key=len)

Or without creating a new list:

li.sort(key=len)

Do you know what is the difference between list.sort() and sorted(list)?

sorted(list) will return a new list (original list doesn't change)

list.sort() will return None but the list is change in-place

sorted() works on any iterable (Dictionaries, Strings, ...)

list.sort() is faster than sorted(list) in case of Lists

Convert every string to an integer: [['1', '2', '3'], ['4', '5', '6']]

nested_li = [['1', '2', '3'], ['4', '5', '6']]
[[int(x) for x in li] for li in nested_li]

How to merge two sorted lists into one sorted list?

sorted(li1 + li2)

Another way:

i, j = 0
merged_li = []

while i < len(li1) and j < len(li2):
    if li1[i] < li2[j]:
        merged_li.append(li1[i])
        i += 1
    else:
        merged_li.append(li2[j])
        j += 1

merged_li = merged_li + merged_li[i:] + merged_li[j:]

How to check if all the elements in a given lists are unique? so [1, 2, 3] is unique but [1, 1, 2, 3] is not unique because 1 exists twice

There are many ways of solving this problem:

# Note: :list and -> bool are just python typings, they are not needed for the correct execution of the algorithm.

Taking advantage of sets and len:

def is_unique(l:list) -> bool:
    return len(set(l)) == len(l)

This one is can be seen used in other programming languages.

def is_unique2(l:list) -> bool:
    seen = []

    for i in l:
        if i in seen:
            return False
        seen.append(i)
    return True

Here we just count and make sure every element is just repeated once.

def is_unique3(l:list) -> bool:
    for i in l:
        if l.count(i) > 1:
            return False
    return True

This one might look more convulated but hey, one liners.

def is_unique4(l:list) -> bool:
    return all(map(lambda x: l.count(x) < 2, l))

You have the following function

def my_func(li = []):
    li.append("hmm")
    print(li)

If we call it 3 times, what would be the result each call?

['hmm']
['hmm', 'hmm']
['hmm', 'hmm', 'hmm']

How to iterate over a list?

for item in some_list:
    print(item)

How to iterate over a list with indexes?

for i, item in enumerate(some_list):
    print(i)

How to start list iteration from 2nd index?

Using range like this

for i in range(1, len(some_list)):
    some_list[i]

Another way is using slicing

for i in some_list[1:]:

How to iterate over a list in reverse order?

Method 1

for i in reversed(li):
    ...

Method 2

n = len(li) - 1
while n > 0:
    ...
    n -= 1

Sort a list of lists by the second item of each nested list

li = [[1, 4], [2, 1], [3, 9], [4, 2], [4, 5]]

sorted(li, key=lambda l: l[1])

li.sort(key=lambda l: l[1)

Combine [1, 2, 3] and ['x', 'y', 'z'] so the result is [(1, 'x'), (2, 'y'), (3, 'z')]

nums = [1, 2, 3]
letters = ['x', 'y', 'z']

list(zip(nums, letters))

What is List Comprehension? Is it better than a typical loop? Why? Can you demonstrate how to use it?
You have the following list: [{'name': 'Mario', 'food': ['mushrooms', 'goombas']}, {'name': 'Luigi', 'food': ['mushrooms', 'turtles']}] Extract all type of foods. Final output should be: {'mushrooms', 'goombas', 'turtles'}

brothers_menu =  \
[{'name': 'Mario', 'food': ['mushrooms', 'goombas']}, {'name': 'Luigi', 'food': ['mushrooms', 'turtles']}]

# "Classic" Way
def get_food(brothers_menu) -> set:
    temp = []

    for brother in brothers_menu:
        for food in brother['food']:
            temp.append(food)

    return set(temp)

# One liner way (Using list comprehension)
set([food for bro in x for food in bro['food']])

Python - Dictionaries

How to create a dictionary?

my_dict = dict(x=1, y=2)
OR
my_dict = {'x': 1, 'y': 2}
OR
my_dict = dict([('x', 1), ('y', 2)])

How to remove a key from a dictionary?

del my_dict['some_key']
you can also use my_dict.pop('some_key') which returns the value of the key.

How to sort a dictionary by values?

{k: v for k, v in sorted(x.items(), key=lambda item: item[1])}

How to sort a dictionary by keys?

dict(sorted(some_dictionary.items()))

How to merge two dictionaries?

some_dict1.update(some_dict2)

Convert the string "a.b.c" to the dictionary {'a': {'b': {'c': 1}}}

output = {}
string = "a.b.c"
path = string.split('.')
target = reduce(lambda d, k: d.setdefault(k, {}), path[:-1], output)
target[path[-1]] = 1
print(output)

Common Algorithms Implementation

Can you implement "binary search" in Python?

Solution

Python Files

How to write to a file?

with open('file.txt', 'w') as file:
    file.write("My insightful comment")

How to print the 12th line of a file?
How to reverse a file?
Sum all the integers in a given file
Print a random line of a given file
Print every 3rd line of a given file
Print the number of lines in a given file
Print the number of of words in a given file
Can you write a function which will print all the file in a given directory? including sub-directories
Write a dictionary (variable) to a file

import json

with open('file.json', 'w') as f:
    f.write(json.dumps(dict_var))

Python OS

How to print current working directory?

import os

print(os.getcwd())

Given the path /dir1/dir2/file1 print the file name (file1)

import os

print(os.path.basename('/dir1/dir2/file1'))

# Another way
print(os.path.split('/dir1/dir2/file1')[1])

Given the path /dir1/dir2/file1

Print the path without the file name (/dir1/dir2)
Print the name of the directory where the file resides (dir2)

import os

## Part 1.
# os.path.dirname gives path removing the end component
dirpath = os.path.dirname('/dir1/dir2/file1')
print(dirpath)

## Part 2.
print(os.path.basename(dirpath))

How do you execute shell commands using Python?
How do you join path components? for example /home and luig will result in /home/luigi
How do you remove non-empty directory?

Python Regex

How do you perform regular expressions related operations in Python? (match patterns, substitute strings, etc.)

Using the re module

How to substitute the string "green" with "blue"?
How to find all the IP addresses in a variable? How to find them in a file?

Python Strings

Find the first repeated character in a string

While you iterate through the characters, store them in a dictionary and check for every character whether it's already in the dictionary.

def firstRepeatedCharacter(str):
    chars = {}
    for ch in str:
        if ch in chars:
            return ch
        else:
            chars[ch] = 0

How to extract the unique characters from a string? for example given the input "itssssssameeeemarioooooo" the output will be "mrtisaoe"

x = "itssssssameeeemarioooooo"
y = ''.join(set(x))

Find all the permutations of a given string

def permute_string(string):

    if len(string) == 1:
        return [string]

    permutations = []
    for i in range(len(string)):
        swaps = permute_string(string[:i] + string[(i+1):])
        for swap in swaps:
            permutations.append(string[i] + swap)

    return permutations

print(permute_string("abc"))

Short way (but probably not acceptable in interviews):

from itertools import permutations

[''.join(p) for p in permutations("abc")]

Detailed answer can be found here: http://codingshell.com/python-all-string-permutations

How to check if a string contains a sub string?
Find the frequency of each character in string
Count the number of spaces in a string
Given a string, find the N most repeated words
Given the string (which represents a matrix) "1 2 3\n4 5 6\n7 8 9" create rows and colums variables (should contain integers, not strings)
What is the result of each of the following?

>> ', '.join(["One", "Two", "Three"])
>> " ".join("welladsadgadoneadsadga".split("adsadga")[:2])
>> "".join(["c", "t", "o", "a", "o", "q", "l"])[0::2]

>>> 'One, Two, Three'
>>> 'well done'
>>> 'cool'

If x = "pizza", what would be the result of x[::-1]?

It will reverse the string, so x would be equal to azzip.

Reverse each word in a string (while keeping the order)
What is the output of the following code: "".join(["a", "h", "m", "a", "h", "a", "n", "q", "r", "l", "o", "i", "f", "o", "o"])[2::3]

mario

Python Iterators

What is an iterator?

Python Misc

Explain data serialization and how do you perform it with Python
How do you handle argument parsing in Python?
What is a generator? Why using generators?
What would be the output of the following block?

for i in range(3, 3):
   print(i)

No output :)

What is yeild? When would you use it?
Explain the following types of methods and how to use them:

Static method
Class method
instance method

How to reverse a list?
How to combine list of strings into one string with spaces between the strings
You have the following list of nested lists: [['Mario', 90], ['Geralt', 82], ['Gordon', 88]] How to sort the list by the numbers in the nested lists?

One way is:

the_list.sort(key=lambda x: x[1])

Explain the following:

zip()
map()
filter()

Python - Slicing

For the following slicing exercises, assume you have the following list: my_list = [8, 2, 1, 10, 5, 4, 3, 9]

What is the result of `my_list[0:4]`?
What is the result of `my_list[5:6]`?
What is the result of `my_list[5:5]`?
What is the result of `my_list[::-1]`?
What is the result of `my_list[::3]`?
What is the result of `my_list[2:]`?
What is the result of `my_list[:3]`?

Python Debugging

How do you debug Python code?

pdb :D

How to check how much time it took to execute a certain script or block of code?
What empty return returns?

Short answer is: It returns a None object.

We could go a bit deeper and explain the difference between

def a ():
    return

>>> None

And

def a ():
    pass

>>> None

Or we could be asked this as a following question, since they both give the same result.

We could use the dis module to see what's going on:

  2           0 LOAD_CONST               0 (<code object a at 0x0000029C4D3C2DB0, file "<dis>", line 2>)
              2 LOAD_CONST               1 ('a')
              4 MAKE_FUNCTION            0
              6 STORE_NAME               0 (a)

  5           8 LOAD_CONST               2 (<code object b at 0x0000029C4D3C2ED0, file "<dis>", line 5>)
             10 LOAD_CONST               3 ('b')
             12 MAKE_FUNCTION            0
             14 STORE_NAME               1 (b)
             16 LOAD_CONST               4 (None)
             18 RETURN_VALUE

Disassembly of <code object a at 0x0000029C4D3C2DB0, file "<dis>", line 2>:
  3           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

Disassembly of <code object b at 0x0000029C4D3C2ED0, file "<dis>", line 5>:
  6           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

An empty return is exactly the same as return None and functions without any explicit return
will always return None regardless of the operations, therefore

def sum(a, b):
    global c
    c = a + b

>>> None

How to improve the following block of code?

li = []
for i in range(1, 10):
    li.append(i)

[i for i in range(1, 10)]

Given the following function

def is_int(num):
    if isinstance(num, int):
        print('Yes')
    else:
        print('No')

What would be the result of is_int(2) and is_int(False)?

Python - Linked List

Can you implement a linked list in Python?

The reason we need to implement in the first place, it's because a linked list isn't part of Python standard library.

To implement a linked list, we have to implement two structures: The linked list itself and a node which is used by the linked list.

Let's start with a node. A node has some value (the data it holds) and a pointer to the next node

class Node(object):
    def __init__(self, data):
        self.data = data
        self.next = None

Now the linked list. An empty linked list has nothing but an empty head.

class LinkedList(object):
    def __init__(self):
        self.head = None

Now we can start using the linked list

ll = Linkedlist()
ll.head = Node(1)
ll.head.next = Node(2)
ll.head.next.next = Node(3)

What we have is:

| 1 | -> | 2 | -> | 3 |

Usually, more methods are implemented, like a push_head() method where you insert a node at the beginning of the linked list

def push_head(self, value):
    new_node = Node(value)
    new_node.next = self.head
    self.head = new_node

Add a method to the Linked List class to traverse (print every node's data) the linked list

def print_list(self):
node = self.head
while(node):
print(node.data)
node = node.next

Write a method to that will return a boolean based on whether there is a loop in a linked list or not

Let's use the Floyd's Cycle-Finding algorithm:

def loop_exists(self):
    one_step_p = self.head
    two_steps_p = self.head
    while(one_step_p and two_steps_p and two_steps_p.next):
        one_step_p = self.head.next
        two_step_p = self.head.next.next
        if (one_step_p == two_steps_p):
            return True 
    return False

Python - Stack

Implement Stack in Python

Python Testing

What is your experience with writing tests in Python?
What is PEP8? Give an example of 3 style guidelines

PEP8 is a list of coding conventions and style guidelines for Python

5 style guidelines:

1. Limit all lines to a maximum of 79 characters.
2. Surround top-level function and class definitions with two blank lines.
3. Use commas when making a tuple of one element
4. Use spaces (and not tabs) for indentation
5. Use 4 spaces per indentation level

How to test if an exception was raised?
What assert does in Python?
Explain mocks
How do you measure execution time of small code snippets?
Why one shouldn't use assert in non-test/production code?

Flask

Can you describe what is Django/Flask and how you have used it? Why Flask and not Djano? (or vice versa)
What is a route?
What is a blueprint in Flask?
What is a template?

zip

Given x = [1, 2, 3], what is the result of list(zip(x))?

[(1,), (2,), (3,)]

What is the result of each of the following:

list(zip(range(5), range(50), range(50)))
list(zip(range(5), range(50), range(-2)))

[(0, 0, 0), (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)]
[]

Python Descriptors

Explain Descriptors

Read about descriptors here

What would be the result of running a.num2 assuming the following code

class B:
    def __get__(self, obj, objtype=None):
        reuturn 10

class A:
    num1 = 2
    num2 = Five()

10 What would be the result of running some_car = Car("Red", 4) assuming the following code

class Print:

    def __get__(self, obj, objtype=None):
        value = obj._color
        print("Color was set to {}".format(valie))
        return value

    def __set__(self, obj, value):
        print("The color of the car is {}".format(value))
        obj._color = value

class Car:

    color = Print()

    def __ini__(self, color, age):
        self.color = color
        self.age = age

An instance of Car class will be created and the following will be printed: "The color of the car is Red"

Python Misc

How can you spawn multiple processes with Python?
Implement simple calculator for two numbers

def add(num1, num2):
    return num1 + num2


def sub(num1, num2):
    return num1 - num2


def mul(num1, num2):
    return num1*num2


def div(num1, num2):
    return num1 / num2

operators = {
    '+': add,
    '-': sub,
    '*': mul,
    '/': div
}

if __name__ == '__main__':
    operator = str(input("Operator: "))
    num1 = int(input("1st number: "))
    num2 = int(input("2nd number: "))
    print(operators[operator](num1, num2))

What data types are you familiar with that are not Python built-in types but still provided by modules which are part of the standard library?

This is a good reference https://docs.python.org/3/library/datatypes.html

Explain what is a decorator
In python, everything is an object, even functions themselves. Therefore you could pass functions as arguments for another function eg;

def wee(word):
    return word

def oh(f):
    return f + "Ohh"

>>> oh(wee("Wee"))
<<< Wee Ohh

This allows us to control the before execution of any given function and if we added another function as wrapper,
(a function receiving another function that receives a function as parameter) we could also control the after execution.

Sometimes we want to control the before-after execution of many functions and it would get tedious to write

f = function(function_1())
f = function(function_1(function_2(*args)))

every time, that's what decorators do, they introduce syntax to write all of this on the go, using the keyword '@'.

Can you show how to write and use decorators?
These two decorators (ntimes and timer) are usually used to display decorators functionalities, you can find them in lots of tutorials/reviews. I first saw these examples two years ago in pyData 2017. https://www.youtube.com/watch?v=7lmCu8wz8ro&t=3731s

Simple decorator:

def deco(f):
    print(f"Hi I am the {f.__name__}() function!")
    return f

@deco
def hello_world():
    return "Hi, I'm in!"

a = hello_world()
print(a)

>>> Hi I am the hello_world() function!
    Hi, I'm in!

This is the simplest decorator version, it basically saves us from writting a = deco(hello_world()).
But at this point we can only control the before execution, let's take on the after:

def deco(f):
    def wrapper(*args, **kwargs):
        print("Rick Sanchez!")
        func = f(*args, **kwargs)
        print("I'm in!")
        return func
    return wrapper

@deco
def f(word):
    print(word)

a = f("************")
>>> Rick Sanchez!
    ************
    I'm in!

deco receives a function -> f
wrapper receives the arguments -> *args, **kwargs

wrapper returns the function plus the arguments -> f(*args, **kwargs)
deco returns wrapper.

As you can see we conveniently do things before and after the execution of a given function.

For example, we could write a decorator that calculates the execution time of a function.

import time
def deco(f):
    def wrapper(*args, **kwargs):
        before = time.time()
        func = f(*args, **kwargs)
        after = time.time()
        print(after-before)
        return func
    return wrapper

@deco
def f():
    time.sleep(2)
    print("************")

a = f()
>>> 2.0008859634399414

Or create a decorator that executes a function n times.

def n_times(n):
    def wrapper(f):
        def inner(*args, **kwargs):
            for _ in range(n):
                func = f(*args, **kwargs)
            return func
        return inner
    return wrapper

@n_times(4)
def f():
    print("************")

a = f()

>>>************
   ************
   ************
   ************

Write a decorator that calculates the execution time of a function
Write a script which will determine if a given host is accessible on a given port
Are you familiar with Dataclasses? Can you explain what are they used for?
You wrote a class to represent a car. How would you compare two cars instances if two cars are equal if they have the same model and color?

class Car:
    def __init__(self, model, color):
        self.model = model
        self.color = color

    def __eq__(self, other):
        if not isinstance(other, Car):
            return NotImplemented
        return self.model == other.model and self.color == other.color

>> a = Car('model_1', 'red')
>> b = Car('model_2', 'green')
>> c = Car('model_1', 'red')
>> a == b
False
>> a == c
True

Explain Context Manager
Tell me everything you know about concurrency in Python
Explain the Buffer Protocol
Do you have experience with web scraping? Can you describe what have you used and for what?
Can you implement Linux's tail command in Python? Bonus: implement head as well
You have created a web page where a user can upload a document. But the function which reads the uploaded files, runs for a long time, based on the document size and user has to wait for the read operation to complete before he/she can continue using the web site. How can you overcome this?
How yield works exactly?

Monitoring

Explain monitoring. What is it? What its goal?

Google: "Monitoring is one of the primary means by which service owners keep track of a system’s health and availability".

What is wrong with the old approach of watching for a specific value and trigger an email/phone alert while value is exceeded?

This approach require from a human to always check why the value exceeded and how to handle it while today, it is more effective to notify people only when they need to take an actual action.
If the issue doesn't require any human intervention, then the problem can be fixed by some processes running in the relevant environment.

What types of monitoring outputs are you familiar with and/or used in the past?

Alerts

Tickets

Logging

What is the difference between infrastructure monitoring and application monitoring? (methods, tools, ...)

Prometheus

What is Prometheus? What are some of Prometheus's main features?
In what scenarios it might be better to NOT use Prometheus?

From Prometheus documentation: "if you need 100% accuracy, such as for per-request billing".

Describe Prometheus architecture and components
Can you compare Prometheus to other solutions like InfluxDB for example?
What is an Alert?
Describe the following Prometheus components:

Prometheus server
Push Gateway
Alert Manager

Prometheus server is responsible for scraping and storing the data

Push gateway is used for short-lived jobs

Alert manager is responsible for alerts ;)

What is an Instance? What is a Job?
What core metrics types Prometheus supports?
What is an exporter? What is it used for?
Which Prometheus best practices are you familiar with?. Name at least three
How to get total requests in a given period of time?
What HA in Prometheus means?
How do you join two metrics?
How to write a query that returns the value of a label?
How do you convert cpu_user_seconds to cpu usage in percentage?

Git

Name	Topic	Objective & Instructions	Solution
My first Commit	Commit	Exercise	Solution
Time to Branch	Branch	Exercise	Solution
Squashing Commits	Commit	Exercise	Solution

How do you know if a certain directory is a git repository?

You can check if there is a ".git" directory.

Explain the following: git directory, working directory and staging area

This answer taken from git-scm.com

"The Git directory is where Git stores the meta data and object database for your project. This is the most important part of Git, and it is what is copied when you clone a repository from another computer.

The working directory is a single checkout of one version of the project. These files are pulled out of the compressed database in the Git directory and placed on disk for you to use or modify.

The staging area is a simple file, generally contained in your Git directory, that stores information about what will go into your next commit. It’s sometimes referred to as the index, but it’s becoming standard to refer to it as the staging area."

What is the difference between git pull and git fetch?

Shortly, git pull = git fetch + git merge

When you run git pull, it gets all the changes from the remote or central
repository and attaches it to your corresponding branch in your local repository.

git fetch gets all the changes from the remote repository, stores the changes in
a separate branch in your local repository

How to check if a file is tracked and if not, then track it?

There are different ways to check whether a file is tracked or not:

git ls-file <file> -> exit code of 0 means it's tracked
git blame <file>
...

How can you see which changes have done before committing them?

`git diff```

What git status does?
You have two branches - main and devel. How do you make sure devel is in sync with main?

git checkout main
git pull
git checkout devel
git merge main

Git - Merge

You have two branches - main and devel. How do you put devel into main?

git checkout main
git merge devel
git push origin main

How to resolve git merge conflicts?

First, you open the files which are in conflict and identify what are the conflicts. Next, based on what is accepted in your company or team, you either discuss with your colleagues on the conflicts or resolve them by yourself After resolving the conflicts, you add the files with `git add ` Finally, you run `git rebase --continue`

What merge strategies are you familiar with?

Mentioning two or three should be enough and it's probably good to mention that 'recursive' is the default one.

recursive
resolve
ours
theirs

This page explains it the best: https://git-scm.com/docs/merge-strategies

Explain Git octopus merge

Probably good to mention that it's:

It's good for cases of merging more than one branch (and also the default of such use cases)
It's primarily meant for bundling topic branches together

This is a great article about Octopus merge: http://www.freblogg.com/2016/12/git-octopus-merge.html

What is the difference between git reset and git revert?

git revert creates a new commit which undoes the changes from last commit.

git reset depends on the usage, can modify the index or change the commit which the branch head
is currently pointing at.

Git - Rebase

You would like to move forth commit to the top. How would you achieve that?

Using the git rebase command

In what situations are you using git rebase?
How do you revert a specific file to previous commit?

git checkout HEAD~1 -- /path/of/the/file

How to squash last two commits?
What is the .git directory? What can you find there?
The .git folder contains all the information that is necessary for your project in version control and all the information about commits, remote repository address, etc. All of them are present in this folder. It also contains a log that stores your commit history so that you can roll back to history.

This info copied from https://stackoverflow.com/questions/29217859/what-is-the-git-folder

What are some Git anti-patterns? Things that you shouldn't do

Not waiting too long between commits
Not removing the .git directory :)

How do you remove a remote branch?

You delete a remote branch with this syntax:

git push origin :[branch_name]

Are you familiar with gitattributes? When would you use it?

gitattributes allow you to define attributes per pathname or path pattern.

You can use it for example to control endlines in files. In Windows and Unix based systems, you have different characters for new lines (\r\n and \n accordingly). So using gitattributes we can align it for both Windows and Unix with * text=auto in .gitattributes for anyone working with git. This is way, if you use the Git project in Windows you'll get \r\n and if you are using Unix or Linux, you'll get \n.

How do you discard local file changes? (before commit)

git checkout -- <file_name>

How do you discard local commits?

git reset HEAD~1 for removing last commit
If you would like to also discard the changes you `git reset --hard``

True or False? To remove a file from git but not from the filesystem, one should use git rm

False. If you would like to keep a file on your filesystem, use git reset <file_name>

Go

What are some characteristics of the Go programming language?

Strong and static typing - the type of the variables can't be changed over time and they have to be defined at compile time
Simplicity
Fast compile times
Built-in concurrency
Garbage collected
Platform independent
Compile to standalone binary - anything you need to run your app will be compiled into one binary. Very useful for version management in run-time.

Go also has good community.

What is the difference between var x int = 2 and x := 2?

The result is the same, a variable with the value 2.

With var x int = 2 we are setting the variable type to integer while with x := 2 we are letting Go figure out by itself the type.

True or False? In Go we can redeclare variables and once declared we must use it.

False. We can't redeclare variables but yes, we must used declared variables.

What libraries of Go have you used?

This should be answered based on your usage but some examples are:

fmt - formatted I/O

What is the problem with the following block of code? How to fix it?

func main() {
    var x float32 = 13.5
    var y int
    y = x
}

The following block of code tries to convert the integer 101 to a string but instead we get "e". Why is that? How to fix it?

package main

import "fmt"

func main() {
    var x int = 101
    var y string
    y = string(x)
    fmt.Println(y)
}

It looks what unicode value is set at 101 and uses it for converting the integer to a string.
If you want to get "101" you should use the package "strconv" and replace y = string(x) with y = strconv.Itoa(x)

What is wrong with the following code?:

package main

func main() {
    var x = 2
    var y = 3
    const someConst = x + y
}

Constants in Go can only be declared using constant expressions.
But x, y and their sum is variable.

const initializer x + y is not a constant