[Docker] Docker Interview 70 Questions

오늘은 Docker 에 관련된 기술 질문 모음을 만들어 보았다.

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1. What is Hypervisor? 

The hypervisor allows you to create a virtual environment in which the guest virtual machines operate. It controls the guest systems and checks if the resources are allocated to the guests as necessary. It is also called Virtual Machine Monitor. It divides the host system and allocates the resources to each divided virtual environment. You can basically have multiple OS on a single host system. There are two types of Hypervisors: 

• Type 1: It’s also called Native Hypervisor or Bare metal Hypervisor. It runs directly on the underlying host system. It has direct access to your host’s system hardware and hence does not require a base server operating system. 

• Type 2: This kind of hypervisor makes use of the underlying host operating system. It’s also called Hosted Hypervisor. 

2. What is virtualization? 

Virtualization is the process of creating a software-based, virtual version of something(compute storage, servers, application, etc.). These virtual versions or environments are created from a single physical hardware system. Virtualization lets you split one system into many different sections which act like separate, distinct individual systems. A software called Hypervisor makes this kind of splitting possible. The virtual environment created by the hypervisor is called Virtual Machine. 

Virtualization is a method of logically dividing mainframes to allow multiple applications to run simultaneously.  

However, this scenario changed when companies and open source communities were able to offer a method of handling privileged instructions. It allows multiple OS to run simultaneously on a single x86 based system. 

3. What is containerization? 

Let me explain this is with an example. Usually, in the software development process, code developed on one machine might not work perfectly fine on any other machine because of the dependencies. This problem was solved by the containerization concept. So basically, an application that is being developed and deployed is bundled and wrapped together with all its configuration files and dependencies. This bundle is called a container. Now when you wish to run the application on another system, the container is deployed which will give a bug-free environment as all the dependencies and libraries are wrapped together. Most famous containerization environments are Docker and Kubernetes. 

4. Difference between virtualization and containerization 

Once you’ve explained containerization and virtualization, the next expected question would be differences. The question could either be differences between virtualization and containerization or differences between virtual machines and containers. Either way, this is how you respond. 

Containers provide an isolated environment for running the application. The entire user space is explicitly dedicated to the application. Any changes made inside the container is never reflected on the host or even other containers running on the same host. Containers are an abstraction of the application layer. Each container is a different application. 

Whereas in Virtualization, hypervisors provide an entire virtual machine to the guest(including Kernal). Virtual machines are an abstraction of the hardware layer. Each VM is a physical machine. 

Container는 hypervisor와 완전히 다름. 궁극적으로는 hypervisor와 유사한 형태의 "가상화"를 목표로 하고 있지만 hypervisor는 OS 및 커널이 통째로 가상화되는 반면에 container는 간단히 보면 filesystem의 가상화만을 이루고 있다. container는 호스트 PC의 커널을 공유하고 따라서 init(1) 등의 프로세스가 떠있을 필요가 없으며, 따라서 가상화 프로그램과는 다르게 적은 메모리 사용량, 적은 overhead를 보임

**overhead : 어떤 처리를 하기위해 들어가는 간접적인 처리시간, 메모리를 말함.

5. What is Docker? 

Docker is an open-source lightweight containerization technology. It has gained widespread popularity in the cloud and application packaging world. It allows you to automate the deployment of applications in lightweight and portable containers.  

Docker is a containerization platform that packages your application and all its dependencies together in the form of containers so as to ensure that your application works seamlessly in any environment, be it development, test, or production. Docker containers, wrap a piece of software in a complete filesystem that contains everything needed to run: code, runtime, system tools, system libraries, etc. It wraps basically anything that can be installed on a server. This guarantees that the software will always run the same, regardless of its environment. 

6. What is a Docker Container? 

Docker containers include the application and all of its dependencies. It shares the kernel with other containers, running as isolated processes in user space on the host operating system. Docker containers are not tied to any specific infrastructure: they run on any computer, on any infrastructure, and in any cloud. Docker containers are basically runtime instances of Docker images. 

7. What are Docker Images? 

Docker image is the source of Docker container. In other words, Docker images are used to create containers. When a user runs a Docker image, an instance of a container is created. These docker images can be deployed to any Docker environment. Every docker images are stored in the Docker registry. 

8. What is Docker Hub? 

Docker images create docker containers. There has to be a registry where these docker images live. This registry is Docker Hub. Users can pick up images from Docker Hub and use them to create customized images and containers. Currently, the Docker Hub is the world’s largest public repository of image containers. 

Docker hub is a cloud-based registry that which helps you to link to code repositories. It allows you to build, test, store your image in Docker cloud. You can also deploy the image to your host with the help of Docker hub.  

9. Explain Docker Architecture? 

Docker Architecture consists of a Docker Engine which is a client-server application with three major components: 

1. A server is a type of long-running program called a daemon process (the docker command). 

2. A REST API specifies interfaces that programs can use to talk to the daemon and instruct it what to do. 

3. A command-line interface (CLI) client (the docker command). 

4. The CLI uses the Docker REST API to control or interact with the Docker daemon through scripting or direct CLI commands. Many other Docker applications use the underlying API and CLI. 

three components of Docker Architecture:

• Client: Docker provides Command Line interface tools to the client to interact with Docker daemon
  
• Docker-Host : contains container, images, and Docker daemon. It offers a complete environment to execute and run your application. 
• Registry
  

10. What is a Dockerfile? 

Docker can build images automatically by reading the instructions from a file called Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build, users can create an automated build that executes several command-line instructions in succession. 

The common instruction in Dockerfile are : FROM, LABEL, RUN and CMD.

11. Tell us something about Docker Compose. 

Docker Compose is a YAML file which contains details about the services, networks, and volumes for setting up the Docker application. So, you can use Docker Compose to create separate containers, host them and get them to communicate with each other. Each container will expose a port for communicating with other containers. 

How do I run multiple copies of Compose file on the same host?  

Compose uses the project name which allows you to create unique identifiers for all of a project's containers and other resources. To run multiple copies of a project, set a custom project name using the -a command-line option or using COMPOSE_PROJECT_NAME environment variable.  

How can you run multiple containers using a single service?  

By using docker-compose, you can run multiple containers using a single service. All docker-compose files uses yaml language.  

12. What is Docker Swarm? 

Docker Swarm is native clustering for Docker. It turns a pool of Docker hosts into a single, virtual Docker host. Docker Swarm serves the standard Docker API, any tool that already communicates with a Docker daemon can use Swarm to transparently scale to multiple hosts. 

Docker Swarm is native gathering for docker which helps you to a group of Docker hosts into a single and virtual docker host. It offers the standard docker application program interface.  

13. What is a Docker Namespace? 

A namespace is one of the Linux features and an important concept of containers. Namespace adds a layer of isolation in containers. Docker provides various namespaces in order to stay portable and not affect the underlying host system. Few namespace types supported by Docker – PID, Mount, IPC, User, Network 

The Namespace in Docker is a technique that offers isolated workspaces called the Container. Namespaces also offer a layer of isolation for the Docker containers.  

14. What is the lifecycle of a Docker Container? 

This is one of the most popular questions asked in Docker interviews. Docker containers have the following lifecycle: 

• Create a container 

• Run the container 

• Pause the container(optional) 

• Un-pause the container(optional) 

• Start the container 

• Stop the container 

• Restart the container 

• Kill the container 

• Destroy the container 

what are the steps for docker container life cycle? Build - Pull - Run

15. What is Docker Machine? 

Docker machine is a tool that lets you install Docker Engine on virtual hosts. These hosts can now be managed using the docker-machine commands. Docker machine also lets you provision Docker Swarm Clusters. 

Docker Basic Commands 

Once you’ve aced the basic conceptual questions, the interviewer will increase the difficulty level. So let’s move on to the next section of this Docker Interview Questions article. This section talks about the commands that are very common amongst docker users. 

16. How to check for Docker Client and Docker Server version? 

The following command gives you information about Docker Client and Server versions: 

$ docker version 

17. How do you get the number of containers running, paused and stopped? 

You can use the following command to get detailed information about the docker installed on your system. 

$ docker info 

You can get the number of containers running, paused, stopped, the number of images and a lot more. 

18. If you vaguely remember the command and you’d like to confirm it, how will you get help on that particular command? 

The following command is very useful as it gives you help on how to use a command, the syntax, etc. 

$ docker --help 

The above command lists all Docker commands. If you need help with one specific command, you can use the following syntax: 

$ docker <command> --help 

19. How to login into docker repository? 

You can use the following command to login into hub.docker.com: 

$ docker login 

You’ll be prompted for your username and password, insert those and congratulations, you’re logged in. 

20. If you wish to use a base image and make modifications or personalize it, how do you do that? 

You pull an image from docker hub onto your local system 

It’s one simple command to pull an image from docker hub: 

$ docker pull <image_name> 

21. How do you create a docker container from an image? what is the command to run image as a container?

Pull an image from docker repository with the above command and run it to create a container. Use the following command: 

$ docker run -it -d <image_name> 

$ sudo docker run -i -t alpine /bin/bash

 

Most probably the next question would be, what does the ‘-d’ flag mean in the command? 

-d means the container needs to start in the detached mode. Explain a little about the detach mode. Have a look at this blog to get a better understanding of different docker commands. 

22. How do you list all the running containers? 

The following command lists down all the running containers: 

$ docker ps 

** Docker container status

• Running 

• Paused  

• Restarting  

• Exited 

23. Suppose you have 3 containers running and out of these, you wish to access one of them. How do you access a running container? 

The following command lets us access a running container: 

$ docker exec -it <container id> bash 

The exec command lets you get inside a container and work with it. 

24. How to start, stop and kill a container? 

The following command is used to start a docker container: 

$ docker start <container_id> 

and the following for stopping a running container: 

$ docker stop <container_id> 

kill a container with the following command: 

$ docker kill <container_id> 

What are the command to control Docker with Systemd?  

systemctl start/stop docker 

service docker start/stop

25. Can you use a container, edit it, and update it? Also, how do you make it new and store it on the local system? 

Of course, you can use a container, edit it and update it. This sounds complicated but its actually just one command. 

$ docker commit <conatainer id> <username/imagename> 

26. Once you’ve worked with an image, how do you push it to docker hub(docker registry)? 

$ docker push <username/image name> 

27. How to delete a stopped container? 

Use the following command to delete a stopped container: 

$ docker rm <container id> 

28. How to delete an image from the local storage system? 

The following command lets you delete an image from the local system: 

$ docker rmi <image-id> 

29. How to build a Dockerfile?  

Once you’ve written a Dockerfile, you need to build it to create an image with those specifications. Use the following command to build a Dockerfile: 

$ docker build <path to docker file> 

The next question would be when do you use “.dockerfile_name” and when to use the entire path? 

Use “.dockerfile_name” when the dockerfile exits in the same file directory and you use the entire path if it lives somewhere else. 

30. Do you know why docker system prune is used? What does it do? 

$ docker system prune 

The above command is used to remove all the stopped containers, all the networks that are not used, all dangling images and all build caches. It’s one of the most useful docker commands. 

Docker Advanced Questions 

Once the interviewer knows that you’re familiar with the Docker commands, he/she will start asking about practical applications This section of Docker Interview Questions consists of questions that you’ll only be able to answer when you’ve gained some experience working with Docker. 

31. Will you lose your data, when a docker container exists? 

No, you won’t lose any data when Docker container exits. Any data that your application writes to the container gets preserved on the disk until you explicitly delete the container. The file system for the container persists even after the container halts. 

32. Where all do you think Docker is being used? 

When asked such a question, respond by talking about applications of Docker. Docker is being used in the following areas: 

• Simplifying configuration: Docker lets you put your environment and configuration into code and deploy it. 

• Code Pipeline Management: There are different systems used for development and production. As the code travels from development to testing to production, it goes through a difference in the environment. Docker helps in maintaining the code pipeline consistency. 

• Developer Productivity: Using Docker for development gives us two things – We’re closer to production and development environment is built faster. 

• Application Isolation: As containers are applications wrapped together with all dependencies, your apps are isolated. They can work by themselves on any hardware that supports Docker. 

• Debugging Capabilities: Docker supports various debugging tools that are not specific to containers but work well with containers. 

• Multi-tenancy: Docker lets you have multi-tenant applications avoiding redundancy in your codes and deployments. 

• Rapid Deployment: Docker eliminates the need to boost an entire OS from scratch, reducing the deployment time. 

33. How is Docker different from other containerization methods? 

Docker containers are very easy to deploy in any cloud platform. It can get more applications running on the same hardware when compared to other technologies, it makes it easy for developers to quickly create, ready-to-run containerized applications and it makes managing and deploying applications much easier. You can even share containers with your applications. 

If you have some more points to add you can do that but make sure the above explanation is there in your answer. 

34. Can I use JSON instead of YAML for my compose file in Docker? 

You can use JSON instead of YAML for your compose file, to use JSON file with compose, specify the JSON filename to use, for eg: 

$ docker-compose -f docker-compose.json up 

35. How have you used Docker in your previous position? 

Explain how you have used Docker to help rapid deployment. Explain how you have scripted Docker and used it with other tools like Puppet, Chef or Jenkins. If you have no past practical experience in Docker and instead have experience with other tools in a similar space, be honest and explain the same. In this case, it makes sense if you can compare other tools to Docker in terms of functionality. 

36. How far do Docker containers scale? Are there any requirements for the same? 

Large web deployments like Google and Twitter and platform providers such as Heroku and dotCloud, all run on container technology. Containers can be scaled to hundreds of thousands or even millions of them running in parallel. Talking about requirements, containers require the memory and the OS at all the times and a way to use this memory efficiently when scaled. 

37. What platforms does docker run on? 

This is a very straightforward question but can get tricky. Do some company research before going for the interview and find out how the company is using Docker. Make sure you mention the platform company is using in this answer. 

Docker runs on various Linux administration: 

• Ubuntu 12.04, 13.04 et al 

• Fedora 19/20+ 

• RHEL 6.5+ 

• CentOS 6+ 

• Gentoo 

• ArchLinux 

• openSUSE 12.3+ 

• CRUX 3.0+ 

It can also be used in production with Cloud platforms with the following services: 

• Amazon EC2 

• Amazon ECS 

• Google Compute Engine 

• Microsoft Azure 

• Rackspace 

38. Is there a way to identify the status of a Docker container? 

There are six possible states a container can be at any given point – Created, Running, Paused, Restarting, Exited, Dead. 

Use the following command to check for docker state at any given point: 

$ docker ps 

The above command lists down only running containers by default. To look for all containers, use the following command: 

$ docker ps -a 

39. Can you remove a paused container from Docker? 

The answer is no. You cannot remove a paused container. The container has to be in the stopped state before it can be removed. 

40. Can a container restart by itself? 

No, it’s not possible for a container to restart by itself. By default the flag -restart is set to false. 

41. Is it better to directly remove the container using the rm command or stop the container followed by remove container? 

Its always better to stop the container and then remove it using the remove command. 

$ docker stop <coontainer_id>  $ docker rm -f <container_id> 

Stopping the container and then removing it will allow sending SIG_HUP signal to recipients. This will ensure that all the containers have enough time to clean up their tasks. This method is considered a good practice, avoiding unwanted errors. 

42. Will cloud overtake the use of Containerization? 

Docker containers are gaining popularity but at the same time, Cloud services are giving a good fight. In my personal opinion, Docker will never be replaced by Cloud. Using cloud services with containerization will definitely hype the game. Organizations need to take their requirements and dependencies into consideration into the picture and decide what’s best for them. Most of the companies have integrated Docker with the cloud. This way they can make the best out of both the technologies. 

43. How many containers can run per host? 

There can be as many containers as you wish per host. Docker does not put any restrictions on it. But you need to consider every container needs storage space, CPU and memory which the hardware needs to support. You also need to consider the application size. Containers are considered to be lightweight but very dependant on the host OS. 

44. Is it a good practice to run stateful applications on Docker? 

The concept behind stateful applications is that they store their data onto the local file system. You need to decide to move the application to another machine, retrieving data becomes painful. I honestly would not prefer running stateful applications on Docker. 

45. Suppose you have an application that has many dependant services. Will docker compose wait for the current container to be ready to move to the running of the next service? 

The answer is yes. Docker compose always runs in the dependency order. These dependencies are specifications like depends_on, links, volumes_from, etc. 

46. How will you monitor Docker in production? 

Docker provides functionalities like docker stats and docker events to monitor docker in production. Docker stats provides CPU and memory usage of the container. Docker events provide information about the activities taking place in the docker daemon. 

47. Is it a good practice to run Docker compose in production? 

Yes, using docker compose in production is the best practical application of docker compose. When you define applications with compose, you can use this compose definition in various production stages like CI, staging, testing, etc. 

48. What changes are expected in your docker compose file while moving it to production? 

These are the following changes you need make to your compose file before migrating your application to the production environment: 

• Remove volume bindings, so the code stays inside the container and cannot be changed from outside the container. 

• Binding to different ports on the host. 

• Specify a restart policy 

• Add extra services like log aggregator 

49. Have you used Kubernetes? If you have, which one would you prefer amongst Docker and Kubernetes? 

Be very honest in such questions. If you have used Kubernetes, talk about your experience with Kubernetes and Docker Swarm. Point out the key areas where you thought docker swarm was more efficient and vice versa. Have a look at this blog for understanding differences between Docker and Kubernetes. 

You Docker interview questions are not just limited to the workarounds of docker but also other similar tools. Hence be prepared with tools/technologies that give Docker competition. One such example is Kubernetes. 

50. Are you aware of load balancing across containers and hosts? How does it work? 

While using docker service with multiple containers across different hosts, you come across the need to load balance the incoming traffic. Load balancing and HAProxy is basically used to balance the incoming traffic across different available(healthy) containers. If one container crashes, another container should automatically start running and the traffic should be re-routed to this new running container. Load balancing and HAProxy works around this concept. 

This brings us to the end of the Docker Interview Questions article. With increasing business competition, companies have realized the importance of adapting and taking advantage of the changing market. Few things that kept them in the game were faster scaling of systems, better software delivery, adapting to new technologies, etc. That’s when docker swung into the picture and gave these companies boosting support to continue the race. 

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51.What are the advantages of using Docker container?  

Here, are a major advantage of using Docker.  

• Offers an efficient and easy initial set up 

• Allows you to describe your application lifecycle in detail 

• Simple configuration and interacts with Docker Compose. 

• Documentation provides every bit of information. 

52. What are the important features of Docker?  

Here are the essential features of Docker:  

• Easy Modeling 

• Version control 

• Placement/Affinity 

• Application Agility 

• Developer Productivity 

• Operational Efficiencies 

53. What are the main drawbacks of Docker? 

Some notable drawbacks of Docker are:  

• Doesn't provide a storage option 

• Offer a poor monitoring option. 

• No automatic rescheduling of inactive Nodes 

• Complicated automatic horizontal scaling set up 

54. What is Docker Engine?  

Docker daemon or Docker engine represents the server. The docker daemon and the clients should be run on the same or remote host, which can communicate through command-line client binary and full RESTful API.   

55. Explain Registries  

There are two types of registry is  

• Public Registry 

• Private Registry 

Docker's public registry is called Docker hub, which allows you to store images privately. In Docker hub, you can store millions of images.  

56. What is memory-swap flag?  

Memory-swap is a modified flag that only has meaning if- memory is also set. Swap allows the container to write express memory requirements to disk when the container has exhausted all the RAM which is available to it.  

57. How can you monitor the docker in production environments?  

Docker states and Docker Events are used to monitoring docker in the production environment.  

58. Explain Docker object labels  

Docker object labels is a method for applying metadata to docker objects including, images, containers, volumes, network, swam nodes, and services.  

59. Write a Docker file to create and copy a directory and built it using python modules?  

FROM pyhton:2.7-slim  WORKDIR /app  COPY . /app  docker build –tag  

60. Where the docker volumes are stored?  

You need to navigate:  

/var/lib/docker/volumes  

61. List out some important advanced docker commands  

Command	Description
docker info	Information Command
docker pull	Download an image
docker stats	Container information
Docker images	List of images downloaded

62. How does communication happen between Docker client and Docker Daemon?  

You can communicate between Docker client and Docker Daemon with the combination of Rest API, socket.IO, and TCP.  

63. Explain Implementation method of Continuous Integration(CI) and Continues Development (CD) in Docker?  

You need to do the following things:  

• Runs Jenkins on docker 

• You can run integration tests in Jenkins using docker-compose 

64. How to include code with copy/add or volumes?  

In docker file, we need to use COPY or ADD directive. This is useful to relocate code. However, we should use a volume if we want to make changes.  

65. Explain the process of scaling your Docker containers  

The Docker containers can be scaled to any level starting from a few hundred to even thousands or millions of containers. The only condition for this is that the containers need the memory and the OS at all times, and there should not be a constraint when the Docker is getting scaled. 

66. What is CNM?  

CNM stands for Container Networking Model. It is a standard or specification from Docker, Inc. that forms the basis of container networking in a Docker environment. This docker's approach provides container networking with support for multiple network drivers.  

67. Does Docker offer support for IPV6?  

Yes, Docker provides support IPv6. IPv6 networking is supported only on Docker daemons runs on Linux hosts. However, if you want to enable IPv6 support in the Docker daemon, you need to modify /etc/docker/daemon.json and set the ipv6 key to true.  

68. What are a different kind of volume mount types available in Docker?  

Bind mounts- It can be stored anywhere on the host system  

69. How to configure the default logging driver under Docker?  

To configure the Docker daemon to default to a specific logging driver. You need to set the value of log-driver to the name of the logging drive the daemon.jason.fie.  

70. Explain Docker Trusted Registry?  

Docker Trusted Registry is the enterprise-grade image storage toll for Docker. You should install it after your firewall so that you can securely manage the Docker images you use in your applications.  

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