Docker Interview Questions

Docker Interview Questions and Answers

As the IT sector witnesses a boom, people who are working in the sector are determined to upskill themselves. They need to keep up with the creation, deployment, and running of applications easier.

 Docker makes such a simplification possible with the help of containers. The containers let a developer package up an application with all the parts it needs. Then, they deploy all the parts as a package.

Docker is basically an open-source tool that uses the Linux Kernel features, such as control groups to create containers at the top of an operating engine. It has been developed by DotCloud. Docker is heavily dependent on cloud technology.

Knowledge of Docker is essential for those who are aspiring for a job in the IT sector. People who know how to use Docker get a lucrative salary. So, it is a good idea to know a few common questions recruiters ask the employees who are familiar with Docker.

Frequently Asked Docker Interview Questions

 1.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.

2.What do you understand about Docker?

Docker may be described as a containerization platform. It packages an application and all its dependencies together in the form of containers. The main aim of packaging everything in containers is to ensure that the application works seamlessly in any environment.

 Basically, a Docker container wraps anything that may be installed in a server. It guarantees that the software will always work the same way, regardless of the environment in which it works.

3.What is Docker Hub?

The Docker Hub is the largest public repository of image containers in the world.  It contains more than a million container images, sourced from individual developers, open-source projects, and commercial software vendors.

 All the Docker Hub users may share their images at will. They may also download predefined base images that may be used as a starting point for any containerized project.

The Docker images create Docker containers. The registry in which the Docker images live is known as the Docker Hub, The users may pick up images from Docker Hub and use them to create customized containers.

4.What do you mean by a Docker container?

Basically, Docker containers are runtime instances of Docker images. They are not restricted to any particular infrastructure. On the contrary, they run on any infrastructure and any cloud. The Docker containers share the kernel with other containers, running as isolated processes on the host operating system. They include an application and all its dependencies.

The Docker images are read-only files. On the other hand, Docker containers are live, executable content. It is possible for users to interact with the Docker containers.

5.What are Docker images?

A Docker image is the source of a Docker container. It is used to create containers. The moment a user runs a Docker image, an instance of a container is created. The Docker image might be deployed to any Docker environment.

A Docker Image contains executable application source code. It also contains libraries and tools that the application source code needs to run as a container. It is possible to create several Docker images from a single base image. All the Docker images will share the common features of their stock in that case.

Docker images are constituted of layers. Every layer corresponds to a version of the image. The moment a developer introduces any changes to a layer, a new top layer is created. This new top layer replaces the earlier top layer as the current version of the image.

6.What is the meaning of Docker Namespace?

Namespace adds a layer of isolation in containers. Docker provides different namespaces to stay portable and avoid affecting the underlying host system. A namespace is one of the most important features of Linux. User, Network and Mount are a few classic types of Namespace supported by Docker.

7.Describe the life cycle of Docker containers?

This is one of the most commonly asked questions during the interview. The life cycle of a Docker container is as follows-

  • Creating a container
  • Running the container
  • Pausing the container
  •  Unpausing the container
  • Starting the container
  •  Stopping the container
  • Restarting the container
  • Killing the container
  • Destroying the container

8.What is Docker Machine?

Docker Machine may be described as a tool that allows the users to install Docker Engine on virtual hosts. Then, the hosts can be managed with the help of Docker-machine commands.

9.What do you mean by Docker Swarm?

Docker Swarm is an important concept of Docker. It is a native clustering of Docker, that turns a pool of Docker hosts into a virtual and single Docker host. Any tool which already communicates with a Docker daemon may use Swarm to scale transparently to several hosts. It serves the standard Docker API.

10.What do you understand by Docker Compose?

Docker Compose is a YAML file. It contains details about the volumes, networks, and services for setting up the Docker application. So, people may use Docker Compose for creating separate containers and getting them to communicate with one another. It also hosts the different containers. Each container exposes a port for communicating with the other containers.

11.Why do you think so many people use Dockers?

The reasons behind the popularity of Dockers are as follows-

  • Better Portability-Docker containers run across any desktop, cloud environment, and data center without any modification. It proves they have better and more seamless portability.
  • Automated Container Creation- On the basis of application source code, Docker can automatically build a container.
  • Lightweight And Has Granular Updates– Only one process can run in each Docker container. As a result, it becomes possible to build an application that continues running even after one of its parts has been taken down for repair.
  • Shared Container Libraries-Developers may access an open-source registry that contains hundreds of user-contributed containers.
  • Container Versioning- Docker may track versions of a container image and roll back to the earlier versions. It may also trace who built a version and how. Moreover, it may even upload only the deltas between an existing and a new container.
  • Container Reuse- Existing containers may also be used as base images. They are essentially like templates for building new templates.

12.Why are containers used?

Containers offer all the advantages of VMS, including cost-effective scalability, and application isolation. However, they also contain an additional level of abstraction at the OS level. As a result, it has the following advantages-

  • Greater Resource efficiency- Containers allow the users to run many more copies of the same application on the same hardware than VMS. As a result, cloud spending is reduced.
  • Lighter Weight- Containers do not carry the payload of an entire OS instance. They only include the OS processes needed for executing the code.
  • Improved Developer Productivity-Containers are easier to deploy and restart, as compared to VMS. They are also faster than VMS. As a result, they are more suitable for development teams that adopt Agile and DevOps practices.

13.Where do you think Docker is being used?

Docker is being used in the following areas-

  • Code Pipeline Management- Different systems are being used for production and development. While traveling from development to testing and production, the code goes through a difference in the environment. Docker helps in the maintenance of code pipeline consistency.
  • Simplifying Configuration-Docker allows the users to put their configuration and environment into code and deploy it.
  • Multi-tenancy- Docker allows the users to have multi-tenant applications, evading redundancy and the deployments and codes.
  • Developer Productivity- Thousands of users use Docker for development. It brings the users closer to production. Also, the development environment is constructed faster.
  • Application Isolation-Basically, containers are applications wrapped together with all the dependencies. So, the apps have a tendency to become isolated. They may work on any hardware which supports Docker on their own.
  • Rapid Deployment-Docker reduces the need to boost the entire operating system from the scratch. As a result, it reduces the deployment time.
  • Debugging capability- Docker supports a variety of debugging tools that are not specific to containers. However, these tools work well with containers.

14.How does Docker stand out from all the other containerization methods?

Docker makes it easier for the developers to create ready-to-run containerized applications very fast. As a result,  it simplifies the process of deploying and managing applications. The users are free to share containers with the applications.

The Docker containers are easy to deploy in any cloud platform. It may get more and more applications running on the same hardware as compared to any other technology.

15.Could you name a few platforms on which Docker works?

Docker can be used in productions with Cloud platforms with the following positions-

  • Amazon EC2
  • Google Computer Engine
  • Microsoft Azure
  • Rackspace

It also runs on different Linux administration, like-

  • ArchLinux
  • Gentoo

16. Is it possible for a container to restart by itself?

 No, it is not possible for a container to restart by itself. The flag restart is set to false by default.

17. Is it possible to remove a paused container from Docker?

It is not possible to remove a paused container from Docker. The container needs to be in the stopped state before it is removed.

18. How far can the Docker containers scale? Is there any requirement for the same?

Platform providers like dotCloud and Heroku work on container technology. Containers may be scaled to thousands or even millions of them running in parallel.

Containers require memory and OS at all times. They also need a way to use this memory efficiently when scaled.

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

There are six possible states a container can be given at any point of time. They are as follows-

  1. Created
  2. Running
  3. Paused
  4.  Restarted
  5. Exited
  6. Dead

20.Do you think that it is a good practice to run stateful applications on Docker? 

The stateful applications store data onto the local file system. The users need to decide to move the application to another machine when it becomes difficult to retrieve the data. So, many veteran Docker users claim that they do not prefer to run stateful applications in Docker.

21. How is it possible to monitor Docker in production?

Docker provides different functionalities like Docker events and Docker stats to monitor Docker in production. Docker events provide information about the activities that take place in the Docker daemon. On the other hand, Docker stats provide CPU and memory usage of the container.

22.What changes do you expect in your Docker compose file while moving it to production?

It is necessary to introduce several changes to the Docker compose file before migrating the application to the production environment. The changes are as follows-

  • Specifying a restart policy
  • Adding extra services such as log aggregator
  • Removing volume bindings so that the code stays within the container. The code cannot be changed from outside the container.
  • Binding to various ports on the host

23.Do you support the system of running Docker compose in production?

The most practical application of Docker compose is to use it in production. It is possible for users to use Docker compose in different stages of production like testing and staging.

24.Do you think that you will lose your data when a Docker container exits?

When a Docker container exits, there is no chance of data loss. All the data written by the system gets preserved on the disk until the user deletes that particular container. The file system for the container continues even after the container stops or exits.

25.What are the major components of Docker Architecture?

Docker architecture has four major components. They are as follows-

  • Daemon Client
  • Host
  • Registry

Let’s understand each of the components-

  1. Daemon- It accepts Docker API requests. Docker daemon also manages Docker objects like containers, images, volumes, and networks. Also referred to as “docked”, Docker Daemon may also communicate with other daemons to manage Docker services.
  2. Client- Docker client sends the Docker commands to Docked Docked actually executes them, using Docker API. It may communicate with more than one daemon.
  3. Host- It refers to the physical host on which Docker Daemon runs. Docker images and containers are also created there.
  4. Registry-Docker registry hosts the Docker images, It is used to push and pull the Docker images from the configured registry.

26. What do you mean by Dockerfile?

A Docker file is a text document that contains all the commands a user may use to assemble an image.


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There is no setlist of Docker interview questions and their answers that will help the aspirants crack the interview. Only in-depth knowledge of Docker and hands-on experience in the relevant domain will help the candidates answer the questions smoothly.