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Docker Interview Questions

Docker was introduced in 2013 and reached the IT industry. With 8 billion container image downloads, it quickly became a huge success at the end of 2017. The growing demand for Docker showed an exponential increase in job openings. Take advantage of all the new vacancies with this article that lists the Top 50 Docker Interview Questions.

Introduction to Docker

Docker is a very popular and powerful open source containerization platform used to build, deploy, and run applications. Docker allows you to decouple the application / software from the underlying infrastructure.

What is a container?

A container is a standard software unit that contains dependencies so that applications can be deployed quickly and reliably on different computer platforms. Docker can be viewed as a large ship (stevedore) that carries huge boxes of products (containers). The Docker container does not require the installation of a separate operating system. Docker simply trusts the kernel resources and their functionality or uses them to allocate CPU and memory to the system).

Why learn Docker?

App development is much more than just writing code! They involve a lot of things behind the scenes, such as: B. Using multiple frameworks and architectures for each phase of their lifecycle, making the process more complex and demanding. Leveraging containerization helps developers simplify and speed up the application workflow efficiently, and gives them the freedom to develop with their own choice of technology and development environment.


All of these aspects form the core of DevOps. Therefore, it is even more important for any developer to understand them in order to improve productivity, accelerate development, and consider factors of application scalability and more efficient resource management.

Think of containers as a very lightweight pre-installed box with all the packages, dependencies, and software required for your application. Easily deploy them to production with minimal configuration changes. Many companies like PayPal, Spotify, Uber, etc. use Docker to simplify operations and bring infrastructure and security closer together to create more secure applications. Because containers are portable, they can be deployed on multiple platforms such as bare instances, virtual machines, Kubernetes platforms, etc., depending on the platform or scale requirements you want.

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

What are Docker images?

They are executable packages (contained in application code and dependencies, software packages, and so on) used to create containers. Docker images can be deployed in any Docker environment and containers can be rotated there to run the application. When you mention Docker images, your very next question will be “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.

What can you tell us about Docker Compose?

It is a YAML file that contains all the details about various services, networks, and volumes required to configure the Docker-based application. So Docker-Compose is used to create multiple containers, host them and communicate with each other. For the purpose of inter-container communication, ports are exposed from each individual container.

Can you tell us something about the Docker namespace?

A namespace is basically a Linux feature that secures the partition of operating system resources in a mutually exclusive manner. This forms the core concept of containerization, as namespaces introduce an isolation layer between containers. In Docker, namespaces ensure that containers are portable and do not affect the underlying host. Examples of namespace types currently supported by Docker: PID, Mount, User, Network, IPC.

What is the Docker command that lists the status of all Docker containers?

To get the status of all containers we run the following command: docker ps -a

Under what circumstances do you lose the data stored in a container?

The data in a container remains in it until you delete the container.

What is the Docker Image Registry?

In simple terms, a Docker image registry is an area where Docker images are stored. Instead of containerizing applications every time, a developer can use images stored in the registry directly. This image registry can be public or private, and Docker Hub is the most popular and well-known public registry available.

What does the docker info command do?

The command gets detailed information about Docker installed on the host system. The information can be, for example, the number of containers or images and what state they are running in, and the hardware specifications such as total memory allocated, processor speed, kernel version, etc.

Can you tell us what the purpose of the docker is to create, run, and launch commands?

With the up command to maintain a docker-compose (ideally always), we can also start or restart all networks, services and controllers

Docker Interview questions: Conclusion

DevOps technologies are growing exponentially. As systems become more and more distributed, developers have turned to containerization because they need to develop software faster and perform better maintenance. They also contribute to an easier and faster deployment and continuous integration process, which is why these technologies have seen tremendous growth. Docker is the best known and most popular tool to achieve the purpose of containerization and continuous integration / development and due to its great support for pipelines also for continuous deployment. With the growing ecosystem, Docker has proven useful for working across multiple use cases, making learning even more exciting.

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.

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.

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.

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.

Explain Docker Architecture?

Docker Architecture consists of a Docker Engine which is a client-server application with three major components: A server which is a type of long-running program called a daemon process (the docker command). A REST API which specifies interfaces that programs can use to talk to the daemon and instruct it what to do. A command line interface (CLI) client (the docker command). 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. Refer to this blog, to read more about Docker Architecture.

What is a Dockerfile?

Let’s start by giving a small explanation of Dockerfile and proceed by giving examples and commands to support your arguments. 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 interviewer does not just expect definitions, hence explain how to use a Dockerfile which comes with experience. Have a look at this tutorial to understand how Dockerfile works.

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.

What is Docker Swarm?

You are expected to have worked with Docker Swarm as it’s an important concept of Docker. 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.

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 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 interview questions 2021

It pays to go to an interview as well prepared as possible. In addition to having your resume and references, you should have a solid understanding of what the position entails and what the company is like. For example, when you research the company, you are showing the interviewer that you have a real interest in the organization and that you took the time to do your homework. However, you need to review the information about the position you are applying for. For example, if you’re applying for a position that relies heavily on Docker cloud computing, you should brush up on your Docker skills. To prepare you for the important job interview, we present a collection of Docker interview questions and answers for the basic, intermediate and experience levels. While there is no way to adequately cover the full spectrum of Docker container interview questions, you can find a good cross-section of the challenges you are likely to encounter. Once you are comfortable with these Docker interview questions, you will be better prepared to approach this interview with a higher level of confidence.

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