Interview Questions and Docker

Top 25 Docker Interview Questions with Answers

Posted in Interview Questions, Docker
Top 25 Docker Interview Questions with Answers

Released in 2013, Docker is a helpful tool for packing, shipping, and running applications within ‘containers’. As many big firms like Google, Amazon, and VMware have looked towards Docker as the go-to container technology, it is the right time to learn the tool and make a career out of it. If you wish to do so, below are the top 25 Docker interview questions that you need to know.

Q: Explain Docker Container?
A: A container is the basic unit of software which holds the code and all its dependencies, in order to make the application run smoothly, quickly and reliably from one computing ecosystem to another. A Docker container may be created using a Docker image. It is an executable package of the software, which holds everything that is required to run an application, which are system tools, libraries, code, runtime, and settings.

Q: Explain the components of Docker Architecture.
A: The components in Docker architecture are given below:

  • Host: This component holds the Docker Daemon, Images, and Containers. While the Docker Daemon establishes a link with the Registry, the Docker Images act as metadata for the applications which are held in the Docker Containers.
  • Client: The Docker Client component runs operations to set up communication with the Docker Host.
  • Registry: This Docker Component is used to store the Docker Images. Docker Hub and Docker Cloud are public registries, which can be utilized by anyone.

Q: Explain Docker Registry in detail.
A: The place where all Docker Images are stored is known as the Docker Registry. The Docker Hub is a public registry which is the default storage for these images. Another public registry is Docker Cloud. The Docker Hub is the most significant public storehouse of the image containers, consistently maintained by a large number of developers, along with many individual contributors.

Q: Briefly explain the Docker Container lifestyle.
A: The lifecycle of a Docker Container is:

  • Creation of the 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

Q: Name some important Docker commands
A: Below are some important Docker commands:

  • build: to build an image file for Docker
  • create: for creation of a new container
  • kill: to kill a container
  • dockerd: for launching Docker daemon
  • commit: for creating a new image from the container changes

Q: What are Namespaces in Docker.
A: Docker Namespaces is a technology providing isolated workspaces knows as container. Once a container is started, a set of namespaces is created for the said container. These namespaces provide a layer of seclusion for these containers as each container functions in a distinct namespace, with its access limited to the mentioned namespace.

Q: What is Docker Swarm?
A: Docker Swarm is a native tool used for clustering and scheduling Docker containers. Using Docker Swarm, developers and IT supervisor can easily establish and manage a bunch of nodes in Docker or a solitary Virtual System (VS).

Q: How to identify the status of a Docker Container?
A: To identify the status of a Docker container, one should run the command

docker ps-a

This command will furnish the list of all available Docker containers with the respective status on the host. From the list one can easily make out the intended container to check its status.

Q: What are the Docker Image and Docker Run Command?
A: A Docker Image is a group of files and an amalgamation of parameters which allow the creation of instances that run in distinct containers as isolated processes. An image is basically built using the instructions for a complete and executable version of an application, which relies on the host OS kernel. The Docker run command can be used to create the instance called container which can be run using the Docker image. When the Docker user runs an image, it becomes one or multiple instances of that container.

Q: State the functionalities and applications of Docker.
A: Below are some functionalities and applications of implementing Docker:

  • It makes the configuration simpler and provides ease of configuration at the infrastructure level
  • By helping the developer concentrate exclusively on business logic, it reduces development time and increases productivity
  • It amplifies the debugging capabilities which provide useful functionalities
  • It allows the isolation of the application
  • It reduces the use of multiple servers in the form of containerization
  • It facilitates rapid deployment at the OS level

Q: What is termed as Docker Objects?
A: Docker Images, Services and Containers are termed as Docker Objects.

  • Images: A read-only template with instructions for creating a Docker container
  • Containers: A runnable instance of an image
  • Services: It allows the scaling of containers across a variety of Docker Daemons, which all work together as a swarm.

Other Docker Objects include Networks and Volumes.

Q: Which is more suitable for Docker Container, Stateless or Stateful application?
A: Stateless applications should be preferred over a Stateful application for Docker Container. For instance, we can create one container from our application and take out the configurable state parameters from the app. Once it is one, we can run the same container with different parameters in production and other environments. Through Stateless application, we can reuse the same image in distinct scenarios. Also, it is easier to scale a Stateless application that a Stateful application when it comes to Docker Containers.

Q: Explain the use of Dockerfile.
A: Dockerfile contains a bunch of instructions which are passed on Docker to make possible the build process of images, which can automatically read these instructions. It can also be termed as a text document containing all the possible commands that a user may call on the command line for assembly of an image.

Q: Which networks all available by default in Docker?
A: The default networks available in Docker are:

  • bridge: Default network which the containers connect to if the network is not specified otherwise
  • none: Connects to a container-specific network stack lacking a network interface
  • host: Connects to the host’s network stack

Q: List the steps in a deploy process for Dockerized Apps stored In A Git Repo.
A: While the deploy process changes with your production environment, a basic deploy process will have the following:

  • Build an application through Docker Build located in the code directory
  • Perform the test of an image
  • Push the new image to registry docker
  • Notify the remote application server to get hold of the image from the registry and run it
  • Port swapping in HTTP proxy
  • Stop the older container

Q: Explain how Docker is different from other container technologies.
A: Docker is one of the latest container technologies and has emerged as one of the most popular. Built in the era of cloud, Docker comes with a whole lot of new features which were missing in older container technologies. One of the finest features of Docker is that it can run on any infrastructure, be it your home machine or the Cloud. Through Docker, more applications can now run on the old servers, and it also allows the process to package and ship programs. Docker also has a Container Hub that acts as a repository for containers, which are easy to download and use. Moreover, these containers can also be shared by your applications. It is also very well documented, which makes it better than other container technologies.

Q: If you were to exit the Docker Container, will you lose your data?
A: When a Docker Container is exited, no data loss occurs as all the data is written to the disk by the application for the sole purpose of preservation. This process is consistently repeated until and unless the container is unequivocally deleted. Moreover, the file system for the Docker container persists even after the Docker container is halted.

Q: How is Docker monitored in production?
A: To monitor Docker in production, tools such as Docker stats and Docker events are available. Through these tools, one can get reports on important statistics. Once Docker stats is called with a container ID, it returns the CPU and memory usage of the container. It is similar to the top command in Linux. On the other hand, Docker Events are commands to see a list of activities in process in Docker Daemon. Some of these events are attached, commit, rename, destroy, die and more. One also has the option to filter the events they are interested in.

Q: Shed some light on the workflow of Docker usage.
A: Below is a brief explanation of the workflow of Docker usage:

  • Since the Dockerfile is the source code of the image, everything starts with it
  • Once it is created, the Dockerfile is used to build the image of the container. This image is only the compiled version of the Dockerfile
  • This image is then redistributed using the registry, which is like a repository of images.
  • Further, the image can be used to run containers. A container, while it is running, is very similar to a VM without the hypervisor.

Q: Explain the disparity between the commands ‘Docker run’ and ‘Docker create’.
A: The primary difference between Docker run and Docker create is that if you use the latter, the container is created in a ‘stopped’ state. Also, Docker creates can be used to store and output container ID for use at a later time. The best way to do it is to use ‘docker run’ with —cidfile FILE_NAME as running it again won’t allow overwriting the file.

Q: What is Virtualisation?
A: Virtualisation, in its earlier days, was termed as a method of logically dividing mainframes to allow multiple applications to run concurrently. But as time progressed and the industry was able to allow for multiple operating systems to be run simultaneously on a single x86 based system, the meaning of virtualization changed considerably.

The net result? Virtualization allows the user to run two different OS on the same hardware. While the primary OS is the administrator, every guest OS goes through the processes such as bootstrapping, loading kernel and more. It is also perfect for security, as every guest OS may not be allowed the full access to host OS, as it may lead to the data breach.

There are primarily three types of virtualization:

  • Paravirtualization
  • Emulation
  • Container-based virtualization

Q: What is the difference between a registry and a repository?
A: The Docker Registry is a service for hosting and distributing images, whereas the Docker Hub is the default registry. On the other hand, the Docker Repository is the collection of Docker images which are related. That is, they have the same name but different tags.

Q: Can JSON be used instead of YAML for the compose file in Docker? If yes, how?
A: Yes, JSON can be used instead of YAML for the Docker compose file. To use the JSON file with composing, the filename should be specified as the following:

“docker-compose -f docker-compose.json up”

Q: Explain CMD and ENTRYPOINT in a Dockerfile?
A: In a Dockerfile, both CMD and ENTRYPOINT instructions define which command will be executed while running a container. For their cooperation, there are some rules, such as:

  • The Dockerfile should specify at least one command from CMD or ENTRYPOINT
  • While using the container as an executable, ENTRYPOINT needs to be defined
  • When running the container with an alternative argument, CMD will be overridden

Q: Explain the process to run an application inside a Linux Container using Docker
A: Below are the steps on how to run an application inside the Linux Container using Docker

  • Install and run Docker
  • Fetch Fedora 21 (Linux based OS) base image from the Docker hub
  • Load your application in the Docker base image
  • Run container using your new image in interactive mode
  • Check the containers in the system
  • Start or stop Docker container
  • Go inside a Docker container
  • Remove container or image

So this was the list of top 25 Docker interview questions. How many did you know the answer to? Let us know in the comment window below. Also, check out our best Docker tutorials, submitted and recommended by the community.

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Sagar Bhatia

Sagar Bhatia

Sagar is an engineering graduate and a technology lover and has been writing across various disciplines for over 5 years now. An avid gamer himself, he wishes to create a venture revolving around the e-sports domain in India. View all posts by the Author

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