Top 23 Docker Interview Questions and Answers

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11/Jul/2020 | 10 minutes to read

Best Answers to Docker Interview Questions

QFLES is listing some Essential Docker Interview Questions and Answers for Freshers and mid level of Experienced Professionals prepared by Industry Experts. All answers for these Docker interview questions are given based on standard documents and tried to explain in simple and easiest way.

1. What is the Docker?

Docker is an open-source Platform that provides the capability to develop, test, ship and run your application in an isolated environment. This isolated environment is called container. Docker enables you to keep your application separate from your Infrastructure and it reduce the delay between developing your application and deploy it in production.
Docker run the applications in an isolation with security, it allows you to run many containers on a host machine (A machine where many virtual machines are created and run). Containers are light weighted as they directly run within host machines's kernel rather than running on hypervisor (It's a system that creates and runs virtual machines). It means you can run more containers on this hardware than if you were using virtual machines. Even you can run docker containers on virtual machines as well.
Docker Certified Associate (DCA) exam is designed by Docker to validate the skills

2. What is the Docker Engine?

Docker Engine is an application which has client-server type architecture with these major components.

• Daemon Process ('dockerd' command) - It's a server which is a type of long running program.
• A Rest API - It provides an interface that programs or CLI use to talk to daemon process and to instruct it.
• A CLI (Command Line Interface) client ('docker' command)

3. What can you use Docker for?

You can use the docker for:

• Fast and consistent delivery of applications- means dockers allows developers to streamline their work by allowing them to work on local containers which provide their applications and services. Containers are best suited for CI (Continuous Integration) and CD (Continuous Delivery)
• Scaling and Responsive deployment - means docker's container based platform allows for highly portable workloads and docker containers can run on developers laptop, on physical or virtual machines, on cloud provider or in a hybrid environment.
• Running more workloads on same hardware - means docker provides cost effective alternative of hypervisor based virtual machines so that you can consume more compute capacity without changing the existing hardware. Docker is very light weight and fast.

4. Give some Docker Example scenarios.

As Docker provides the consistent and fast delivery of applications, these are the below examples for that.

• Developer write the code and can share that with their team members using dockers containers.
• Docker is used to push the code and execute automated or manual tests in a Test environment.
• Developers can fix the bugs in development environment and can push them to test environment for testing.
• Giving fixes or updated application to customer is easy as you can push the updated image to the Production.

5. Explain the Docker architecture.

Docker is based on client-server architecture. Docker client talks to docker daemon (A long running program) via REST API over a network interface or UNIX sockets. Docker client and daemon can run on same system or remote. Docker daemon is responsible for building, running and distributing the containers.

Source: Docker Architecture

• Docker Daemon - responsible to listen API calls and manage docker objects. To manage Docker services it can communicate with other docker daemon as well.
• Docker Client - is used to communicate with docker daemon via REST APIs and can communicate with more than one daemon.
• Docker Registries - Docker images are stored in Docker registry. Docker Hub is a public registry and a default location for docker images configured in docker.
• Docker Objects - When you are using docker means you are going to use many things like images, containers, volume, networks and many other, these are called docker objects.

6. What is DTR (Docker Trusted Registry)?

If you are using Docker Data Center (DDC), then Docker provide enterprise grade image storage solution called Docker Trusted Registry (DTR). DTR can be installed on virtual private network or on-premises so that you can store your images in a secure way and behind your firewall. DTR also provides a User Interface that can be accessed by authorized users to view and manage the repositories.

7. What are the common Docker objects?

With Docker you use many things like Images, Containers, Registries, Services, Volumes etc. These all are Docker objects.

8. Explain the Docker Images.

An Image is a read-only template with instructions that forms the basis of a container. It's an order collection of file-system changes. Image is based on another image with some customizations. for example, you can create an image that is based on 'ubuntu' image but you can install other application dependencies.
Images contains a set of run parameters (starting executable file) that will run when container starts.

9. Why Images are light weight, fast and small?

Docker allow you to create your own image using a docker file with simple instructions needed to create an image. Each instruction create layers in docker image. So when you make any changes in docker file then only changed layers are rebuilt not all, this is the reason that docker images are fast, small and light weight in compare to other virtualization systems.

10. Describe Docker Containers?

Containers are created from Images or you can say Container is a runnable instance of an image. When you build your image and deploy the application with all the dependencies then multiple containers can be instantiated. Each container is isolated from one another and from host machine as well. So A Container is defined by an image and other configurations provided when you start or create it.

11. Explain the underlying technology of Docker.

Docker is developed in Go Language and has capability to use many Linux kernel features to deliver the functionality. Docker is also relies on many other below technologies as well.

• Namespaces - Docker provides an isolated environment called container, that container is managed by namespaces. So Namespaces provide that isolation layer.
• Control groups - Control groups (cgroups) allow docker engine to share hardware resources to the containers.
• Union file systems - Building blocks for a container is managed by Union file system or UnionFS
• Container format - Namespaces, Control groups and UnionFS are combined into a wrapper by Docker Engine that's called Container Format.

Docker Quick Facts:

Docker is being popular with the time and here is what you should know about docker.

• Docker was launched by Solomon Hykes in 2013. Now Solomon Hykes is the CTO and chief Architect of Docker.
• Docker allows to build, ship, run and orchestrate the applications in an isolated environment.
• Docker is much faster than starting a virtual machine, but virtual machines are not obsolete yet.
• Docker Hub offers free options to host public repositories by developers and paid options for private repositories.
• Docker Desktop and Docker Compose usage has reduced the local development environment setup time that helps the developers to be productive.

Future of Docker:

Docker offers a quick way to build, develop, ship and orchestrate the distributed applications in isolated environment.
Docker is being used by many companies to make developer's process faster. Docker also provide automatic deployment management. So Mostly companies are adopting this containerized approach for their application development and deployment.
Docker also provide integration with many hundreds of tools like Bitbucket, Jenkins, Kubernetes, Ansible, Amazon EC2 etc. So there are lot of jobs in the market for Docker skills and Docker Professionals are being paid very good salary.

Some General Interview Questions for Docker:

1. How much will you rate your self in Docker?

When you attend an interview, Interviewer may ask you to rate your self in specific Technology like Docker, So It's depend on your knowledge and work experience in Docker.

2. What challenges did you face while working on Docker?

This question may be specific to your technology and completely depends on your past work experience. So you need to just explain the challenges you faced related to Docker in your Project.

3. What was your role in last Project related to Docker?

It's based on your role and responsibilities assigned to you and what functionality you implemented using Docker in your project. This question is generally asked in every interview.

4. How much experience do you have in Docker?

Here you can tell about your overall work experience on Docker.

5. Have you done any Docker Certification or Training?

It's depend on candidate like you have done any Docker training or certification. Certifications or trainings are not essential but good to have.