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Docker Interview Questions

These questions are a good mix of basic and intermediate concepts, which will test your knowledge of Docker in both practical and theoretical aspects.

Docker Interview Questions

1. What is Docker?

Answer: Docker is an open-source platform that automates the deployment, scaling, and management of applications. It uses containers, which are lightweight, portable, and self-sufficient units that package an application and its dependencies. Docker helps in solving the "it works on my machine" problem by ensuring consistency across different environments.

2. What is a Docker container?

Answer: A Docker container is a lightweight, standalone, executable package that includes everything needed to run a piece of software: the code, runtime, system tools, libraries, and settings. Containers are isolated from the host system, providing consistency across multiple environments.

3. What is the difference between a Docker image and a container?

Answer:

  • Docker Image: A Docker image is a read-only template used to create containers. It contains the application code, dependencies, and the necessary runtime environment.
  • Docker Container: A container is a running instance of an image. It is the live execution of a Docker image that can be started, stopped, and managed.

 

4. What is Dockerfile?

Answer: A Dockerfile is a text file that contains a series of instructions on how to build a Docker image. It defines the base image, the files to copy into the image, the commands to run, and other settings like exposed ports and environment variables.

5. Explain Docker Compose.

Answer: Docker Compose is a tool for defining and running multi-container Docker applications. Using a docker-compose.yml file, you can define services, networks, and volumes, and then use a single command (docker-compose up) to bring up the entire application stack with all its components (e.g., frontend, backend, database).

6. What is the difference between CMD and ENTRYPOINT in a Dockerfile?

Answer:

  • CMD: Specifies the default command to run when a container starts. If a command is provided when running the container, it will override the CMD.
  • ENTRYPOINT: Defines the main command that should always be executed when the container starts. You can provide additional arguments to ENTRYPOINT, but it can’t be overridden like CMD.

 

7. What is the purpose of docker volumes?

Answer: Docker volumes are used to persist data created or used by Docker containers. Volumes are independent of the container lifecycle, so data stored in a volume persists even if the container is removed. Volumes also allow sharing data between multiple containers.

8. What is the difference between EXPOSE and VOLUME in Docker?

Answer:

  • EXPOSE: This instruction is used to indicate which ports the container will listen on during runtime. It doesn't publish the port but acts as documentation.
  • VOLUME: This defines a mount point with a specified path and indicates that data stored in this location should be persisted or shared between containers.

 

9. What are Docker networks?

Answer: Docker networks allow containers to communicate with each other. Docker supports several types of networks (e.g., bridge, host, overlay, none), and they determine how containers are connected and communicate both within the same host and across different hosts.

10. What is the difference between docker run and docker exec?

Answer:

  • docker run: Creates and starts a container from a specified image. It is used to run a new container.
  • docker exec: Executes a command inside a running container. It's useful for interacting with a container that is already running.

 

11. How do you monitor Docker containers?

Answer: You can monitor Docker containers using:

  • docker stats: Displays resource usage stats for running containers.
  • Logs (using docker logs <container_id>).
  • Third-party monitoring tools like Prometheus, Grafana, Datadog, etc.

 

12. What is a Docker registry?

Answer: A Docker registry is a service that stores Docker images. The most commonly used registry is Docker Hub, but you can set up private registries as well (e.g., using Docker Registry or cloud providers like AWS ECR, Google Container Registry).

13. What are the benefits of using Docker?

Answer:

  • Portability: Containers can run anywhere: on your local machine, on-premises, or in the cloud.
  • Consistency: Ensures the same environment for development, testing, and production.
  • Isolation: Containers run in isolation, which improves security and avoids conflicts between different applications.
  • Scalability: Docker simplifies the process of scaling applications.

 

14. What is the Docker "bridge" network?

Answer: The bridge network is the default network mode when no other network is specified. It allows containers to communicate with each other on the same Docker host, and it connects them to the host system's network through a NAT (Network Address Translation) interface.

15. Explain Docker Swarm.

Answer: Docker Swarm is Docker’s native clustering and orchestration tool. It allows you to deploy and manage a cluster of Docker nodes as a single virtual system. It helps with scaling applications, load balancing, and high availability across multiple machines.

16. How do you scale Docker containers?

Answer: Scaling can be achieved by running multiple instances of the same container. In Docker Swarm or Kubernetes, this can be done easily by specifying the desired number of replicas. You can also scale using Docker Compose by adjusting the number of replicas for a service.

17. How can you share data between two Docker containers?

Answer: Data can be shared between Docker containers using Docker volumes or through shared networks. Volumes provide a persistent storage mechanism, while networks allow communication between containers.

18. What is Docker Hub and how is it used?

Answer: Docker Hub is a public Docker registry where you can find and share Docker images. It allows you to pull images from public repositories or push your own images for others to use. Docker Hub also allows you to create private repositories for your images.

19. How can you handle secrets and sensitive information in Docker?

Answer: Secrets can be managed in Docker using environment variables, Docker Secrets (in Docker Swarm), or external tools like Vault. Avoid storing secrets directly in Dockerfiles or image layers to maintain security.

20. How do you troubleshoot Docker containers?

Answer: To troubleshoot Docker containers:

  • Use docker logs <container_id> to view logs.
  • Use docker inspect <container_id> to get detailed information about the container's configuration.
  • Check the resource usage with docker stats.
  • Use docker exec to interact with the running container and run debugging tools.
  • Ensure that the network and volume configurations are correct.