Docker Interview Questions & Answers (2024)

  Dockers is a software providing Platform As A Service ( PaaS ) helping to host the container through docker-engine behind the background. It isolates the containers providing a separate network namespace. It bundles all the dependencies with required software, libraries, and configuration files with bootable software provided as a docker image. Docker launches the containers with the help of these images. It allows containers to communicate with each other in well-defined channels.

In this article, we have compiled a list of 50 Docker interview questions and answers. These questions cover a wide range of topics, from the basics of Docker containers to more advanced concepts such as container orchestration, Docker Compose, and Docker networking. Whether you’re a beginner exploring containerization or an experienced professional (5 years or 10 years of Experience) preparing for a Docker-focused interview, this guide equips you with the insights and knowledge necessary to succeed in your Docker interview.

Docker containers are lightweight a single server or VM can run several containers simultaneously. It implements a high-level API to provide these lightweight containers that run processes in isolation. The magic of docker lies in making the process as containers (Operating system) by providing separate namespaces, security groups, and unique hostnames.

Docker comes up with popular tools such as docker-compose for defining and running, docker swarm providing the native functionality for docker containers and docker volume facilitating independent persistence of data.

Basic Docker Interview Questions

1. What is Docker ?

Docker is a containerization platform that allows to package an application with all its dependencies into one single entity as single container which can be easily deployed and run on any machine that supports docker. This makes it easier to devlop , test , deploy applications in different environments. It uses container technology to isolate processes and provide a lightweight, portable solution for application deployment.

2. What are the Features of Docker?

Docker features containerization for providing consistent deployment , using resources efficient shared kernel utilization, and provides seamless portability across environments. It enhances the security through isolation of containers supporting versioning and automated builds. It offers a rich number of pre-built images for streamlined application development and deployment.

3. What are the Pros and Cons of Docker?

Pros of Docker

• Portability: It enables consistent deployment across various environments.
• Resource Efficiency: Optimizing of resource usage with a shared kernel will be done effectively.
• Isolation: It provides security through isolation of process and file system.
• Automation: it supports automated builds and streamlining development workflow

Cons of Docker

• Learning Curve: Initial learning of the containerization concepts will bit new to understand.
• Additional Resources: Containers use some more resources compared to running applications directly on host.
• Security Concerns: Misconfigurations may lead to the security risks if not properly managed.
• Container Orchestration Complexity: Management of orchestration tools will be complex for larger-scale deployments.

4. Name and Explain the Components of Docker.

Docker consists of the following as a docker components :

• Docker Engine: Docker engine is the runtime that executes containers.
• Docker Images: are lightweight, readable templates containing executable packages that include the application with its dependencies.
• Docker Containers: are standardized , encapsulated environments that run applications/instances of Docker images.
• Docker Compose: It is a tool for defining and running multi-containered Docker applications.

5. Name and Explain the State of a Docker Container.

A state of a docker container directly influences its runtime characteristics and how it interacts with the underlying Operating system. A Docker container will be in one of these three states:

• A docker container will be in running state it is when actively executing.
• A container in paused state means that container is temporarily halted.
• The container will be in stopped state when it is inactive.

6. Can You tell What is the Functionality of a Hypervisor?

A Hypervisor is a virtualization software that helps in running multiple operating systems (Guest OS) on a single physical host system by providing an isolation between the virtual machines (VMs) and manages their resources.

7. Difference between Docker and Virtualization?

Docker uses containerization concept, which shares the host OS kernel for efficiency and speed whereas Virtualization involves running complete OS instances (Guest Operating systems) on a hypervisor, which may have more overhead on using resources. The following figure illustrate on both the Docker and Virtualization Architectures.

8. On What Circumstances Will You Lose Data Stored in a Container?

The Data in a container can be lost whenever the container is deleted, or if docker non-persistence storage ( Ephemeral storage ) is used without proper data management. To make the data persistent , it is recommended to use Docker volumes or volume binding ( volume mounts ) are recommended.

9. What is Docker Hub?

Docker Hub is container registry that serves as a centralized repository for Docker Image. It built for developers and open source contributors to find , use , share and download container images. Docker Hub can be used either host public repos that can be used for free, or docker private repo for teams and enterprises.

10. What Command Can You Run to Export a Docker Image As an Archive?

You can use this following command to export a Docker image as an archive:

docker save -o   <output_file_name>.tar    <image_name>

11. What Command Can Be Run to Import a Pre-Exported Docker Image Into Another Docker Host?

We can use this following command to import a pre-exported Docker image into another host:

docker load -i <input_file_name>.tar

12. Can a Paused Container Be Removed From Docker?

Yes, a paused container can be removed using the command with rm option:

docker rm <container_id>

13. How Do You get the Number Of Containers Running, Paused, and Stopped?

For obtaining the number of running, paused, and stopped containers in Docker you can use the command such as `docker ps -q` for knowing the list of running containers and `docker ps -q -f “status=paused”` for paused ones. Stopped containers can be counted using `docker ps -aq -f “status=exited”`. These commands will the provide the list of container IDs , and you have to can further process the output to get the counts programmatically like `docker ps -q | wc -l`.

The following command is used to know number of container are in running state:

docker ps -q | wc -l

The following command is used to know number of container are in paused state:

docker ps -aq -f  "status=paused" | wc -l

The following command is used to know number of containers are in stopped state:

docker ps -aq -f  "status=exited" | wc -l

14. How to Start, Stop, and Kill a Container?

In Docker to start , stop and kill a container we using start , stop and kill options on association with the docker command , the usage is given below.

To start the docker container use this command:

docker start < container_name >

To stop the docker container use this command:

docker stop < container_name >

To kill the docker container use this command:

docker kill < container_name >

Intermediate Docker Interview Questions

15. Can You Tell the Difference Between CMD and ENTRYPOINT?

CMD vs ENTRYPOINT

• CMD is used for setting default commands and arguments that will be executed at the start of runtime of the containers. It is oftenly overridden by providing command-line arguments during container startup.
• ENTRYPOINT configures a container to run as an executable, defining the command that has to be executed when the container starts. It is more strict than CMD and is oftenly used when the container should have to behave like an executable.

16. What are the Key Distinctions Between Daemon Level Logging and Container Level Logging in Docker?

Daemon Level Logging vs. Container Level Logging

• Daemon Level Logging: It involves with configuring the Docker daemon’s logging behavior globally. This will affect all containers that are running on the host. Configuration settings will be applied at the daemon level.
• Container Level Logging: This will be connected to logs specific to an individual container. They can be accessed using the docker logs <container_id> command , providing clear understanding regard the container’s runtime activities.

docker logs <container_id>

17. What Does the Docker Info Command Do?

docker info provides detailed information regarding the Docker system. It includes information such as the number of containers, images, storage driver that are used and much more. It’s a valuable command for gaining details on overview of the Docker environment.

18. Where are Docker Volumes Stored in Docker?

Docker volumes will be stored on the host machine in the directory /var/lib/docker/volumes . This ensures persistance of the data storage even if the container is removed.

19. Can You Tell the Differences Between a Docker Image and Layer?

A Docker Image can be considered as a snapshot of a file system and application dependencies. It is composed of multiple layers, where each layer will represent a set of filesystem changes. These Layers facilitate in efficient image creation and sharing common components among the images.

20. Can a Container Restart By Itself?

Yes, a container itself can restart automatically by setting up the –restart option during the creation period of time. For example using `docker run –restart` always. This will ensure that the container restarts irrespective of its exit status.

docker run --restart

21. Name the Essential Docker Commands and What They Do.

The essential Docker Commands are listed here:

• docker run:  is used to run the docker image as a docker container.
• docker ps: Docker os command will list all the running container in the docker.
• docker exec: It helps in execute the commands in a running container.
• docker stop: It will stops a container which is running in the docker.

22. What are Docker Object Labels?

Docker object labels are key-value mapping pair applied to the docker objects for better organizational and metadata purposes. For example, `docker run –label environment=production <image_name>` adds a label to a container.

docker run --label environment=production <image_name>

23. How Do You Check the Versions of Docker Client and Server?

Use `docker version`command to obtain the detailed information about the Docker client and server, including their respective version numbers.

docker version

24. Why is Docker System Prune Used? What Does It Do?

It is used for removal of unused data on inclusion of stopped containers, docker networks and dangling image. It helps in freeing up the disk space on cleaning unnecessary resources.

docker system prune

25. What is Docker Swarm?

Docker Swarm is an inherented native clustering that comes up with a orchestration solution for the Docker software. It helps in simplifying the management of a swarm of Docker nodes on allowing the seamless scaling of the applications across various multiple nodes within the network. It provides built-in load balancing and will ensure the high availability of containerized applications.

26. How Do You Scale Docker Containers Horizontally?

Horizontal scaling is achieved through replicating the services across multiple nodes. Tools like Docker Compose or Docker Swarm facilitate this process. For example, using `docker-compose up –scale web=3` command will replicates the “web” service to three instances, distributing the workload across them horizontally.

docker-compose up --scale web=3

27. What Is the Difference Between Docker Restart Policies “no”, “on-failure,” And “always”?

These restart policies will provide flexibility in managing the container behavior based on specific requirements. The restart policy “no” gives full control over restarts, “on-failure” handles irregular issues, and “always” will ensures the constant availability. Choose the appropriate policy based over the nature and importance of that particular containerized application.

Restart Policy	Description
“no”	No automatic restart will be done. The container will not restart automatically, in any case of the exit status. It will be suitable for the scenarios where manual intervention is preferred or when the container is perfered for a one-time execution. Example: docker run –restart no my_container
“on-failure”	It will restarts the container only if it exited with a non-zero status. This policy is useful when anticipating occasional failures and wanting the container to automatically recover from such failures. Example: docker run –restart on-failure:3 my_container ( restarts up to 3 times on failure )
“always”	The container will restart regardless of its exit status. It will be useful in critical services that should be always running and ensuring continuous operation even if the container exits. Example: docker run –restart always my_container

28. How Do You Inspect the Metadata of a Docker Image?

By using the `docker inspect <image_name>` command , you can examine into detailed metadata about the Docker image. This contains the information regarding labels, layers, and the configuration settings.

docker inspect <image_name>

29. How Do You Limit the CPU and Memory Usage of a Docker Container?

Om using the –cpus option you can set the CPU limits and with -m option you can set memory limits. The following example illustrates usuage of CPU and memory for a docker container.

docker run --cpus=3 -m  1024M   <image_name>

30. What are the Differences Between Docker Community Edition (CE) and Docker Enterprise Edition (EE)?

Usage of Docker Community Edition will be peferable for individuals and small-scale projects, It provides the essential features of containerization for free. On the other hand, Docker Enterprise Edition deals in providing the enterprise needs with advanced features and support for the large-scale projects in production environments. The choice between these two will depends on the scale, requirements, and support needed for the Docker deployment.

Feature	Docker Community Edition (CE)	Docker Enterprise Edition (EE)
Pricing	Free for individual use and will be suitable for development and testing.	It requires the subscription and offers advanced features for the production environments.
Support	Provides community support through forums and community resources	Provides enterprise-grade level support through service-level agreements (SLAs).
Certification	Limited certification for specific platforms.	These are Certified and tested for a wide range of operating systems, cloud providers, and plugins.
Security	Contains Basic security features	Enhanced security with additional features like image signing and scanning are available.
Orchestration Tools	Has only Basic orchestration capabilities	Advanced orchestration tools like Docker Swarm and Kubernetes for large-scale deployments are available.
Image and Container Management	Core image and container management features.	Additional management tools and features, including role-based access control (RBAC).
Environment Support	Ideal for development and small-scale deployments.	Tailored for large-scale enterprise environments with optimized performance.
Networking	Basic networking capabilities.	Advanced networking features, including multi-host networking and DNS.
Plugins and Extensions	Plugins and extensions are limited in the community edition.	A wide range support of certified plugins and extensions are available for various integrations.

Advanced Docker Interview Questions

31. What Is the Purpose of the “docker checkpoint” Command?

The “docker checkpoint” command is vital for the creation of snapshots of a running container’s state , including its file system and the memory. It is particularly useful for experimental mode of scenarios such as debugging or migration.
For example to checkpoint a container named “my_container,” the command would be:

docker checkpoint create my_container checkpoint_name

32. Can We Use JSON Instead Of YAML While Developing a Docker-Compose File in Docker?

Yes, Docker Compose has support for both YAML and JSON formats for defining the configuration of services. While YAML is more commonly used due to its readability and clearness , you can also use JSON as an alternative. The choice between of two will depends on personal preference on requirements of the projects. To use JSON, simply try on creating a `docker-compose.json` file instead of a `docker-compose.yml` file, and define your services in JSON format.

33. Describe the Lifecycle of a Docker Container.

In the lifecycle of docker container , it goes through the following states:

• Creation: On using `docker run` container is created.
• Execution: In this state specific required commands are executed inside the container.
• Pausing/Unpausing: These are optional states for temporarily halting a container.
• Stopping: On using the `docker stop` command container is gracefully halted in this state.
• Removal: Using `docker rm` command the container can be deleted.
• Restarting: With the `docker restart` command containers can be restarted.

34. How Will You Ensure Container 1 Runs Before Container 2 While Using docker-compose?

In Docker Compose, the order of the services startup is determined by their dependencies. By specifying container dependencies with the “depends_on” key in the docker-compose.yml file, you can ensure the desired startup order.
A sample example on usage of depends_on provided here , In this even though container1 is listed firsted because of the depends_on key container2 will be startup and then container1 will be queue order.

services:
  container1:
    depends_on:
     - container2
   ...
  container2:
  ....

35. How Does Docker Handle Container Isolation and Security?

Docker uses containerization concept to isolate the processes by imiting their access to the host system. Features like namespaces and cgroups provides the resource isolation for the containers and Docker Security Scanning helps in identifying the vulnerabilities in images.

36. How do the Docker Daemon and the Docker Client Communicate With Each Other?

The Docker daemon and client communicate on using REST APIs. The Docker Clinet will send the commands to the daemon using the API, and the daemon will execute those commands on managing containers, images, and other Docker objects.

37. Can You Implement Continuous Development (CD) and Continuous Integration (CI) in Docker?

Yes, Docker is an integral part to the CICD pipelines. Developers can use Docker images for the consistent environments, and CI tools can perform the automate testing and deployment using Docker containers for ensure reproducibility.

38. Is it a Good Practice to Run Stateful Applications on Docker?

Docker is primarily designed for the stateless applications. On using Docker volumes or persistent storage stateful applications can be runnable but it’s crucial to carefully manage data persistence and backup to avoid data loss.

39. What Is the Purpose of Docker Secrets?

Docker secrets are used mostly to securely store sensitive information, such as passwords or API keys in Docker swarm. Secrets are encrypted and can only be accessible by services that have explicit permission to use them.
Example:

docker secret create db_password mysecretpassword

40. How Do You Create a Multi-stage Build in Docker?

A multi-stage build in Docker involves with using multiple “FROM” instructions in a Dockerfile. Each “FROM” instruction will begin a new stage, allowing you to build and copy the artifacts from previous stages for reducing the final image size.
Example of Dockerfile with multi-stage build:

FROM builder as build
# Build stage

FROM alpine
# Final stage
COPY --from=build /app /app

41. How Do You Update a Docker Container Without Losing Data?

To update a Docker container without losing data, you can try on using a combination of Docker volumes or bind mounts to make the data persistant outside the container. When updating, create a new container with the updated image and then link it to the existing data volume.

42. How Do You Manage Network Connectivity Between Docker Containers And the Host Machine?

Docker provides several ways to manage network connectivity between containers and the host machine. The choice of networking options depends on the specific requirements of the application and the desired level of isolation.

• Bridge Networks: Bridge networks are the default networks , these are created when a Docker daemon starts. Through this network containers on the same bridge network can communicate with each other.
• Host Networks: In this containers will share the host’s network namespace. so that containers can directly use the host machine’s network interfaces.
• Custom Networks: Custom bridge networks can used to create isolate containers and control their communication. Containers on custom networks can communicate with each other with the help of container names as hostnames.
• Overlay Networks (Swarm Mode): Overlay networks are used in Docker Swarm mode for communication between the services that running on different nodes. They provide multi-host networking for orchestration of containers.
• Macvlan Networks: Macvlan networks will allow containers to have their own MAC addresses on the physical network, providing them with the direct access to the host’s network.

Examples for creating the networks are listed here:

# Create a bridge network
docker network create my_bridge_network

# To Run a container with host network
docker run --name container1 --network host -d my_image

# To create a custom bridge network
docker network create my_custom_network

# To create an overlay network
docker network create --driver overlay my_overlay_network

43. How Do You Debug Issues in a Docker Container?

Debugging techniques will provide a comprehensive approach for troubleshoot and to the resolve issues within Docker containers. Depending on the nature of the problem on following these guided commands you can understands the details of the container’s behavior.

• Container Logs: On running this `docker logs <container_id>` command you can view the standard output and error logs.

docker logs <container_id>

• Interactive Shell: Helps in accessing the interactive shell inside the container for detailing.

docker exec -it <container_id> /bin/bash

• Inspect Container Details: helps in retrieving the detailed information about the container.

docker inspect <container_id>

• Process Listing: Provide the list of running processes inside the container.

docker exec -it <container_id> ps aux

• Network Troubleshooting: Check network connectivity of the container through the hostname

docker exec -it <container_id> ping <hostname>

• Resource Utilization: Helps in monitoring the CPU and memory usage.

docker stats <container_id>

44. How Does Docker Handle Service Discovery in Swarm Mode?

In Docker Swarm Mode, service discovery is automatically handled through maintaining an internal DNS Service that automatically assigns DNS names to the containers on enabling easy service discovery within the swarm.

Scenario-Based Docker Interview Questions

45. Which Is the Preferred Method for Removing a Container: Using the “docker stop” Command Followed By the “docker rm” Command, or Simply Using the “docker rm” Command By Itself?

The recommend approach for the container removal is to use the combined “docker stop” and “docker rm” commands, as it makes sure a safely stopping of the container before removing it. This two-step process will helpful in avoiding potential issues related to the active processes within the container. However, if you are sure about that the container is not running, then you can go for using the “docker rm” command alone to remove it.

46. Suppose You Have 3 Containers Running, and Out Of These, You Wish to Access One Of Them. How Do You Access a Running Container?

To access the running container, you can use the “docker exec” command. Here is the general syntax:

docker exec -it <container_id_or_name> /bin/bash

The 'docker exec' command is useful for the execute of a command inside a running container. It provides a interactive session for pseudo terminal -TTY with option -it, allowing you to interact with the container <container_id_or_name>. Try on replacing this with the actual ID or name of the container that you want to access. The /bin/bash program specifies the command that to be executed in the container. In this case, it starts with interactive Bash shell, but you can also replace it with the appropriate required command for your needs. This will opens up the bash shell inside the specified container on enabling you to run commands, inspecting the container’s filesystem, or troubleshoot. After once you are done with it, you can exit from the shell and it won’t affect the container’s running state.

47. Considering a Server With 16 GB RAM and a Quad-core CPU, What Factors Determine the Maximum Number Of Containers You Can Run on the Host for a Microservices App?

The maximum number of containers a host can support will mainly depends on the available resources like RAM and CPU cores. With the 16 GB of RAM, on assuming each container utilizes 512 MB efficiently, you could potentially run on around nearly 32 containers. However, cautious allocation, monitoring tools, and the container orchestration are essential factors for optimal resource utilization and scalability.

48. How Will You Monitor Docker in Production?

For monitoring the docker in production environment, generally utilize tools like Docker Stats, cAdvisor, and the Prometheous for real-time insights retriving for container performance. Implementation of centralized logging with solutions such as ELK Stack or Splunk to track container logs are used in common. Additionally, consideration of using container orchestration platforms like Kubernetes or Docker Swarm, which offer better built-in monitoring and scaling capabilities. Regularly review metrics such as CPU, memory usage, and network activity to ensure optimal performance and address potential issues promptly.

49. Are You Aware of Load Balancing Across Containers And Hosts? How Does It Work?

Load Balancing across the containers and hosts is critical for distributing traffic efficiently in the containerized environment. Container orchestration tools like Kubernetes or Docker Swarm employes load balancers to evenly distribute the requests among the container instances or nodes. This will enhance the scalability, fault tolerance, and resource utilization by directing the traffic to healthy containers or hosts. The Load balancing plays a vital role in maintaining the stability and optimization on overall system performance in both the dynamic and scalable containerized applications.

50. How Do You Share Data Between Containers in Docker?

In Docker, you can share data between the containers on using volumes or by utilizing the `–volumes-from` option. Volumes will provide a persistent and the shared storage mechanism, allowing the data to be accessed and modified by multiple containers. Alternatively, the –volumes-from option allows the container to access the volumes of another container. This provides the seamless data sharing and collaboration between the containers, through facilitating the communication and coordination in complex multi-container setups.

51. Can a Container Restart By Itself?

Yes, The containers can be configured to restart by themselves automatically. Docker provides a restart policy that allowing you to define the container behavior when it exits. On using the options such as “–restart always”.
you can instruct Docker to restart the container automatically, in any case of the exit status. This will be useful for ensuring the continuous availability of critical services within the containerized environment. Other restart policies include “unless-stopped” and “on-failure” offering the flexibility in handling the container lifecycle events.

52. How Do You Perform a Live Migration Of Docker Containers Between Hosts?

For performing a live migration of docker containers between hosts can be achieved through using container orchestration tools like Docker Swarm or Kubernetes. These tools will manage the seamless containers movements across the hosts on ensuring minimal downtime. Through Utilizing the features such as Swarm’s `docker service update` or Kubernetes’ `kubectl drain` and `kubectl uncordon` commands, you can initiate the live migrations by allowing containers to be moved to different hosts while maintaining the availability of application.