Businesses want to develop projects quickly to boost their overall growth. With the increasing demand for efficient software development tools and methods, this notion is strengthening more and more. As a result, various modern and highly efficient tools to support business functions have emerged today.
Docker is one such tool that can help businesses to develop, test, and deploy applications faster. In general, it is useful to pack, ship, and run applications in containers. Docker is an open-source containerization platform that helps you in developing, installing, and running software applications. It helps you to isolate the software from the infrastructure that supports it.
The year 2013 witnessed the release of Docker, and it had over 8 billion container image downloads by the end of 2017. This has resulted in a major increase in the demand for Docker-trained professionals.
If you are applying for a job role that relies heavily on Docker, you need to brush up on your Docker skills. Well, we have compiled a list of the frequently asked Docker interview questions for developers of all levels. Going through these Docker interview questions will help you refresh your knowledge of various Docker concepts and thus prepare you better for your upcoming interview.
But before we start discussing the Docker interview questions and answers, let's understand why one should learn Docker.
Why Should You Learn Docker?
There's a lot more to software development than just writing code. The software development process entails a lot of behind-the-scenes work, such as selecting appropriate frameworks and technologies at different stages of the software development life cycle (SDLC), which eventually makes the overall process more complicated.
Containerization offered by Docker allows developers to simplify and accelerate application development workflow while having the flexibility to choose technologies and development environments. This makes it necessary for developers to understand containerization in order to increase efficiency and speed up the development process.
Consider containers as pre-installed boxes containing all the packages and dependencies of applications that can be readily deployed in a production environment with minimum modifications.
Many businesses, such as PayPal and Uber, use Docker to streamline operations and put infrastructure and security together to create more stable applications. Containers can be installed on a variety of systems, including bare metal, virtual machines, and the Kubernetes network, depending on the size or preferred platform.
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Top Docker Interview Questions and Answers
Now, let us begin with some simple Docker interview questions and move to more difficult questions. We have divided the Docker interview questions into three levels; Basic Docker interview questions, intermediate Docker interview questions, and advanced Docker interview questions.
Basic-Level Docker Interview Questions
1. What do you understand by the term Docker?
Answer: Docker can be described as a containerization framework that bundles all of our applications into a single package, allowing us to run them in any environment. This means that our application can run smoothly in any environment, making it simple to build a production-ready app.
This framework bundles the appropriate software in a file system that includes everything required to run the code, including the runtime, libraries, and system resources.
Containerization technology, such as Docker, uses the same OS kernel as the machine on which it runs, thus making it very fast. This means we only need to run Docker once, and the rest of the process will run smoothly because the operating system of our system is already running.
2. What do you understand by virtualization?
Answer: Virtualization is the process of creating a software-based, simulated version of something. A single physical hardware device is used to build these simulated versions or environments. Virtualization allows you to divide a single system into several parts that function as separate, independent systems. This form of splitting is possible thanks to a program called a hypervisor. We can refer to the Hypervisor's environment as a virtual machine.
3. Name the latest version of Docker.
Answer: The latest version of Docker is 20.10.7, released on June 2, 2021.
4. What do you understand by containerization?
Answer: Because of dependencies, code built on one computer may not work properly on another machine. The containerization principle helps to solve this issue. With containerization, an application is packaged and wrapped with all of its system settings and dependencies as it is built and deployed.
When you want to run the program on a different device, you can use the container, which provides a bug-free environment because all the modules and dependencies are bundled together. Docker and Kubernetes are two of the most well-known containerization environments.
5. Explain hypervisors used in VMs in simple terms.
Answer: A hypervisor is a piece of software that enables virtualization. Virtual Machine Monitor is another name for the hypervisor. In general, it splits the host system into virtual environments and allocates resources to each one. On a single host system, you can essentially run multiple operating systems with the help of a hypervisor.
Hypervisors are divided into two categories, namely Type 1 and Type 2. Type 1 hypervisor is also known as a native hypervisor, and it operates on the host device directly. A Type 1 hypervisor doesn't need a base server operating system because it has immediate access to the host's system hardware.
On the other hand, the underlying host OS is used by the Type 2 hypervisor, which is also known as a hosted hypervisor.
6. Differentiate containerization and virtualization.
Answer: Containers help to create a separate environment in which software can run. The software has exclusive use of the entire user area. Any modifications made inside a container have no effect on the server or other containers on the same host.
Moreover, containers are an application layer abstraction, and each container represents a distinct application. In the case of virtualization, hypervisors provide the guest with an entire virtual machine, like Kernel. VMs are a hardware layer abstraction, and each virtual machine is a physical machine.
7. Explain Docker containers in simple terms.
Answer: A Docker container provides packaged, isolated, and contained environment for applications, including all of their dependencies. Also, a container runs on the same OS kernel as the other containers on a machine. Each container is independent of the other, and they run in separate processes within the operating system in userspace.
Docker isn't connected to any specific IT infrastructure, so it can run on any device or in the cloud. We can create a Docker container from scratch using Docker images. Also, it is possible to use images from the Docker Hub. You can assume that Docker containers are only runtime instances of the environment's template or Docker image.
8. Explain Docker Images in simple terms.
Answer: Docker images are templates that contain the libraries, dependencies, config, system files, and so forth, needed to create Docker containers. They contain several read-only layers of intermediate Images. You can download Docker images from registries such as Docker Hub or create them from scratch. Docker images are generally used to create Docker containers by running the docker run command within them.
9. What do you understand by Docker Hub?
Answer: We can think of the Docker Hub as a cloud registry that allows us to connect code repositories, create images, and test images. We can also save images locally or attach them to the Docker Cloud to deploy them to the host.
Moreover, it employs a centralized resource discovery mechanism that we can use for team collaboration, process automation, delivery, and change management by establishing a production pipeline.
Docker Hub contains tons of official and vendor-specific images that users can pull to their local machines. Also, the users have the flexibility to modify those images or create containers associated with them. Some of the popular Docker images on Docker Hub are Ubuntu, Alpine Linux, CentOS, MySQL, and Nginx.
10. Explain in simple terms the architecture of Docker.
Answer: The Docker architecture utilizes Docker Engine, which is a client-server framework and consists of three components, namely a server, a REST API, and a command-line interface (CLI).
- A server can be considered as a daemon operation, which in turn is a kind of long-running process.
- A REST API describes interfaces that applications can use to communicate with the daemon and send instructions.
- The CLI uses the Docker REST API to monitor or interact with the Docker daemon via scripting or direct CLI commands. The underlying API and CLI are used by many other Docker applications.
11. Explain the uses of a Dockerfile in Docker.
Answer: A Dockerfile is a simple text file without any extension that allows a user to define instructions that would be executed while building an image. When we run the docker build command, the daemon looks for the dockerfile inside the build context and starts executing the instructions inside it one by one. Usually, the first instruction inside a dockerfile is the FROM instruction.
You can use this instruction to pull a base image, and each instruction after that adds a new intermediate image layer on top of the base image. It's quite important to understand the build cache mechanism that the build process uses to write the instructions in the best possible sequence.
Instructions should be written in such a way that the least frequently changing instruction is at the top and the one which changes frequently is at the bottom. This ensures that the build process can utilize the cache from the previous build to save time and resources.
12. Explain the Docker container life cycle.
Answer: The Docker container life cycle involves the following processes:
- The primary purpose of Docker registries is to download Docker images. It may be a public Docker image or one of the many private Docker images that are downloadable. Anyone can pull a Docker image on their own server by executing the docker pull command .
- Users may use the docker create command to build a runtime instance called container after pulling the Docker Image. The newly formed container is now in a state called created. This indicates that the container has been produced but is not yet operational.
- Once the users create a container, they can start it by running the docker start command on it. The container will now be active on the host machine and will be in the started state.
- Rather than using the start or create commands, you can use the docker run instruction on the images to launch the container and bring it to the running state.
You have three choices from this stage. The first is the state of being paused. You may use the docker pause command to pause all processes running within the docker container and then use the unpause command to restart them in the same state as before.
Also, you may use the docker stop command to stop the container completely, which will bring it to a halt. As a result, it enters into the stopped state. You may use the docker start instruction to restart the container from scratch.
The final state is the dead state, which occurs when a container ceases to run and gets terminated despite the daemon's best efforts due to some form of malfunction, such as a busy resource or system.
13. What do you understand by Docker Compose?
Answer: Docker Compose is a CLI tool that takes several containers and assembles them into applications that can run on a single host using a specially formatted descriptor file. YAML files are useful in configuring the application's services. The fact that it enables users to run commands on multiple containers simultaneously is unquestionably a plus. This means that developers can create a YAML config script for an application's services and then start it with a single command.
This tool, which was originally created for Linux, is now available for all major operating systems, including macOS and Windows. One of the key advantages of Docker Compose is its portability. Docker-compose up is sufficient to put up a complete development platform, which can then be taken down with docker-compose down. As a result, developers can easily centralize their application development.
14. What do you understand by Docker Swarm?
Answer: It's a Docker container orchestration platform that manages multiple containers deployed across multiple machines. It primarily assists end users with the development and deployment of a Docker node cluster. As a result, Docker Swarm provides the basic capabilities of managing and organizing multiple Docker containers in a Docker environment.
Docker daemons are all the nodes in a Docker Swarm that communicate using the Docker API. Also, Docker Swarm containers can be deployed and managed from nodes in similar clusters. In Docker, a swarm is a set of Docker hosts that are operating in the Docker Swarm mode. The hosts could act as workers who run the services or as managers who oversee member relationships.
In some cases, a particular Docker host can act as both a manager and a worker. Users may specify the desired state of a particular service during the development process. The service state requirements may include the service's ports, the number of reproductions, and network and storage resources.
Docker may also demonstrate productivity in maintaining the desired state by rescheduling or restarting unavailable tasks, as well as maintaining load balancing across several nodes.
15. Explain the functionalities of Docker Swarm.
Answer: Docker Swarm mode allows for automatic load balancing in the Docker environment, as well as scripting for writing and structuring the Swarm environment. It also allows you to easily roll back environments to a previous state.
Above all, Docker Swarm emphasizes high-security features. It improves connectivity between the Swarm's worker and manager nodes while increasing stability with the use of load balancing. In general, load balancing ensures that the Swarm environment becomes easily scalable.
The Docker CLI's direct integration with Docker Swarm eliminates the need for external orchestration software. For the development and handling of a swarm of Docker containers, you don't need to use any other tool.
16. List the compatible system for Docker to run.
Answer: Docker runs on Windows (x86-64) and Linux (on x86-64, ARM, and other CPU architectures).
17. Explain the working of Docker Swarm.
Answer: The manager node in a dysfunctional cluster is aware of the state of the worker nodes and assigns tasks to the worker nodes. Agents on the worker nodes communicate the status of their tasks with the manager node. As a consequence, the manager node can guarantee that the cluster's desired state is maintained.
In Docker Swarm, any node in a similar cluster may deploy or receive services. During the service development process, users must decide the container image they would like to use. Users may create instructions and services for one of the two scenarios: global or replicated.
A global server could operate on all the nodes of the Swarm, while a replicated service's manager node could assign tasks to worker nodes. Although service in Docker Swarm is typically a definition of a state or task, the actual task defines the job to be completed. Docker could make it possible for a user to build services that can start tasks. Tasks assigned to a node, on the other hand, may not be delegated to other nodes.
A Docker Swarm environment may also become a container for several manager nodes. The CLI is the foundation for creating a service. The API linked in the Swarm environment helps in coordinating all the resources.
Task assignment allows users to assign jobs to tasks based on particular IP addresses. The dispatcher and scheduler are in charge of assigning tasks and guidelines to worker nodes in order to complete tasks. The worker node, therefore, communicates to the manager node to see if any new tasks have been assigned. Finally, in the Swarm environment, tasks allocated to the worker nodes are carried out.
18. Why should you use Docker?
Answer: The following are the major reasons that specify why you should use Docker:
- Docker allows you to use the same versioning and packaging that platforms like Git and NPM offer for server applications. Since Docker containers are just a single instance of Docker images, version tracking different builds of your container is a breeze. Also, it's much easier to manage all of your dependencies because everything is contained.
- With Docker, your build environment would be similar to your production environment. Also, there won't be any dependency issues when running the same container on other machines.
- You wouldn't have to think about reconfiguring the server or reinstalling any of the dependencies if you were to connect another server to your cluster. With container orchestration or management platforms like Swarm and Kubernetes, Docker can run multiple servers with ease.
- Docker makes it possible to prepare the code for deployment on new services. You can bundle up your web server and simply run it by creating an Nginx container, bundle up your API server and deploy it in a Node.js container, and run your database as its own container. You can run all three of these Docker containers on the same computer. Also, it is simple to switch these containers to a new server. You can switch one of these containers to a new server or allocate it across a cluster if you need to scale.
- If you want to run different applications on a single VPS, Docker will help you save money. If each app has its own set of dependencies, the server will quickly become cluttered. Docker allows you to run several different containers on the same server without worrying about a container affecting the other containers or the host.
19. What can you use Docker for?
- Docker simplifies the development process by allowing developers to operate in structured environments and using local containers to deliver the software and services. CI/CD workflows benefit greatly from containers. Take the following scenario as an example: A group of programmers run programs locally and use Docker containers to share the programs with their colleagues. They use Docker to deploy their applications and run automated and manual tests in a test environment. Also, the programmers fix bugs in the development environment before deploying the programs to the test environment for further testing or validation. When the testing is done, it's only a matter of pushing the modified Image to the manufacturing environment to bring the patch to the consumer.
- The container-based Docker framework enables highly portable workflows. Docker containers can run on a desktop, data center, cloud, or hybrid environment.
- Because of Docker's portability and lightweight design, it can handle workloads dynamically and scale them up or down in real-time.
- It provides a better, cost-effective solution to hypervisor-based VMs, allowing you to make better use of your computing resources. Docker is ideal for high-density environments as well as medium and small deployments where limited resources are available.
Intermediate-Level Docker Interview Questions
20. What is the Docker Engine?
Answer: Docker Engine is a free containerization platform for developing and deploying applications. It is a client-server program running on top of dockerd, a long-running daemon process. It includes APIs that define interfaces for software to communicate with and instruct the Docker daemon as well as the CLI client docker.
With scripting or CLI commands, the CLI client uses Docker APIs to monitor or communicate with the Docker daemon. Many other Docker applications use the underlying API and CLI. Also, it is the daemon that creates and controls Docker objects such as images, volumes, and so on.
21. Explain namespaces in Docker.
Answer: A namespace is a Linux functionality that ensures the partitioning of OS resources is mutually exclusive. Namespaces provide a layer of separation between containers, which is the central principle behind containerization. The namespaces in Docker ensure that containers are portable and have no effect on the underlying host. PID, User, Mount, and Network are examples of namespaces that Docker currently supports.
22. How can you scale containers in Docker?
Answer: We can scale and expand Docker containers to any number of containers, from a few hundred to thousands or millions. The only stipulation is that the containers need memory and an operating system at all times. Also, there should be no limitation on memory and OS as Docker scales. We can use Docker Compose or Docker Swarm to manage the scaling of Docker containers.
23. Explain default networks in Docker.
Answer: Bridge network, host, and none are the default networks in Docker. If no network is defined, the bridge network is the default one to which all the containers link. The server network aids in connecting to the host's network stack. Without a network interface, the none network ensures access to a network stack that is specific to a container.
24. Can cloud overtake containers?
Answer: Docker containers are becoming more common, but the cloud is also becoming more and more popular. However, it is likely that Docker may never be overshadowed by the cloud. Using cloud computing in conjunction with containerization would be a great option.
Organizations must understand their needs and dependencies and determine what is best for them. The majority of businesses have Docker incorporated with the cloud to get the most out of both technologies.
25. Is it okay to execute stateful apps in Docker?
Answer: The idea behind stateful apps is that they collect their data on the local file system. While you transfer an application to another machine, retrieving its data becomes difficult. Hence, it's better not to prefer running stateful apps on Docker.
26. What are the advantages of using Docker?
Answer: The following are the key advantages of Docker:
- Makes it easier to create, run, and manage containers.
- Supports version control.
- Facilitates agile development.
- Makes the application portable.
- Allows scaling of applications.
- Increases productivity of developers.
27. Give some downsides of Docker.
Answer: The following list enumerates some of the major downsides of using Docker:
- There is no option for data storage.
- It is not an ideal option for monitoring containers.
- There is no auto-rescheduling of nodes that are not active.
- Horizontal scaling is quite complex.
28. Explain the memory-swap flag.
Answer: Memory-swap is a flag that has no effect unless --memory option is also set. When a container has used up all of the RAM available to it, swap allows it to assign express memory specifications to the disc.
29. Can you monitor Docker in production?
Answer: Docker has certain features, including docker stats or docker events, that can help to track Docker in development. Docker stats display the container's CPU and RAM consumption. Docker events include information on what's going on inside the docker daemon.
30. Give some applications of Docker in real life.
Answer: There are several instances where we can use Docker, such as:
- Management of pipelines of codes.
- Rapid deployment of applications.
- Creation of isolated environments for applications.
31. What are Docker objects?
Answer: There are several key components that are essential to run Docker, including containers, images, networks, volumes, services, and swarm nodes. All these components are collectively termed Docker objects.
32. What is the path of Docker volumes storage?
Answer: The default path for Docker volumes is as follows:
33. How do Docker clients and daemon communicate?
Answer: They communicate with a combination of tools such as TCP, socket.IO, and Restful APIs.
34. How can you integrate CI/CD with Docker?
Answer: We can run Jenkins with Docker, connect Docker to git repositories, and perform integration tests on several Docker containers using Docker compose.
Advanced-Level Docker Interview Questions
35. How can you create Docker images?
Answer: The following are the two ways of creating Docker images:
- The first one is to pull an image directly from any Docker registry using the Docker pull command. We need to be logged in through the command line to do so.
- The second one is to create customized Docker images by specifying instructions inside a Dockerfile and then use the Docker build command to create the Image.
36. How can you control Docker using systemd?
Answer: We can use the following commands if we want to control Docker using the systemd:
$ systemctl start/stop docker $ service docker start/stop
These commands help us to start and stop Docker services in our machines.
37. How can we use a JSON file for Docker compose instead of a YAML file?
Answer: To do so, we need to execute the following command:
$ docker-compose -f docker-compose.json up
Advanced-Level Docker Interview Questions
38. How can you ensure persistent storage in Docker?
Answer: Once we exit or delete a container, we lose the entire data stored in it. However, if we still want access to any kind of data inside such a Docker container, we can mount volumes to it. We can use a directory inside our local machine and mount it as a volume to a path inside the container. Also, we can simply create Docker volumes using the Docker volume create command and share a volume with multiple containers simultaneously.
39. How to access the bash of a Docker container?
Answer: To access the bash of a Docker container, we need to run the container in interactive mode. We can use the interactive and pseudo-TTY options to allow the terminal to let us input commands using a terminal driver. You can use the following command:
$ docker run -i -t <image-name> bash
40. What do you mean by CNM in Docker?
Answer: Container Networking Model (CNM) is a Docker, Inc. standard or specification that governs the networking of containers in a Docker environment. It provides provisions for multiple drivers for container networking.
41. Does Docker support IPv6?
Answer: Docker does, in reality, support IPv6 . Only Docker daemons running on Linux hosts support IPv6 networking. However, if you want the Docker daemon to support IPv6, you must edit the /etc/docker/daemon.json file and change the ipv6 key to true.
42. How can you backup Docker Images?
Answer: To backup Docker images, we can either upload them onto a registry like Docker Hub or convert them into a tarball archive file. We can upload a Docker image to a registry using the Docker push command mentioned below:
$ docker push <image-name>
To save the Docker image into an archived tarball file, we can use this command:
$ docker save -o <name-of-tar-file> <name-of-container>
43. How can you restore Docker Images?
Answer: If we have stored or backed up a Docker image into a registry, we can use the Docker pull command to restore that image:
$ docker pull <image-name>
If we have saved the Docker image as a tarball file using the Docker save command, we can use the Docker load command to extract it back as an image in our local machine.
$ docker load -i <tarball-file-name>
44. When can we not remove a container?
Answer: It is not possible to remove a container if it is paused or running. We need to stop the container before we can kill or remove it.
45. Differentiate between Docker ADD and COPY instructions.
Answer: The Docker ADD command enables us to copy files and directories from the local machine to the containers. It can copy files from a directory, a URL, Git, Bitbucket, or even an archived file. If we specify the source directory as an archive file, the ADD command will extract it and then load it to the container. The Docker COPY command also copies files to the container. However, it only copies from directories and does not support copying from URLs. If we specify an archived file, the COPY command will copy it as it is without extracting it.
46. Explain the difference between Docker start and run commands.
Answer: The Docker start command simply starts a container without running it. It simply creates an instance of the Image and maintains the container in the start state. However, the Docker run command allows to run a container and keep it in a running state. When the container is running, we can execute commands inside it or access its file system.
47. How can we run commands inside Docker containers?
Answer: There are multiple ways to run commands inside Docker containers. We can use a Dockerfile and use the RUN instruction along with a command that we want to run inside the Docker container. We can also use the Docker run command to start the Docker container and access the bash of the container. Inside the bash, we can directly run commands. If the container is running in the background or detached mode, we can use the Docker exec command to run commands inside that container.
48. How can you remove a running container or Image?
Answer: We can remove a running container or Image using the force option along with the container or Image remove command. We can use the following commands:
$ docker rm -f <container-name> $ docker rmi -f <image-name>
49. How can we identify the status of a container?
Answer: We can use the Docker container list or ps command to do so. To display all the running containers, we can use the following command:
$ docker ps
To display all the containers in the machine, we can use the following command:
$ Docker ps -a
$ docker container ls -a
50. What is a build cache?
Answer: When we create Docker images using Dockerfiles, we specify instructions inside them. Each instruction creates a new intermediate image layer. When we first build the Image, it executes all the instructions one by one.
When we try to build it once again after making any changes, it uses the cache from the previous build for all unchanged instructions. As soon as it encounters a changed instruction, the cache is broken for this, and all subsequent instructions are executed freshly.
51. What are the two types of registries?
Answer: There are two types of Docker registries - private and public. Private registries can be in a local machine or hosted on a cloud, and only their owners can access them. Public registries are those from which any authenticated user can pull images. Docker Hub supports both private and public registries.
Docker is the most popular platform for containerization, continuous integration, continuous development, and continuous deployment, all thanks to its excellent pipeline support. Also, Docker's active community has demonstrated that this platform is useful for a variety of use cases, which makes it even more exciting to learn this containerization platform.
This ends our discussion of Docker Interview Questions. We hope that this guide helped you to learn the most commonly asked Docker interview questions along with their answers effectively.
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