Docker Interview Questions

Last updated on Nov 07, 2023

In this blog, we're presenting you with the Top 30 Docker interview Questions and Answers, where all those questions and answers are covered from basic to advanced concepts of Docker, such as definitions,  features, components, architecture, and lifecycle of Docker. 

Our team segregated frequently asked Docker interview questions as follows: 

Most Frequently Asked Docker Interview Questions and Answers

Docker Freshers Interview Questions and Answers

1. What is Docker?

Docker is a containerization technology that encapsulates all of our programs into a single package, allowing us to execute them in any environment. This means that our application will run in any environment, making it simple to have a product-ready application. Docker wraps the required software in a file system that contains everything needed to run the code, including the runtime, libraries, and system utilities. Containerization technology, such as Docker, uses the same operating system kernel as the machine, making it very fast. This means we only need to launch Docker once, and since our OS is already operating, we will have a seamless and smooth transition.

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2. What is containerization?

Because of dependencies, code produced on one system may not work flawlessly on another machine during the software development process. The containerization technique solved this difficulty. So, an application is bundled and wrapped with all of its system settings and components as it is produced and deployed. A container is the name for this bundle. When you want to execute the program on another system, you can use the container, which provides a bug-free environment because all the components and modules are wrapped together. Docker and Kubernetes are two of the most well-known containerization environments.

3. What can I use Docker for?

Docker simplifies the development process by allowing developers to work in systematic settings while employing local containers to provide your apps and services. Continuous delivery and continuous integration (CI/CD) operations benefit greatly from containers.

Consider the following hypothetical situation:

  • Your developers write code remotely and use Docker containers to share it with their peers.
  • They utilise Docker to do automated and human tests on their applications in a test environment.
  • Developers can repair defects in the development environment before deploying them to the test environment for evaluation.
  • When testing is complete, just push the revised image to the production system to deliver the patch to the customer.

Responsive Deployment and Scaling:

The container-based Docker platform enables immensely portable workflows. Docker containers can run on a developer's workstation, in a data centre on real or virtual computers, in cloud services, or in a hybrid environment.

Docker's portability and minimal weight make it simple to dynamically handle operations, scaling up or down apps and services in real-time as business demands dictate.
Running more workloads on the same hardware:
Docker is a compact and efficient application. It offers a practical, cost-effective replacement to hypervisor-based virtualization, allowing you to make better use of your computational resources. Docker is suitable for the high instances as well as small and medium implementations where more may be accomplished with fewer resources.

4. What are the features of Docker?

Docker has a number of features, some of which are listed and explained further below:

Increase in productivity: It increases productivity by making analytical configuration easier and swiftly deploying apps. It also saves resources in addition to providing an integrated environment in which to operate applications.
Services: The behaviour of a container within a cluster is specified by a series of tasks called services. Swarm distributes the container instances listed from each job in the Services across the clusters.
Security Management: It manages the swarm's secrets and chooses which ones to grant access to services, including a few important engine functions like secret create and secret inspect.
Better Software Deliver: It is claimed that software delivery using containers is more efficient. Containers are portable, self-contained, and have a separate disk volume. As the container matures and is deployed to other environments, this isolated volume travels with it.
Reduce the Size: Docker can lower the size of the development since it delivers a smaller OS footprint via containers.
Fast and Easier Configuration: It is one of Docker's important features that enables you to build the system more quickly and easily. Codes can be implemented in much less time and with less work compared to this functionality. Docker is utilised in a different context, therefore the infrastructure is irrelevant to the application's environment.
Swarm: Swarm is a tool for clustering and managing Docker containers. On the front end, it uses the Docker API, which helps to control it with an array of tools. It's a self-organising set of engines that can be used to create pluggable backends.

Rapid Scaling of a System: Containers use fewer pieces of computational gear and produce greater results. They allow data centre operators to squeeze more activity onto less hardware, resulting in cost savings and hardware sharing.
Software-defined networking: Software-defined networking is supported by Docker. The Docker Engine and CLI allow operators to define segregated networks for containers without modifying a single network. Operators and developers create and configure sophisticated network topologies in configuration files. It also serves as a security benefit because the application's containers can run in a virtual computing network with regulated egress and ingress paths.
Application isolation: Docker creates containers for running applications in a controlled environment. Docker can run any program because each container is self-contained. 

5. What are the limits of Docker?

The following are the limitations of Docker:

Containers don't operate at full speed: Virtual machines utilise resources more effectively than containers. However, due to overlay networking, the interaction between the host and the containers system, and other factors, containers still have performance overhead. You must use bare metal rather than containers if you want 100 percent bare-metal performance.
The container ecosystem is in shambles: Although the Docker platform as a whole is open source, some docker products aren't compatible with others, mainly owing to competition among the companies who support them. Red Hat's container-as-a-service platform, OpenShift, for example, only operates with the Kubernetes orchestrator.
Persistent data storage is tricky: unless you store it somewhere else first, all of the information inside a container vanishes when the container goes down. Although there are solutions to keep data persistently in Docker, such as Docker Large Datasets, this is likely a difficulty that has yet to be fully addressed.
Graphical applications don't work well: Docker was created as a way for installing server applications that do not require a graphical interface. While there are several innovative ways (such as X11 video streaming) for running a GUI software inside a container, they are at best clumsy.
Containers are not appropriate for all applications: Containers are particularly useful for programs that are meant to run as a collection of discrete microservices. The only true advantage of Docker is that it makes application distribution easier by offering a simple packaging method.

6. Who is Docker for?

Docker is a tool that serves both system administrators and developers and is thus included in several DevOps (Developers+Operations) toolchains. It allows developers to concentrate on writing code rather than worrying about the system on which it will operate. Furthermore, companies can get a head start by incorporating one of the thousands of programs that are often built to run in a Docker container as part of their applications.

Docker enables flexibility and decreases the number of systems required in operations because of its minimal overhead and compact footprint.

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7. What is virtualization, and how does it work?

The process of generating a software-based, virtual replica of something is known as virtualization (applications, servers, compute storage, etc.). A single physical and logical system is used to create these virtual versions or settings. Virtualization allows you to divide a single system into multiple pieces that function as separate, independent systems. This type of splitting is made feasible by a programmer called Hypervisor. A virtual machine refers to the digital environment produced by the hypervisor.

8. Can You Differentiate Virtualization and Containerization?

Containers create a separate environment in which the application can run. The application has exclusive use of the full user area. Any modifications made inside the container have no effect on the host or other containers on the same host. Containers are an application layer abstraction. Each container corresponds to a specific application.

In virtualization, hypervisors provide the guest with a complete virtual computer (including Kernel). Networks are an abstraction of the hardware layer. Every virtual machine has a physical counterpart.

9. What is Hypervisor?

Virtualization is made possible by hypervisor software. Virtual Machine Monitor is the other name for it. It divides the host system into virtual environments and assigns resources to each of them. On a single host machine, you can effectively run multiple operating systems. Hypervisors come in two varieties:

Type 1: This hypervisor is also known as a bare-metal hypervisor. It operates on the host system directly. It doesn't require a base server operating system because it has direct access to your host's system hardware.

Type 2: The implicit host operating system is used by hypervisors, also known as the "hosted hypervisor."

10. What is a Docker Container?

A container is an image that can be executed. The Docker CLI and API can be used to create, start, stop, move, and delete containers. You can associate memory with a container or create a new image compression based on its present state. 

A container is separated from other containers and the host machine by default. You can specify how segregated a container's network, storage, and other supporting components are from other containers and the host machine.

When you start or create a container, you define it by its image and any configuration choices you provide it. Any modifications to a container's state that aren't saved when it's removed are lost.

11. What is the Docker Registry?

The Docker Registry is the location where all Docker images are maintained. The Docker Hub is a public registry where these images are stored by default. The Docker Cloud is another public registry. The Docker Hub is the most important public repository of image containers, with a large number of developers and many individual contributors contributing regularly.

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12. Explain about Docker Image?

An image is a read-only framework with Docker container deployment instructions. Usually, an image is based on another picture with some modifications. For example, you could create an illusion that focuses on the Ubuntu image but also includes the Apache web server, your app, and the configuration files needed to run it.

You can either make your own photos or rely on those generated by others and stored in a registry. Generate a Dockerfile with a simple syntax for outlining the procedures required to create and run your image. A layer in the image is created for each instruction in a Dockerfile. Only those tiers that have been modified are recreated when you edit the Dockerfile and recreate the image. When subjected to other virtualization systems, this is one of the reasons pictures are so light, tiny, and fast

13. What is meant by Docker Hub?

Docker Hub is a Docker service that lets you find and share container images within your organisation. It is the largest library of container images in the world, featuring content from a variety of sources, including open-source projects, independent software vendors (ISVs), and container network developers who build and distribute their code in containers.

Users can choose between paying for private repositories

or using free public repositories for collecting and storing images.

14. Explain about Storage and various storage types in Docker?

Users can store information in a container's accessible layer, but you'll need a storage driver. It terminates if the container is not operational since it is non-persistent. Furthermore, it is difficult to convey this information. Docker provides four persistent storage options:

Data Volumes: It enables you to create and rename persistent storage volumes, as well as list volumes and identify which containers are linked with each intensity. Data volumes exist on the host storage device outside of containers and enable a rather efficient copy-on-write method.

Data Volume Container: It is an important alternative that incorporates hosting a volume in its own container and exposing it to other containers. Because the volume container is different from the application volume, it can be transmitted across multiple containers in this situation.

Directory Mounts: It has another option to use a container to install the local directory of a host. The volumes in the situations mentioned above must be in the Docker containers folder. Directory Mounts, on the other hand, can use any repository on the Host device as a resource for the container.

Storage Plugins: It is used to link external repository platforms. These plugins move data from the server to a storage array or device.

Docker Intermediate Interview Questions and Answers

15. What are the features of Docker Hub?

The following are the main features of Docker Hub:

Repositories: they enable you to maintain and share Docker images with your organisation or the rest of the Docker network. Each library can handle a large number of annotated and structured photos.

Teams and Organisations: You may create an organisational entity in Docker Hub and it can contain more than one team of Docker Hub users, allowing you to regulate significant exposure to your specific images or private repositories.

Official Docker Images: The Docker images with basic operating systems, procedural language runtime environments, and open-source file storage that have been carefully monitored. Official Docker Images are reviewed and published by the Docker Library Management System. They adhere to Dockerfile best practices, provide thorough documentation, and constantly update the site.

Software developers employ the Docker Verified Publisher Program to create official Docker Hub repositories for publishing certified container images. The Verified Publisher mark shows that the repository was created and signed by a reputable software vendor, reducing the chance of extracting fake or vulnerable images.

The Automated Builds: This functionality in Docker Hub helps to create pictures from program code saved in a remote repository. You can define tags and branches in a Git repository to specify which code must be created into a Docker image, and a web server will initiate a new development on Docker Hub when you upload code.

Webhooks: When an image is uploaded to a Docker Hub repository, you can have it trigger an action in another service. This might be used to run application evaluations on all new images.

16. Explain Docker Architecture?

Docker is built on a client-server model. The Docker daemon, which oversees the creation, execution, and distribution of your Docker containers, communicates with the Docker client. You can run the Docker daemon and client on the same platform or transmit a Docker client to a Docker daemon that is executing remotely. A REST API is used by the Docker client and daemon to communicate across a UNIX network interface or a socket. Docker Compose is another Docker client that allows you to communicate with multi-container applications.

 Docker Client: Users can communicate with Docker using the Docker client. The Docker client can communicate with a daemon running on another system or run on the same machine. A Docker client has the ability to communicate with multiple daemons at the same time. The Docker client is a CLI that permits you to create, operate, and terminate Docker daemon applications.

The primary function of the Docker Client is to enable customers to download pictures from a repository and execute them on a Docker host.

 Docker Daemon: The Docker host includes a comprehensive environment for executing and running the apps. Networks, images, storage, and containers are all components of the Docker daemon. The daemon is responsible for all container-related operations and processes command through the REST API or CLI, as previously described. It can also interact with other daemons to manage its services. Based on the client's request, the Docker daemon obtains and produces container images. It employs a set of instructions known as a "build file" to produce a working model for the container after receiving the necessary image.

Docker Desktop: Docker Desktop is a simple-to-use application for building and sharing virtual machines and microservices on your Windows or  Mac computer. Docker Desktop includes (Dockerd), Docker Daemon, Docker Content Trust, Credential Helper, Docker Compose, and Kubernetes.

Docker Object: You can build and use networks, plugins, images, volumes, containers, and other things using Docker. A quick outline of a handful of such items is provided in this section.

Image: A Docker image is a read-only framework with specifications for building a Docker container. An image is frequently based on another image with minor bits of modification.

You can take your own images or rely on those taken by others and saved in a database. You make your image by creating a Dockerfile with a simple syntax for outlining the procedures needed to make and execute it. Each Dockerfile command creates a pattern in the image. When you alter the Dockerfile and recreate the image, only the relevant containers are recreated. This is part of what makes images so light, small, and speedy when compared to other virtualization systems.

Containers: A container is an image that can be run. The Docker API or CLI can be used to create, pause, transfer, or remove containers. You can give a container more storage, connect it to one or more connections, or even make a new image from its current setup.

A container is frequently properly segregated from other containers and its network machine by default. You have complete control over how isolated a container's storage, network, and other essential components are from other containers and the host server.
Docker Registry: Docker registries are solutions that let you upload and acquire images in one place. In other words, a Docker registry is a set of Docker repositories, each of which hosts one or more container images. Public registries such as Docker Hub and Docker Cloud are available, although private registries can also be used.

17. Can you tell me about Dockerfile?

A Dockerfile is software that employs the Docker platform to automatically build containers. A Dockerfile is a text file that stores all of the commands for creating an image from the command line. The Docker platform, which runs entirely on Linux, enables developers to construct and run self-contained apps, systems and containers that are independent of the runtime environment. Docker, which is built on the Linux kernel's resource separation characteristics, enables system administrators and developers to deploy applications to different platforms by executing them within containers.

18. Explain about Docker Engine?

The Docker Engine is a client-server technique used to generate and run containers using Docker's services and components. When people talk about Docker, they're referring to either Docker Engine (which includes a REST API, a CLI, and a Docker daemon that communicates with the Docker daemon via the API) or Docker Inc., which sells multiple Docker Engine editions.

The Docker Engine makes it easier to create, deploy, and run container-based programmes. Containers, storage volumes, networks, and images are managed by a server-side daemon process created by the engine. The Docker Engine API also has a client-side command-line interface (CLI) that allows clients to communicate with the daemon. Because the engine is explicit, an administrator can programme a particular set of circumstances to represent the intended state. The Docker Engine modifies conditions and settings automatically to ensure that the current state and the ideal state are always the same.

19. Explain the Docker lifecycle?

 The Docker Container Lifecycle explains the many steps involved in building a Docker container. The following are some of the states:

Create: In this state, a container is created but processes have not yet started.

Run: In this state, it runs the container with all of its processes.

Pause: In this state, all container processes are paused. 

Stop: In this state, all container processes are stopped.

Delete: In this state, a container is in a dead state. 

20. What are the commands in Docker Container LifeCycle Management?

Docker Container Lifecycle Management is the method of monitoring the states of Docker repositories. We must guarantee that the containers are operable or, if they are no longer functional, that they are destroyed. The following are some common commands for regulating the Docker lifecycle.

Create Containers: The docker create command is used to create a new Docker container with the given Docker image.

Syntax: $ docker create

Start Container: The docker start command is used to restart a paused container.

Syntax: $ docker start

Run Container: This command performs the functions of both the "docker create" and "docker start" commands. And also create a new container and run the image inside it.

Syntax: $ docker run

Pause Container: We can use the "docker pause" command to pause the operations operating inside the container.

Syntax: $ docker pause

Stop Container: Stopping an operating Container effectively prevents all processes in that Container. Stopping does not imply death or the end of the process.

Syntax: $ docker stop

Delete Container: Deleting or removing the container entails first terminating all processes running within it and then removing the Container. It is preferable to remove the container only if it is in a stopped condition rather than violently destroying the operating container.

Syntax: $ docker stop

$ docker rm

Kill Container: This docker command can terminate one or more operating containers.

Syntax: $ docker kill

21. How to Explain Docker Namespace?

Namespaces are a component of the Linux kernel and a crucial aspect of Linux containers. Namespaces, on the other hand, give a layer of separation. Docker employs a variety of namespaces to offer the isolation that containers require to stay portable and unaffected by the rest of the host system. Each component of a container runs in its own namespace, and access to that namespace is restricted. The following are the various namespace types: 

  • Process ID
  • Mount
  • IPC (Interprocess communication)
  • User (currently experimental support for)
  • Network

22. What is Docker Swarm?

A Docker Swarm is a cluster of virtual or physical machines that have been configured to form a cluster and execute the Docker application. Once a collection of machines has been clustered, you can still run Docker commands, but they will now be executed by the systems in your cluster. A swarm manager is responsible for managing the cluster's activities, and nodes are systems that have bound the cluster.

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23. What is meant by

Docker handles networking in an application-driven way, providing developers with a variety of options while maintaining a sufficient level of abstraction. The two types of networks available are the user-defined and default docker networks. By default, when you deploy Docker, you get three networks: host, none, and bridge. The host and none networks are part of Docker's network stack. The bridge network creates an IP subnet and a gateway automatically, enabling all packages on the network to interact using IP addresses. Because it is not scalable and hinders network functionality and service discovery, this network is rarely used.

24. Differentiate Docker Engine and Docker Machine?

he Docker Engine was originally created for Linux operating systems, but later versions made it possible to run it directly on Windows and Mac OS. The Docker Machine is a program that allows you to manage and install the Docker Engine on numerous virtual hosts or previous Windows and Apple operating systems. Commands submitted via Docker Machine, which is deployed on the internal network, will not only build virtual hosts but will also deploy Docker and set up its clients.

Even though Docker Engine now functions directly on Apple and Windows, Docker Machine may still be used to maintain virtual hosts on Linux and OSes, as well as on enterprise networks, storage systems, and cloud service resources such as Microsoft Azure, Amazon Web Services, and Digital Ocean.

Docker Experienced Interview Questions and Answers

25. What is Docker Compose and what can it be used for?

Docker Compose is a method of defining numerous containers and their specifications using a JSON  or YAML file.

Docker Compose is most commonly used when your implementation has one or more requirements, such as MySQL or Redis. Normally, these requirements are installed locally during development—a step that must be repeated when transitioning to a production setup. By using Docker Compose, you may skip the installation and configuration steps.

Once configured, you may start all of these containers/dependencies with a solitary docker-compose up command.

26. What are the differences between a Docker file and a Docker Compose?

Dockerfiles are simply texted documents that contain the commands that a client can use to create a picture, whereas Docker Compose is a device for creating and operating a multi-container Docker app. Docker Compose defines the components that comprise your application in docker-compose.yml so that they can run in isolation. It starts an app with a single command, docker-compose up.

If you add the create command to your project's docker-compose.yml, Docker-compose will use the Dockerfile. Your Docker process should consist of creating a Dockerfile for each picture you want to generate, then using compose to construct the pictures using the build command.

27. What does the volume parameter do in the Docker “run ” command?

The volume argument synchronises a container repository with a hosted repository.

docker run -v vxiniv-sites:/etc/vxiniv/sites-available vxiniv

The vxiniv-sites repository on the host is mounted to the /etc/vxiniv/sites-available repository with this command. This allows users to connect vxiniv sites without rebooting the container in which they are running. You can also use a directory on the host to safeguard data generated in your container. Otherwise, deleting your container will result in the deletion of any data created and stored in it.

You can use the same information that was created in a prior container using the same command when you use the volume argument.

28. Is it possible to run multiple Docker containers at the same time?

Yes, you can run various processes within a Docker container, however, this is not recommended for most use scenarios. It is usually recommended that you use one service per container to isolate areas of concern. Each container must address one particular area of concern for optimum efficiency and isolation. If you need to operate numerous services within a separate container, technologies like Supervisor can help.

Supervised is a relatively heavy-weight technique that needs you to deploy supervised and its environment, as well as the many programs it supervises, in your picture (or base your picture on one that contains supervised). Then you run the supervised, which automates your operations.

29. What is the .dockerignore file?

We have Dockerignore files, which are similar to.gitignore files in that they allow you to specify the documents and/or repositories that you want to exclude while generating the image. This would undoubtedly minimise the size of the image while also speeding up the Docker installation and configuration.

Before sending the perspective to the docker daemon, the docker CLI looks for a file named.dockerignore in the context's root directory. If this file exists, the CLI updates the context to exclude files and folders whose patterns match those in it. This avoids sending large or vulnerable files and documents to the daemon and possibly adding them to pictures via ADD or COPY.

30. What is the maximum number of containers you can run per host?

This is entirely dependent on your surroundings. The number of containers that can execute in your environment is affected by the size of your programs as well as the quantity of available resources (such as CPU). Unfortunately, containers are not magical. They are unable to construct a new Processor from scratch. They do, however, offer a more efficient manner of allocating your resources. The containers themselves are extremely light (remember, shared OS vs. separate OS per container) and only last as lengthy as the process in which they are executed.

31. What is meant by CNM in Docker?

The container Networking Model is abbreviated as CNM. The Container Network Model (CNM) is a Docker, Inc. specification or standard that serves as the foundation for container networking in a Docker context. It is Docker's technique for container networking that includes models for various network providers. The CNM establishes the following agreement between containers and the network:

All containers in the same cluster can freely interact with one another. Multiple networks are a good approach to partitioning traffic across containers and thus should be provided by all drivers.

Numerous endpoints per container allow a container to connect to various networks.

To provide network connectivity, an endpoint is deployed to a network sandbox.

32. What are the major components of the CNM network?

Sandbox is a catch-all name for OS-specific techniques that are used to segregate network infrastructures on a Docker host. Docker on Linux provides this sandbox functionality through the usage of kernel namespaces. Network "stacks" within sandboxes comprise interfaces, routing tables, DNS, and so on. In CNM, a network is defined as one or more endpoints that may communicate with one another. All network endpoints can communicate with one another. Without external routing, endpoints on separate networks can not communicate. The following are the components of the CNM Model: 

  • Network
  • Sandbox
  • Endpoint

33. What are the Docker networking drivers?

The following are the Docker Networking drivers: 

Bridge: The standard network driver. This is the sort of network you are building if you do not provide a driver. Bridge networks are typically used when your apps operate in separate containers and need to communicate with one another.

Host: Eliminate network segmentation between the Docker host and container for solitary containers and use the host's networking explicitly. Docker 17.06 and above only support hosts for swarm services.

Overlay: Overlay networks enable many Docker daemons and allow swarm functions to interact with one another. Overlay systems can also be used to communicate and interact between a swarm service and a single container, or between two solitary containers running on distinct Docker daemons. This approach eliminates the requirement for OS-level connectivity between these containers. 

MacVLan: Macvlan networks enable you to provide a MAC address to a container, transforming it into a portable computer on your network. The MAC addresses of warehouses are used by the Docker daemon to redirect communication to them. When interacting with traditional apps that expect to be directly connected to the network connection rather than filtered through the Docker host's network stack, using the macvlan driver can often be the best option.

None: Turn off all networking for this container. Typically used in tandem with a specific network driver. None of the swarm services are available.

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Conclusion: The blog has come to an end. Our team tried to explain all of the concepts, and we hope that the above Docker interview questions and answers will assist you in cracking the interview. Please leave a comment if you have any questions or recommendations.

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As a senior technical content writer for HRK tainings, srivalli patchava has a greater understanding of today's data-driven environment, which includes key aspects of data management and IT organizations. She manages the task of creating great content in the areas of software testing, DevOps, Robotic process automation. Connects with her on Linkedin and Twitter.

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