Docker Interview Q/A

1. What is Hypervisor?

   A hypervisor is software that enables virtualization, also known as a Virtual Machine Monitor. It divides the host system's resources to create isolated virtual environments, allowing multiple operating systems to run on a single host system. There are two types of hypervisors: Type 1, or Native/Bare metal Hypervisor, which runs directly on the host's hardware without needing a base server operating system, and Type 2, or Hosted Hypervisor, which relies on the underlying host operating system.

2. What is virtualization?

   Virtualization is the process of creating virtual versions of various resources, such as computing, storage, and servers, using software. These virtual environments are generated from a single physical hardware system, allowing for the isolation and separation of multiple distinct systems. A hypervisor, like VMware or Hyper-V, plays a key role in enabling virtualization by creating and managing these virtual environments known as Virtual Machines.

3. What is containerization?

   Containerization is a software development and deployment technique that bundles an application, along with all its dependencies and configuration files, into a single package called a container. Containers are designed to be portable and ensure that the application runs consistently across different environments, such as development, testing, and production. Docker and Kubernetes are well-known containerization environments that facilitate this process.

4. Difference between virtualization and containerization

   Containers provide isolated environments for running applications, and changes made within a container do not affect the host system or other containers on the same host. Each container represents a different application. In contrast, virtualization relies on hypervisors to provide complete virtual machines (VMs) to guests, including their kernels. VMs are abstractions of the hardware layer, where each VM can be considered a distinct physical machine.

5. What is Docker?

   Docker is a containerization platform that packages applications and their dependencies into containers. These containers are self-contained and include everything required to run the application, such as code, runtime, system tools, libraries, and configurations. Docker ensures that applications run consistently in any environment, whether it's development, testing, or production. It has gained popularity for its ease of use and portability.

6. What is a Docker Container?

   A Docker container is a self-contained unit that includes an application and all its dependencies. Containers share the host's kernel but run as isolated processes in user space. They are not tied to a specific infrastructure, making them highly portable and able to run on any computer, infrastructure, or cloud platform. Docker containers are instances of Docker images.

7. What are Docker Images?

   Docker images serve as the source for creating Docker containers. These images contain everything needed to run an application, including the code, libraries, and configurations. When a user runs a Docker image, it creates an instance of a container. Docker images can be deployed to any Docker environment and are instrumental in ensuring consistency across different stages of the software development and deployment process.

8. What is Docker Hub?

   Docker Hub is a registry where Docker images are stored and made available to users. Users can access images from Docker Hub to create customized containers and images for their applications. Docker Hub is one of the largest public repositories of Docker images, making it a central hub for sharing and distributing container images.

9. Explain Docker Architecture.

   Docker's architecture comprises a client-server model with three primary components. First, there is a Docker server, which is a long-running daemon process that manages containers. Second, the server provides a REST API that specifies interfaces for interacting with the Docker daemon. Finally, the Docker command line interface (CLI) allows users to control and interact with the Docker daemon. Other Docker applications use the same API and CLI to communicate with the Docker daemon, making it highly extensible and versatile.

10. What is a Dockerfile?

    A Dockerfile is a text document that contains a set of instructions for building a Docker image. It outlines the commands that a user would typically execute on the command line to assemble an image. Docker can automatically build images by reading and executing these instructions. Dockerfiles are instrumental in defining the environment and configurations needed for an application to run in a container.

11. Tell us something about Docker Compose.

    Docker Compose is a tool that uses YAML files to define and manage multi-container Docker applications. It allows users to specify the services, networks, volumes, and other configurations required for their application. Docker Compose simplifies the process of creating, managing, and scaling applications composed of multiple interconnected containers. It is especially useful for defining and running complex applications with multiple dependencies.

12. What is Docker Swarm?

    Docker Swarm is a native clustering and orchestration solution for Docker containers. It transforms a group of Docker hosts into a single, virtual Docker host, providing high availability and load balancing for containerized applications. Docker Swarm makes use of the standard Docker API, allowing tools that interact with a Docker daemon to transparently scale to multiple hosts.

13. What is a Docker Namespace?

    Docker employs various Linux namespaces to provide isolation for containers and prevent them from interfering with the underlying host system. Namespaces include PID (process ID), Mount (file system), IPC (interprocess communication), User (user and group IDs), and Network (network stack). These namespaces ensure that containers remain isolated and portable across different host systems.

14. What is the lifecycle of a Docker Container?

    The lifecycle of a Docker container involves several stages, including creating, running, pausing (optional), un-pausing (optional), starting, stopping, restarting, killing, and ultimately destroying the container. Each stage serves a specific purpose in managing the execution and state of a Docker container.

15. What is a Docker Machine?

    Docker Machine is a tool that allows users to install Docker Engine on virtual hosts. These virtual hosts can be managed using Docker Machine commands, making it easier to set up and manage Docker environments. Docker Machine is also used to provision Docker Swarm clusters, simplifying the process of creating container orchestration clusters.

16. How to check for Docker Client and Docker Server versions?

    To check the versions of Docker Client and Docker Server, you can use the following command: $ docker version. This command provides information about the installed Docker Client and Docker Server versions.

17. How do you get the number of containers running, paused, and stopped?

    This question asks how to obtain information about the number of running, paused, and stopped containers.

    • You can use the following command to get detailed information about the Docker installation, including the number of running, paused, and stopped containers: $ docker info.

18. If you vaguely remember the command and you'd like to confirm it, how will you get help on that particular command?

    This question inquires about the method to obtain help on a Docker command.

    To get help on a Docker command, you can use the following command: $ docker --help. This command lists all Docker commands. If you need help with a specific command, you can use the following syntax: $ docker <command> --help.

19. How to log in to a Docker repository?

    This question asks for the command to log in to a Docker repository.

    • To log in to hub.docker.com, you can use the following command: $ docker login. This command prompts you to enter your username and password, allowing you to log in to the Docker repository.

20. If you wish to use a base image and make modifications or personalize it, how do you do that?

    This question explores the process of using a base image and making modifications.

    • To use a base image and make modifications or customize it, you can pull the base image from Docker Hub to your local system using the following command: $ docker pull <image_name>. Once the base image is on your local system, you can modify it and create a new image.

21. How do you create a Docker container from an image?

    This question asks for the command to create a Docker container from an image.

    • To create a Docker container from an image, you can use the following command: $ docker run -it -d <image_name>. The -d flag indicates that the container should start in detached mode, allowing it to run in the background.

These answers continue to provide explanations for Docker-related concepts and commands as requested in the interview questions.

1. How do you list all the running containers?

   This question inquires about the command to list all running Docker containers.

   • To list all the running Docker containers, you can use the following command: $ docker ps. This command provides a list of running containers along with their details.

2. Suppose you have 3 containers running, and out of these, you wish to access one of them. How do you access a running container?

   This question asks for the command to access a specific running container among multiple running containers.

   • You can access a specific running container using the following command: $ docker exec -it <container id> bash. This command allows you to enter the running container and work within its environment.

3. How to start, stop, and kill a container?

   This question seeks the commands to start, stop, and kill a Docker container.

   • To start a Docker container, you can use the following command: $ docker start <container_id>. To stop a running container, you can use the command: $ docker stop <container_id>. If you need to forcefully terminate a container, you can use the command: $ docker kill <container_id>.

4. Can you use a container, edit it, and update it? Also, how do you make it a new and store it on the local system?

   This question explores whether you can edit and update a container and how to create a new image from it.

   • Yes, you can use a container, make changes to it, and update it. To create a new image from a modified container, you can use the following command: $ docker commit <container_id> <username/imagename>. This command creates a new image with your changes and allows you to store it on your local system.

5. Once you've worked with an image, how do you push it to Docker Hub?

   This question asks for the command to push an image to Docker Hub.

   • To push an image to Docker Hub, you can use the following command: $ docker push <username/image name>. This command uploads your image to your Docker Hub repository, making it available for others to use.

6. How to delete a stopped container?

   This question seeks the command to delete a stopped Docker container.

   To delete a stopped container, you can use the following command: $ docker rm <container id>. This command removes the specified stopped container from your system.

7. How to delete an image from the local storage system?

   This question asks for the command to delete a Docker image from the local system.

   • To delete a Docker image from the local system, you can use the following command: $ docker rmi <image-id>. This command removes the specified image from your local Docker repository.

8. How to build a Dockerfile?

   This question inquires about the command to build a Docker image from a Docker file.

   • To build a Docker image from a Dockerfile, you can use the following command: $ docker build <path to docker file>. This command reads the instructions in the Dockerfile and creates an image based on those specifications.

These answers continue to provide commands and explanations for working with Docker containers and images.