Day 21 Docker Important interview Questions.

Day 21 of #90daysofdevops

Hey Techies! Welcome to this blog

In this blog, we are going to start the Docker Important Interview Questions

Docker Interview

Docker is a good topic to ask in DevOps Engineer Interviews, mostly for freshers. One must surely try these questions to be better in Docker.

Questions

1. What is the Difference between an Image, Container and Engine?

Docker Image - A Docker image is a file used to execute code in a Docker container. Docker images act as a set of instructions to build a Docker container, like a template. Docker images have intermediate layers that increase reusability, decrease disk usage, and speed up docker build by allowing each step to be cached. Image is an executable package of software that includes everything needed to run an application. This image informs how a container should instantiate, determining which software components will run and how.

Docker Container - A Docker Container is a virtual environment that bundles application code with all the dependencies required to run the application. The application runs quickly and reliably from one computing environment to another. Applications running in containers can be deployed easily to multiple different operating systems and hardware platforms. Containers allow applications to be more rapidly deployed, patched, or scaled.

Docker Engine - Docker Engine is an open-source containerization technology for building and containerizing your applications. Docker Engine acts as a client-server application with a server with a long-running daemon process dockerd.

1. What is the Difference between the Docker command COPY vs ADD?

The COPY and ADD commands in Docker are used to add files and directories to a container image.

The COPY command is used to copy local files and directories to the container file system. It is a more basic command and only supports local file and directory paths.

COPY is a docker file command that copies files from a local source location to a destination in the Docker container. The instruction can be used only for locally stored files. Therefore, you cannot use it with URLs to copy external files to your container.

The ADD command also copies local files and directories to the container file system, but it has some additional features. It can automatically decompress files and supports copying files from remote URLs.

ADD command is used to copy files/directories into a Docker image. It can also copy files from a URL and provides tar extraction support.

It is recommended to use the COPY command for most cases as it's more secure and predictable, and the ADD command has some features that may increase the chance of introducing security vulnerabilities.

1. What is the Difference between the Docker command CMD vs RUN?

   RUN is an image build step, the state of the container after a RUN command will be committed to the container image. A Dockerfile can have many RUN steps that layer on top of one another to build the image.

   CMD is the command the container executes by default when you launch the built image. A Dockerfile will only use the final CMD defined. The CMD can be overridden when starting a container with docker run $image $other_command.

The RUN command is used during the image-building process to execute commands and commit the results, while the CMD command sets the command that will be run when the container starts up.

1. How Will you reduce the size of the Docker image?

   There are several ways to reduce the size of a Docker image:

   1. Use a Smaller Base Image(Alpine)

   Alpine Linux is a lightweight Linux distribution that is popular for creating small Docker images. It is smaller than most other Linux distributions and has a smaller attack surface.

   2. Use a .dockerignore file

   A .dockerignore file allows you to specify files and directories that should be excluded from the build context sent to the Docker daemon. This helps to exclude unnecessary files from the build context, which in turn reduces the size of the image.

   3. Utilize the Multi-Stage Builds Feature in Docker

   It allows users to divide the Dockerfile into multiple stages. Multi-stage builds allow you to use multiple FROM statements in your Dockerfile. This allows you to use one image as a builder image and then copy only the necessary files to a smaller image.

   4. Avoid Adding Unnecessary Layers

   A Docker image takes up more space with every layer you add to it. Therefore, the more layers you have, the more space the image requires. Each RUN instruction in a Dockerfile adds a new layer to your image. Remove unnecessary files and dependencies from the image by using the RUN apt-get autoremove, RUN apt-get clean and RUN rm commands in your Dockerfile

   5. Use Squash

   Squash is a technique that allows you to combine all the layers of an image into a single layer. This can significantly reduce the size of an image.

   6. Use official images

   Official images are images that are maintained by the upstream software maintainers. These images are usually smaller in size and more secure than images built by other parties.

   7. Keep Application Data Elsewhere

   Storing application data in the image will unnecessarily increase the size of the images. It’s highly recommended to use the volume feature of the container runtimes to keep the image separate from the data.

2. Why and when to use Docker?

   Why

   1. Portability:

      • Containers encapsulate applications and their dependencies, making them highly portable. An application that runs in a Docker container on one machine is guaranteed to run the same way on another machine.

   2. Consistency:

      • Docker ensures consistency across development, testing, and production environments. The same containerized application can run on a developer's laptop, a testing server, or in a production environment without modification.

   3. Isolation:

      • Containers provide process and file system isolation. This isolation helps prevent conflicts between dependencies and ensures that an application and its dependencies run in their isolated environment.

   4. Resource Efficiency:

      • Containers share the host OS kernel, which makes them more lightweight than virtual machines. This results in better resource utilization, as multiple containers can run on a single host machine without the overhead of separate OS instances.

   5. Scalability:

      • Docker makes it easy to scale applications horizontally by running multiple instances of containers. This is particularly useful in microservices architectures, where different components of an application can be containerized and scaled independently.

   6. Version Control:

      • Docker images can be versioned, providing a way to track changes to an application's environment over time. This makes it easier to roll back to previous versions or deploy specific versions of an application.

   7. DevOps Practices:

      • Docker is often used in conjunction with continuous integration and continuous deployment (CI/CD) pipelines. Containers can be automatically built, tested, and deployed, streamlining the development and release process.

When

1. Multi-Platform Development:

   • When developing applications that need to run on different operating systems, Docker allows developers to create a consistent environment across platforms.

2. Microservices Architecture:

   • In microservices architectures, where applications are composed of small, independently deployable services, Docker containers provide a lightweight and scalable way to package and deploy each service.

3. Continuous Integration/Continuous Deployment (CI/CD):

   • Docker is often used in CI/CD pipelines to create a standardized and automated process for building, testing, and deploying applications.

4. Isolation of Dependencies:

   • When an application has complex dependencies, or when you want to avoid "it works on my machine" issues, Docker can isolate dependencies within containers.

5. Scaling Applications:

   • For applications that need to scale horizontally, Docker makes it easy to deploy and manage multiple instances of containers.

6. Efficient Resource Utilization:

   • When you want to maximize resource utilization by running multiple containers on a single host, Docker's lightweight nature helps achieve better efficiency.

In summary, Docker is valuable in scenarios where you need consistency, portability, scalability, and efficient resource utilization across different stages of the development and deployment lifecycle. It has become an essential tool in modern software development and deployment practices.

1. Explain the Docker components and how they interact with each other.

   Docker is composed of several components that work together to enable the creation, deployment, and management of containerized applications. The main components include:

   1. Docker Daemon:

      • The Docker daemon (dockerd) is a background process that manages Docker containers on a host system. It is responsible for building, running, and monitoring containers. The daemon listens for Docker API requests and communicates with the Docker CLI (Command Line Interface) or other tools to execute container-related commands.

   2. Docker Client:

      • The Docker client (docker) is the primary interface through which users interact with Docker. It accepts commands from the user and communicates them to the Docker daemon. Users interact with Docker by using the Docker CLI, which provides commands to manage containers, images, networks, and other Docker resources.

   3. Docker Images:

      • Docker images are lightweight, standalone, and executable packages that contain everything needed to run a piece of software, including the code, runtime, libraries, and system tools. Images are used as the basis for creating Docker containers. They are often built from a Dockerfile, which is a text file that contains instructions for assembling an image.

   4. Docker Containers:

      • Containers are instances of Docker images. They encapsulate the application and its dependencies, providing a consistent and isolated runtime environment. Containers run on a host system and share the host OS kernel but have their own file system, processes, and network space. Containers can be started, stopped, and deleted, providing a lightweight and portable way to run applications.

   5. Docker Registry:

      • Docker registries are repositories for storing and sharing Docker images. The default public registry is Docker Hub, but organizations often use private registries to store proprietary or sensitive images. Users can push images to a registry, and other users can pull those images to deploy containers. Docker Hub and other registries support versioning and access control for images.

   6. Docker Compose:

      • Docker Compose is a tool for defining and managing multi-container Docker applications. It allows you to define an entire application stack, including services, networks, and volumes, in a single docker-compose.yml file. With a single command, you can then deploy the entire application stack, making it useful for complex applications with multiple components.

   7. Docker Networking:

      • Docker provides a networking model that allows containers to communicate with each other and with the outside world. Each container can be assigned its own network or can connect to existing networks. Docker supports various network drivers, enabling different types of communication, such as bridge, host, overlay, and macvlan.

   8. Docker Volumes:

      • Docker volumes provide a way to persist data generated by and used by Docker containers. Volumes are mounted inside containers and can be shared among multiple containers. They are useful for storing databases, configuration files, and other data that needs to persist beyond the lifecycle of a single container.

The interaction between these components typically follows these steps:

• A user interacts with the Docker CLI or another tool to issue commands.

• The Docker client sends the commands to the Docker daemon.

• The Docker daemon performs the requested actions, such as building an image, creating a container, or managing networks.

• Docker images can be pulled from or pushed to a Docker registry.

• Containers are created and run on the host system, based on Docker images.

• Containers can communicate with each other through Docker networking.

• Data can be persisted using Docker volumes.

1. Explain the terminology: Docker Compose, Docker File, Docker Image, Docker Container?

   1. Docker Compose:

      • Docker Compose is a tool for defining and managing multi-container Docker applications. It allows you to define an entire application stack, including services, networks, and volumes, in a single docker-compose.yml file. With a single command, you can then deploy the entire application stack, making it useful for complex applications with multiple components.

   2. Docker File

      • A Dockerfile is a text file that contains a set of instructions for building a Docker image. It specifies the base image, sets up the environment, installs dependencies, copies application code, and configures the runtime settings. When the Docker image is built using the Dockerfile, it encapsulates the entire application and its dependencies, making it portable and reproducible across different environments.

   3. Docker Image

      • A Docker image is a file used to execute code in a Docker container. Docker images act as a set of instructions to build a Docker container, like a template. Docker images have intermediate layers that increase reusability, decrease disk usage, and speed up docker build by allowing each step to be cached. Image is an executable package of software that includes everything needed to run an application. This image informs how a container should instantiate, determining which software components will run and how.

   4. Docker Container

      • A Docker Container is a virtual environment that bundles application code with all the dependencies required to run the application. The application runs quickly and reliably from one computing environment to another. Applications running in containers can be deployed easily to multiple different operating systems and hardware platforms. Containers allow applications to be more rapidly deployed, patched, or scaled.

2. In what real scenarios have you used Docker?

   Docker use cases for real scenarios:

   1. Simplifying Configuration: The primary use case Docker advocates is simplifying configuration. One of the big advantages of VMs is the ability to run any platform with its config on top of your infrastructure. The same Docker configuration can also be used in a variety of environments.

   2. Code Pipeline Management: Docker provides a consistent environment for the application from development through production, easing the code development and deployment pipeline. Docker supports different computing environments. Given that, when developed codes are transferred between different systems to production, there would be many environments it passes through.

   3. Developer Productivity: With the help of Docker, a development environment can be built with low memory capacity without adding an unnecessary memory footprint to the memory that the host repository already holds. A dozen services can run at low memory with Docker.

   4. App infrastructure isolation: When you install a software package created on one machine on another, there are issues that the DevOps team may face in running apps with specific versions, libraries, dependencies, and so much more.

   With the help of Docker, you can run multiple apps or the same app on different machines without letting something like versions or other such factors affect the development process. This is made possible as Docker uses the kernel of a host system and yet runs like an isolated application.

   5. Consolidation of server requirements: Docker provides a powerful consolidation of multiple servers even when the memory footprint is available on various OSs. Not to mention, you can also share unused memory across numerous instances. With such a possibility, you can even use Docker to create, deploy and monitor a multi-tier app with any number of containers. The ability to isolate the app and its development environments is a boon offered by Docker.

   6. Multi-tenancy support: Using Docker, it was easy and inexpensive to create isolated environments for running multiple instances of app tiers for each tenant. Development teams will have the power to run multiple instances of the application tiers on different tenants, respectively. The increased speed of Docker to spin up operations also serves as an easy way to view or manage the containers provisioned on any system.

   7. Continuous rapid deployment: With Docker, you can easily run the app on any server environment, and evidently, it also provides version-controlled container images. Furthermore, stage environments can be set up via the Docker engine, which enables CD abilities.

3. Docker vs Hypervisor?

   

It is true that Dockers and hypervisors are majorly different, even though they might seem similar to the layman.

They both serve different segments of the IT world based on the applications.

It is up to the organization to choose which one to opt for based on what suits them the best.

Dockers help run multiple instances of the same application, whereas hypervisors with the VMs help run multiple instances of multiple applications.

The capability to run an entire OS does come in handy. Often, organizations opt for both and leverage the advantages of both hypervisors and Dockers to extract the highest level of productivity possible.

1. What are the advantages and disadvantages of using docker?

   Advantages of Docker:

   1. Portability:

      • Docker containers are highly portable, ensuring consistent application behavior across different environments.

   2. Efficiency:

      • Containers are lightweight and share the host OS kernel, resulting in efficient resource utilization and quick startup times.

   3. Consistency:

      • Docker ensures consistency between development, testing, and production environments, reducing deployment issues.

   4. Isolation:

      • Containers provide isolation, preventing conflicts between dependencies and enhancing security.

   5. Scalability:

      • Docker facilitates easy scaling of applications by deploying multiple instances of containers.

Disadvantages of Docker:

1. Learning Curve:

   • Adopting Docker may have a learning curve for users unfamiliar with containerization concepts.

2. Resource Overhead:

   • While lighter than traditional virtualization, Docker introduces some resource overhead.

3. Persistent Storage:

   • Managing persistent storage in Docker can be challenging, especially when containers are ephemeral.

4. Security Concerns:

   • If misconfigured, containers may pose security risks, and sharing the host kernel raises certain security considerations.

5. Complex Networking:

   • Networking configurations in Docker, especially in complex setups, may require additional expertise.

1. What is a Docker namespace?

   In Docker, a namespace is a technology that provides process isolation by creating separate instances of certain system resources for each container. It ensures that containers have their own view of processes, network interfaces, and file systems, contributing to the isolation and independence of containerized applications.

2. What is a Docker registry?

   In Docker, a registry is a repository for storing and sharing Docker images. It serves as a centralized location where Docker images can be pushed, pulled, and managed. The default public registry is Docker Hub, but organizations often use private registries for proprietary or sensitive images.

3. What is an entry point?

   In Docker, an entry point is a command or script specified in the Dockerfile that is executed when a container is started. It defines the default executable for the container. The entry point can be set to an application, script, or binary, and it can include default arguments.

4. How to implement CI/CD in Docker?

   1)Create a Dockerfile: A Dockerfile is a script that contains instructions for building a Docker image. It is a simple text file that contains commands such as FROM, RUN, COPY, EXPOSE, ENV, etc. These commands are executed by the Docker daemon during the build process to create an image.

   2)Create a build pipeline: Set up a build pipeline that automatically builds the image from the Dockerfile whenever there is a change in the source code. This can be done using tools like Jenkins, CircleCI, etc.

   3)Automate testing: Set up automated testing for the image, such as unit tests, integration tests, and acceptance tests, to ensure that the image is working as expected.

   4)Push the image to a registry: Once the image is built and tested, it can be pushed to a Docker registry, such as Docker Hub, so that it can be easily distributed to other systems.

   5)Deploy the image to production: Use a container orchestration tool like Kubernetes, Docker Swarm, or Amazon ECS to deploy the image to a production environment.

   6)Monitor and scale: Monitor the deployed image and scale it as needed to handle increased.

5. Will data on the container be lost when the docker container exits?

   Yes, by default, data within a Docker container is ephemeral, and it will be lost when the container exits. To persist data beyond the container's lifecycle, you can use Docker volumes or bind mounts to connect specific directories on the host machine to directories in the container. This allows data to be stored outside the container and remain accessible even after the container stops.

6. What is a Docker swarm?

   Docker Swarm is a native clustering and orchestration solution for Docker. It enables the creation and management of a swarm of Docker nodes, allowing deployment and scaling of containerized applications across a cluster of machines. Docker Swarm provides a simple and built-in way to manage the orchestration of containers, making it easier to scale and maintain distributed applications.

7. What are the docker commands for the following:

   • view running containers

     docker ps -a

   • command to run the container under a specific name

     docker run --name <container_name> <docker_image>

   • command to export a docker

     docker export <container_id_or_name> > <output_file.tar>

   • command to import an already existing docker image

     docker import <path/to/tarball> <repository>:<tag>

   • commands to delete a container

     docker rm <container_id_or_name>

   • command to remove all stopped containers, unused networks, build caches, and dangling images?

     docker system prune -a

8. What are the common Docker practices to reduce the size of Docker Image?

   Same as question no. 4.

Thank you so much for taking the time to read till the end! Hope you found this blog informative and helpful.

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~kritika :)

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