Docker Interview Questions

Docker is an open-source containerization platform that allows developers to package applications and their dependencies into lightweight, portable containers. These containers ensure consistent behavior across development, testing, and production environments, regardless of the underlying operating system.

• It uses OS-level virtualization to isolate applications within containers.
• Each container runs independently and can be started, stopped, moved, or deleted without affecting other containers.
• This ensures high efficiency, faster deployment, and simplified configuration.

Docker comes with several key features that make it a powerful tool for modern software development and deployment:

• Containerization: Encapsulates applications along with their environment and dependencies.
• Portability: Containers can run on any system that has Docker installed—be it local, staging, or production.
• Isolation: Processes run in separate containers, improving security and stability.
• Efficiency: Containers share the host OS kernel, which reduces overhead compared to traditional virtual machines.
• Versioning and Reusability: You can version control Docker images and reuse them across teams.
• Docker Hub: Offers a large repository of pre-built container images for quick deployment.

Docker has become widely adopted due to its flexibility and performance, but like any technology, it has both advantages and disadvantages:

Pros of Docker:

• Portability: Applications packaged in Docker containers can run seamlessly across different environments.
• Resource Efficiency: Containers share the host OS kernel, using fewer resources than traditional virtual machines.
• Isolation: Each container runs in its own isolated environment, improving security and reducing conflicts.
• Automation: Docker supports CI/CD pipelines and automated builds, simplifying development workflows.

Cons of Docker:

• Learning Curve: Understanding containerization, image creation, and orchestration can be challenging for beginners.
• Additional Resources: Containers, while lightweight, still consume system resources and may not match bare-metal performance.
• Security Concerns: Misconfigured containers or insecure images can introduce vulnerabilities.
• Orchestration Complexity: Managing containers at scale requires tools like Kubernetes, which add complexity.

A container is a lightweight, standalone, executable package that includes everything needed to run a piece of software: code, runtime, system tools, libraries, and settings. Containers ensure that applications run consistently regardless of the environment.

Unlike virtual machines, containers do not require a full operating system. Instead, they share the host OS kernel and use features like resource isolation and namespaces to run isolated processes.

You can visualize Docker as a cargo ship and the containers as the individual boxes it transports. Each container holds all the dependencies and configuration needed to run an app, making deployment across environments fast and reliable.

There are three main components of Docker:

• Docker Client: Executes build and run operations and communicates with the Docker Host.
• Docker Host: Contains the Docker daemon, manages containers and images, and connects to Docker registries.
• Docker Registry: Stores Docker images. Registries can be public or private, with Docker Hub and Docker Cloud being popular public options.

Docker images are executable packages that contain application code, dependencies, and configuration needed to run an application in a container. They can be deployed to any Docker environment, and containers are created from these images to run applications.

Docker Compose is a tool for defining and managing multi-container Docker applications. It uses a YAML file to specify services, networks, and volumes required by the application. It allows you to run multiple containers, connect them, and manage their communication using exposed ports.

Namespaces are a Linux feature used by Docker to provide isolation between containers. They ensure that containers are portable and do not interfere with the host system. Types of namespaces in Docker include PID, Mount, User, Network, and IPC.

Docker Hub is a public cloud-based container registry for storing and sharing Docker images. Developers can push images using the docker push command, and use public or private repositories for collaboration.

Data in a container can be lost if the container is deleted or if non-persistent (ephemeral) storage is used without proper management. To make data persistent, Docker volumes or bind mounts should be used.

You can export a Docker image to a tar archive using:

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

You can import a saved Docker image into another Docker host using:

docker load -i <input_file_name>.tar

Yes, a paused container can be removed using:

docker rm <container_id>

Use the following commands to get container counts:

• Running: docker ps -q | wc -l
• Paused: docker ps -aq -f "status=paused" | wc -l
• Stopped: docker ps -aq -f "status=exited" | wc -l

Docker provides commands to manage container lifecycle:

• Start: docker start <container_name>
• Stop: docker stop <container_name>
• Kill: docker kill <container_name>

The Docker client and Docker daemon work together but have distinct roles:

• Docker Client: The tool that accepts and sends user commands (such as docker run) to the daemon.
• Docker Daemon: The background service that performs the actual operations, such as creating, running, and managing containers.

Example: When you run docker run from the CLI, the Docker client sends the request to the Docker daemon, which then starts the specified container.

Volumes are used in Docker to store data outside of container filesystems, ensuring data persistence even when containers are deleted. They also make it easier to manage, back up, and share data between containers.

Docker allows limiting CPU and memory usage at container runtime:

• CPU limit: Use the --cpus flag to specify the number of CPU cores available to the container.
• Memory limit: Use the -m (or --memory) flag to set the maximum RAM allocation.

Example:

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

The key components of Docker architecture include:

• Docker Client: The command-line interface (CLI) that developers use to interact with Docker.
• Docker Daemon (dockerd): The background service running on the host machine that manages Docker objects such as images, containers, networks, and volumes.
• Docker Images: Read-only templates used to create containers, often built from a Dockerfile and containing application code and dependencies.
• Docker Containers: Runnable instances of Docker images, containing everything needed to execute an application in an isolated environment.
• Docker Registry: A repository for storing and distributing Docker images, such as Docker Hub.

A Dockerfile is a text-based script containing a series of instructions to assemble a Docker image. It specifies the base image, application dependencies, environment variables, configuration, and commands to run the application.

Docker builds an image by executing each instruction in the Dockerfile in sequence, creating intermediate layers that are cached for efficiency.

Example:

FROM node:18-alpine
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
CMD ["npm", "start"]