Day 21: Docker Important interview Questions.

In a DevOps interview focused on Docker, concise and precise answers are essential. Let's address each question succinctly:

Difference between an Image, Container, and Engine:

• Image: A blueprint for creating containers. It contains all the dependencies needed to run an application.

• Container: A runtime instance of an image. It encapsulates the application and its dependencies, isolated from the host system.

• Engine: The software that enables building, managing, and running containers. Docker Engine is a popular containerization platform.

Difference between Docker command COPY vs ADD:

• COPY: Copies files or directories from the host into the container filesystem.

• ADD: Similar to COPY but with additional features like remote URL support and automatic tar extraction, which makes it more versatile.

Difference between Docker command CMD vs RUN:

• CMD: Specifies the default command to execute when a container starts. It's often used to define the primary application process.

• RUN: Executes commands during image build time. It's typically used for installing dependencies or setting up the environment.

How to reduce the size of the Docker image:

• Use multi-stage builds.

• Minimize the number of layers.

• Remove unnecessary files and dependencies.

• Utilize Alpine Linux as a base image for smaller footprint.

• Optimize Dockerfile instructions.

Why and when to use Docker:

• Docker enables consistent development, shipping, and deployment of applications across different environments.

• It's useful in microservices architecture, continuous integration/continuous deployment (CI/CD), and scaling applications.

Explanation of Docker components and terminology:

• Docker Compose: Tool for defining and running multi-container Docker applications.

• Docker File: Text file containing instructions for building Docker images.

• Docker Image: Blueprint for containers, including application code and dependencies.

• Docker Container: Runnable instance of a Docker image.

Real scenarios of using Docker:

• Development environments.

• Continuous integration and deployment pipelines.

• Microservices architecture.

• Scaling applications.

Docker vs Hypervisor:

• Docker containers share the host OS kernel, making them lightweight and faster to start compared to virtual machines managed by hypervisors.

Advantages and disadvantages of Docker:

• Advantages: Portability, scalability, resource efficiency.

• Disadvantages: Limited support for Windows environments, security concerns with shared kernel.

Docker namespace:

• A namespace provides isolation for resources within a container, such as network interfaces, process IDs, and filesystem mounts.

Docker registry:

• A centralized repository for storing and distributing Docker images.

Entry point:

• The initial command or script executed when a container starts.

Implementing CI/CD in Docker:

• Use Docker images for building, testing, and deploying applications.

• Automate Docker image builds and deployments using CI/CD tools like Jenkins or GitLab CI.

Data persistence in Docker containers:

• Data within a container is typically lost when the container exits unless specifically stored in persistent volumes or external storage.

Docker swarm:

• Docker swarm is a clustering and orchestration tool used to manage a cluster of Docker hosts.

Common Docker commands:

• View running containers: docker ps

• Run container under a specific name: docker run --name <name>

• Export a Docker image: docker save

• Import an existing Docker image: docker load

• Delete a container: docker rm

• Remove all stopped containers, unused networks, build caches, and dangling images: docker system prune

Common Docker practices to reduce image size:

• Use lightweight base images.

• Minimize the number of layers.

• Remove unnecessary files and dependencies.

• Optimize Dockerfile instructions.

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