Forgetful Notes

Docker

Docker is an open-source platform that enables developers to automate the deployment, scaling, and management of applications within lightweight, portable containers. A container packages up an application’s code, runtime, system tools, and libraries, ensuring it runs consistently across different computing environments.

Info

Docker’s core innovation is making containerization accessible and easy to use, which has revolutionized modern software development and DevOps practices.

Core Philosophy

Docker’s philosophy is centered on the idea that applications should be portable and self-contained.

  • Consistency: “It works on my machine” is no longer a problem. A container runs the same everywhere.
  • Isolation: Containers run in isolated environments, preventing conflicts between dependencies.
  • Efficiency: Containers are lightweight because they share the host system’s kernel, starting faster and using fewer resources than virtual machines.

Key Concepts

Dockerfile

A Dockerfile is a text file that contains instructions for building a Docker image. It specifies the base image, dependencies, commands, and other configuration needed for the application.

# Use an official Node.js runtime as a parent image
FROM node:18-alpine
 
# Set the working directory in the container
WORKDIR /usr/src/app
 
# Copy package.json and package-lock.json
COPY package*.json ./
 
# Install app dependencies
RUN npm install
 
# Bundle app source
COPY . .
 
# Make port 8080 available to the world outside this container
EXPOSE 8080
 
# Define the command to run the app
CMD [ "node", "server.js" ]

Image

A Docker image is a read-only template used to create containers. Images are built from a Dockerfile and can be stored and shared via a registry like Docker Hub.

Container

A container is a runnable instance of an image. You can create, start, stop, move, or delete containers using the Docker API or CLI.

Docker Compose

Docker Compose is a tool for defining and running multi-container Docker applications. With a docker-compose.yml file, you can configure all of your application’s services, networks, and volumes in one place.

version: '3.8'
services:
  web:
    build: .
    ports:
      - "8080:8080"
  redis:
    image: "redis:alpine"

Essential Commands

# Build an image from a Dockerfile
docker build -t my-app .
 
# Run a container from an image
docker run -p 8080:8080 my-app
 
# List running containers
docker ps
 
# List all images
docker images
 
# Stop a container
docker stop <container_id>
 
# Use Docker Compose to start services
docker-compose up
 
# Stop services with Docker Compose
docker-compose down

Use Cases

  • Microservices: Each service can be deployed in its own container, allowing for independent scaling and updates.
  • CI/CD Pipelines: Docker ensures a consistent environment for building, testing, and deploying applications.
  • Local Development: Replicate the production environment on a developer’s machine to reduce inconsistencies.

Docker has become an essential tool in modern software development, providing a standardized way to build and run applications. It integrates seamlessly with version control systems like Git and is a cornerstone of CI/CD workflows.

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