Docker Compose Documentation

Introduction

Docker Compose is a tool for defining and running multi-container Docker applications. It uses a YAML file to configure services and starts the entire stack with a single command.

What Docker Compose Does

  1. Creates a default bridge network automatically for all services.
  2. Adds all services to the same network, allowing communication using service names as hostnames.

Example:

  • web service can access db using hostname db
  1. Built-in DNS resolution allows containers to communicate without IP addresses.

Basic docker-compose.yml Structure

version: '3.8'
services:
  service_name:
    image: image_name:tag  # Replace with your actual image name/tag
    ports:
      - "host_port:container_port"  # Map host port to container port
    environment:
      - ENV_VAR=value  # Environment variable
    volumes:
      - "host_path:container_path"  # Volume mapping
    depends_on:
      - dependency_service  # Service dependency

Example 1: PostgreSQL and Redis

version: '3.8'
services:
  postgres:
    image: postgres:15
    ports:
      - "5432:5432"
    environment:
      POSTGRES_USER: postgres
      POSTGRES_DB: review
      POSTGRES_PASSWORD: password

  redis:
    image: redis:7
    ports:
      - "6379:6379"

Example 2: Django Application with Comments

version: '3.8'

services:
  django-app:
    container_name: django-app
    image: django-app:latest  # Replace with your actual Django image name/tag
    ports:
      - "8000:8000"  # Expose Django app on port 8000
    environment:
      DEBUG: "1"  # Enable debug mode for development
      POSTGRES_HOST: postgres  # PostgreSQL service name as hostname
      POSTGRES_DB_NAME: nirajan  # Database name
      POSTGRES_DB_PASSWORD: nirajan@9845  # Database password
      REDIS_URL: redis://redis:6379/0  # Redis connection URL
    depends_on:
      - postgres  # Start postgres first
      - redis  # Start redis first
    volumes:
      - static_volume:/code/static  # Store static files
      - media_volume:/code/media  # Store media files

  postgres:
    container_name: postgres
    image: postgres:15
    environment:
      POSTGRES_DB: nirajan
      POSTGRES_USER: nirajan
      POSTGRES_PASSWORD: nirajan@9845
    volumes:
      - postgres_data:/var/lib/postgresql/data  # Persist database data

  redis:
    container_name: redis
    image: redis:7


volumes:
  static_volume:
  media_volume:
  postgres_data:

Where Volumes Are Stored

Docker stores volumes on the host machine:

/var/lib/docker/volumes/

Example:

/var/lib/docker/volumes/static_volume/_data

Do not modify this manually.

Types of Volumes

Named Volume

volumes:
  - static_volume:/code/static

Docker manages storage automatically. Best for production.

Bind Mount

volumes:
  - ./static:/code/static

Stored inside project directory. Best for development.

Named Volumes Declaration

Named volumes must be declared at the bottom of the file.

volumes:
  static_volume:
  media_volume:
  postgres_data:

This tells Docker to create persistent storage managed by Docker.

Docker Networking

Docker Compose creates a default private network for all services.

All services can communicate using service names.

Example:

  • django-app connects to postgres using “postgres”
  • django-app connects to redis using “redis”
  • redis connects to postgres using “postgres”

This is possible because all services are on the same network.

Default Network Behavior

All containers are on the same network by default, meaning they can communicate freely with each other.

Custom Networks

You can isolate services using custom networks.

networks:
  backend:
  cache:

Assign services:

services:
  django-app:
    networks:
      - backend
      - cache

  postgres:
    networks:
      - backend

  redis:
    networks:
      - cache

Docker Compose Commands

docker compose up

Start services

docker compose up -d

Start in background

docker compose down

Stop and remove services

Conclusion

Docker Compose simplifies multi-container applications by combining services, networks, volumes, and environment configuration into a single YAML file.