Docker Compose for Multi-Service Applications

What is Docker Compose?

Docker Compose lets you define and run multi-container applications with a single YAML file. Instead of managing each container separately, you describe your entire stack.

Basic Structure

# docker-compose.yml
services:
  web:
    image: nginx:alpine
    ports:
      - "80:80"
    volumes:
      - ./html:/usr/share/nginx/html
    depends_on:
      - app

  app:
    build: .
    environment:
      - DATABASE_URL=postgres://user:pass@db:5432/myapp
    depends_on:
      - db

  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Common Commands

Environment Variables

services:
  app:
    env_file:
      - .env
    environment:
      - NODE_ENV=production
      # Reference host env vars
      - API_KEY=${API_KEY}

.env file:

DATABASE_URL=postgres://user:pass@db:5432/myapp
REDIS_URL=redis://cache:6379
SECRET_KEY=your-secret-here

Danger Never commit .env files to version control. Add .env to .gitignore.

Health Checks

services:
  db:
    image: postgres:16-alpine
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user -d myapp"]
      interval: 10s
      timeout: 5s
      retries: 5

  app:
    build: .
    depends_on:
      db:
        condition: service_healthy

Networking

Services communicate by service name:

# From the "app" service, connect to "db" by name
connection = psycopg2.connect("host=db port=5432 dbname=myapp")

# Connect to "cache" (Redis)
redis = Redis(host="cache", port=6379)

Production Tips

services:
  app:
    restart: unless-stopped
    deploy:
      resources:
        limits:
          cpus: '2.0'
          memory: 512M
    logging:
      driver: json-file
      options:
        max-size: "10m"
        max-file: "3"

Tip Use docker compose up -d --remove-orphans to clean up containers from services you've removed from the compose file.