Docker Compose Guide

Docker Compose Guide

Write well-structured Docker Compose configurations for local development, testing, and production-like environments with proper networking, health checks, and dependency management.

When to Use

• User creates or edits docker-compose.yml
• User sets up a local development environment with multiple services
• User asks about service dependencies, networking, or volumes
• User needs to coordinate database, cache, and app containers
• User asks about environment variable management in Docker

Core Patterns

Service Definition with Health Checks

Define services with explicit health checks so dependent services wait for readiness, not just container start.

services:
  postgres:
    image: postgres:16-alpine
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: myapp
      POSTGRES_PASSWORD_FILE: /run/secrets/db_password
    volumes:
      - postgres_data:/var/lib/postgresql/data
      - ./init.sql:/docker-entrypoint-initdb.d/init.sql:ro
    ports:
      - "5432:5432"
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U myapp -d myapp"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s
    networks:
      - backend

  redis:
    image: redis:7-alpine
    command: redis-server --maxmemory 256mb --maxmemory-policy allkeys-lru
    volumes:
      - redis_data:/data
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s
      timeout: 3s
      retries: 5
    networks:
      - backend

Application Service with Dependencies

Use depends_on with condition: service_healthy to ensure proper startup order.

services:
  api:
    build:
      context: .
      dockerfile: Dockerfile
      target: development
    volumes:
      - .:/app
      - /app/node_modules
    ports:
      - "3000:3000"
    environment:
      DATABASE_URL: postgres://myapp:secret@postgres:5432/myapp
      REDIS_URL: redis://redis:6379
      NODE_ENV: development
    depends_on:
      postgres:
        condition: service_healthy
      redis:
        condition: service_healthy
    networks:
      - backend
      - frontend
    restart: unless-stopped

Profiles for Optional Services

Use profiles to group services that are only needed in specific scenarios.

services:
  app:
    build: .
    ports:
      - "3000:3000"

  mailhog:
    image: mailhog/mailhog:latest
    ports:
      - "1025:1025"
      - "8025:8025"
    profiles:
      - debug

  pgadmin:
    image: dpage/pgadmin4:latest
    environment:
      PGADMIN_DEFAULT_EMAIL: admin@local.dev
      PGADMIN_DEFAULT_PASSWORD: admin
    ports:
      - "5050:80"
    profiles:
      - debug

  prometheus:
    image: prom/prometheus:latest
    volumes:
      - ./prometheus.yml:/etc/prometheus/prometheus.yml:ro
    ports:
      - "9090:9090"
    profiles:
      - monitoring

Start with profiles: docker compose --profile debug --profile monitoring up

Networks and Volumes

Explicitly define networks for service isolation and named volumes for data persistence.

networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge
    internal: true  # No external access

volumes:
  postgres_data:
    driver: local
  redis_data:
    driver: local

Environment Variable Management

Use .env files for defaults and environment for overrides. Never commit secrets.

services:
  api:
    env_file:
      - .env
      - .env.local  # Overrides .env, not committed
    environment:
      LOG_LEVEL: ${LOG_LEVEL:-info}
      APP_VERSION: ${APP_VERSION:?APP_VERSION is required}

Example .env file:

# .env (committed, defaults only)
COMPOSE_PROJECT_NAME=myapp
LOG_LEVEL=info
NODE_ENV=development

Anti-Patterns

• Using depends_on without health checks: depends_on only waits for container start, not service readiness. Always pair with condition: service_healthy.
• Bind-mounting node_modules: Overwriting container node_modules with host volume causes platform mismatches. Use anonymous volume exclusion: /app/node_modules.
• Hardcoding passwords in compose file: Use env_file, Docker secrets, or environment variables. Never commit credentials.
• Using links: Links are legacy. Use Docker networks instead -- services on the same network resolve each other by service name.
• Not using named volumes: Anonymous volumes are hard to manage and back up. Always use named volumes for persistent data.
• Exposing database ports in production: Only expose database ports for local development. In production, keep databases on internal networks.

Quick Reference

# Start all services
docker compose up -d

# Start with specific profiles
docker compose --profile debug up -d

# Rebuild and start
docker compose up -d --build

# View logs
docker compose logs -f api

# Scale a service
docker compose up -d --scale worker=3

# Stop and remove volumes
docker compose down -v

# Execute command in running container
docker compose exec api sh

# Run one-off command
docker compose run --rm api npm test