Docker Compose

You are a Docker DevOps engineer. The user wants to create a docker-compose.yml file for running multiple interconnected services locally.

What to check first

• Run docker --version and docker-compose --version to confirm both are installed
• Verify the services you need (web app, database, cache, etc.) and their image names from Docker Hub or your registry

Steps

1. Create a docker-compose.yml file in your project root with version: '3.8' at the top (compatible with Docker Engine 20.10+)
2. Define the services: section and list each service as a key (e.g., web:, db:, redis:)
3. For each service, specify the image: field or use build: with a Dockerfile path if building locally
4. Set ports: to map container ports to host ports using "HOST:CONTAINER" format (e.g., "8080:3000")
5. Add environment: variables or reference a .env file with env_file: .env for sensitive data
6. Configure networks: and volumes: for inter-service communication and persistent data storage
7. Use depends_on: to control startup order (e.g., web service depends on db service)
8. Run docker-compose up -d to start all services in detached mode, then use docker-compose logs -f to monitor

Code

version: '3.8'

services:
  web:
    build: .
    container_name: myapp_web
    ports:
      - "8080:3000"
    environment:
      - NODE_ENV=production
      - DATABASE_URL=postgres://user:password@db:5432/myapp
      - REDIS_URL=redis://cache:6379
    depends_on:
      - db
      - cache
    volumes:
      - ./src:/app/src
    networks:
      - app-network
    restart: unless-stopped

  db:
    image: postgres:15-alpine
    container_name: myapp_db
    environment:
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=password
      - POSTGRES_DB=myapp
    ports:
      - "5432:5432"
    volumes:
      - db_data:/var/lib/postgresql/data
    networks:
      - app-network
    restart: unless-stopped
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user"]
      interval: 10s
      timeout: 5s
      retries: 5

  cache:
    image: redis:7-alpine
    container_name: myapp_cache
    ports:
      - "6379:6379"
    volumes:
      - cache_data:/data
    networks:
      - app-network
    restart: unless-stopped
    command: redis-server

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change