DevOps Lesson 5: Docker Compose

Docker Compose runs multi-container applications with one command. Instead of starting app + database + Redis separately, compose starts everything together.

docker-compose.yml

# docker-compose.yml
version: "3.9"

services:
  # Node.js API
  api:
    build: .
    ports:
      - "3000:3000"
    environment:
      - DATABASE_URL=postgresql://user:password@db:5432/mydb
      - REDIS_URL=redis://cache:6379
    depends_on:
      db:
        condition: service_healthy
    volumes:
      - .:/app          # mount local code (for development)
    restart: unless-stopped
  
  # PostgreSQL Database
  db:
    image: postgres:16
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
    volumes:
      - postgres_data:/var/lib/postgresql/data  # persist data!
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user"]
      interval: 5s
      timeout: 5s
  
  # Redis Cache
  cache:
    image: redis:7-alpine
    ports:
      - "6379:6379"

volumes:
  postgres_data:  # named volume (data persists across restarts!)

Compose Commands

docker compose up -d        # start all services (detached)
docker compose down         # stop and remove containers
docker compose logs -f api  # tail api logs
docker compose exec api sh  # shell into running api
docker compose restart api  # restart one service
docker compose ps           # status of all services
docker compose build        # rebuild images