Docker ComposeLesson 4.2

Writing your first docker-compose.yml file

services block, image vs build, ports, environment, volumes, depends_on, restart policy, networks block

Anatomy of a Compose File

Every Compose file defines services, and optionally networks and volumes. Here is a complete, real example for a Node.js API backed by PostgreSQL:

services:
  api:
    build: .
    ports:
      - "3000:3000"
    environment:
      - DB_HOST=db
      - DB_PASSWORD=secret
    depends_on:
      - db
    restart: unless-stopped

  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
      POSTGRES_DB: appdb
    volumes:
      - pgdata:/var/lib/postgresql/data
    restart: unless-stopped

volumes:
  pgdata:

networks:
  default:
    name: app-network

Key points: build: . tells Compose to build from the local Dockerfile. depends_on ensures db starts before api — but it does not wait for Postgres to be ready, only for the container to start. Use a health check or retry logic in your app for true readiness. Compose auto-creates a default network for all services, so they can reach each other by service name out of the box.