Docker Compose

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Lesson 06 / 11

Real apps are rarely one container. An API, a database, maybe a cache — Compose describes that whole stack in one YAML file and brings it up or down as a single unit, instead of you juggling five separate docker run commands by hand.


# compose.yaml
services:
  api:
    build: .
    ports:
      - "3000:3000"
    environment:
      DATABASE_URL: postgres://app:app@db:5432/app
    depends_on:
      - db

  db:
    image: postgres:16
    environment:
      POSTGRES_USER: app
      POSTGRES_PASSWORD: app
      POSTGRES_DB: app
    volumes:
      - db-data:/var/lib/postgresql/data

volumes:
  db-data:


docker compose up -d
docker compose ps
docker compose logs -f api
docker compose down

The api service reaches the db service by name — Compose resolves it on the shared app-net network.

Notice the API's DATABASE_URL points at host db, not an IP address. Services on the same Compose network reach each other by service name — Compose sets up that DNS resolution automatically.

Common gotcha: depends_on controls start order, not readiness. Postgres can still be booting when the API's first connection attempt fires. Add a healthcheck + condition: service_healthy, or retry logic in the app, for anything that matters.

Try it yourselfRun docker compose up -d with the file above, then docker compose exec api sh and try ping db from inside it. It resolves — no IP address in sight.

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