Database service

Add a database service to the compose file.

The database service

The database service uses a ready image, is configured with environment variables, and keeps its data in a named volume. The volume must also be declared under volumes: at the end of the file:

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

volumes:
  pgdata:

Dependency and health check

depends_on sets the start order. But the database container starting doesn't mean it's ready to accept connections; with healthcheck and condition: service_healthy, the API starts when the database is really ready:

  db:
    image: postgres:16
    healthcheck:
      test: ["CMD", "pg_isready", "-U", "app"]
      interval: 5s
      retries: 5
  api:
    build: .
    depends_on:
      db:
        condition: service_healthy

Exercise Add the database

Write the db service in compose.yaml: image postgres:16; POSTGRES_PASSWORD: gizli under environment; the pgdata volume mounted at /var/lib/postgresql/data. At the end of the file, add the pgdata: declaration under a top-level volumes:.

Show example solution

# compose.yaml
services:
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: gizli
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Test yourself

With which address does the API service connect to the database?
- By the service name: db:5432
- localhost:5432
- With the host's IP
- It can't connect without publishing the port
Show answer

By the service name: db:5432 — Compose puts services on a shared network; the service name is the DNS name.

Start the API when the database is healthy.

    depends_on:
      db:
        condition: ____

Show answer

    depends_on:
      db:
        condition: service_healthy

Match.

depends_on

Start order
healthcheck

Readiness test
pg_isready

PostgreSQL ready command
db:5432

Address by service name

Next lesson: Volume and network

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