Lab 9: Compose healthchecks and dependencies

Objective

Create a Compose stack with a database, an API, and a worker. The API and worker wait for the database to become healthy before starting. Verify the start order is correct.

Tasks

Task 1: Project setup

mkdir -p ~/compose-health-lab && cd ~/compose-health-lab

Task 2: Compose file

# compose.yml
services:
  db:
    image: postgres:16
    environment:
      POSTGRES_DB: app
      POSTGRES_USER: app
      POSTGRES_PASSWORD_FILE: /run/secrets/db_pw
    secrets:
      - db_pw
    volumes:
      - pgdata:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U app -d app"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s

  api:
    image: myorg/api:1.0.0
    environment:
      DATABASE_URL: postgres://app:$(cat /run/secrets/db_pw)@db:5432/app
    secrets:
      - db_pw
    depends_on:
      db:
        condition: service_healthy
    healthcheck:
      test: ["CMD", "wget", "-qO-", "http://localhost:8080/health"]
      interval: 5s
      timeout: 3s
      retries: 5

  worker:
    image: myorg/worker:1.0.0
    environment:
      DATABASE_URL: postgres://app:$(cat /run/secrets/db_pw)@db:5432/app
    secrets:
      - db_pw
    depends_on:
      db:
        condition: service_healthy

secrets:
  db_pw:
    file: ./db_pw.txt

volumes:
  pgdata:

echo 'supersecret' > db_pw.txt
chmod 0400 db_pw.txt

Task 3: Bring up and observe

docker compose up -d
sleep 15
docker compose ps

The output shows the state for each service. The db should be healthy; api and worker should be running.

Task 4: Verify ordering with timestamps

docker compose ps --format json | jq -r '.[] | "\(.Service): started \(.State)\n"'

The db StartedAt should precede api and worker StartedAt.

Task 5: Force a failure

Stop the database:

docker compose stop db
docker compose ps

api and worker keep running (they are decoupled at runtime). But on next docker compose up, they will wait for db to be healthy before starting.

Task 6: Cleanup

docker compose down --volumes