Wait until healthy: depends_on

Morning rush. The API starts too early. It calls a database that is still waking up. Tap each moment.

  web:
    depends_on: [api]
  api:
    depends_on: [db]

depends_ondepends_on: Who waits for whom. With condition: service_healthy, a dish waits until the other passes its health check. sets the start order. A dish starts after the dishes it depends on. Put the start order right.

  api:
    depends_on:
      db:
        condition: service_healthy
  db:
    image: postgres:18-alpine
    healthcheck:
      test: ["CMD", "pg_isready", "-U", "postgres"]
      interval: 2s
      retries: 10

~/kedai-kancil $ docker compose up -d[+] up 5/5 ✔ Network kedai-kancil_default Created                                     0.1s ✔ Volume kedai-kancil_db-data  Created                                     0.0s ✔ Container kedai-kancil-db-1  Healthy                                     0.3s ✔ Container kedai-kancil-api-1 Started                                     0.3s ✔ Container kedai-kancil-web-1 Started                                     0.3s

Started is not ready. A health check is a taste test. pg_isready asks db: ready yet? service_healthy makes api wait for a yes.

Right now api waits only until db has started. Make it wait until db is healthy. Then order the set meal.

1services:
2  web:
3    build: ./web
4    ports:
5      - "8080:80"
6    depends_on:
7      api:
8        condition: service_started
9  api:
10    build: ./api
11    environment:
12      DB_HOST: db
13    depends_on:
14      db:
15        condition: service_started
16  db:
17    image: postgres:18-alpine
18    environment:
19      POSTGRES_PASSWORD: example
20    volumes:
21      - db-data:/var/lib/postgresql
22    healthcheck:
23      test: ["CMD", "pg_isready", "-U", "postgres"]
24      interval: 2s
25      retries: 10
26volumes:
27  db-data: {}

Someone deleted db’s password. Postgres exits at once. What does docker compose up -d do now?

Run it and read the red line. Then ask db’s logs why.

Real words: dependency failed to start. Plain words: db stopped, so the dishes waiting for it never started.

You can now make a dish wait until another is healthy.