Docker Compose: my app container starts before the database is ready and fails with 'connection refused' (depends_on doesn't wait). How do I make it wait for Postgres/MySQL to be ready?

Cause: plain depends_on only controls start order: Compose starts the database container first, but the database process inside may need several seconds (or longer on first start, while it initializes) before it accepts connections. The app starts immediately, tries to connect, fails and often exits or crashes. depends_on with the default condition (service_started) waits for 'running', not 'ready'. Fix: 1. Give the database a healthcheck that tests real readiness, e.g. for Postgres: healthcheck: test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"], with interval, timeout, retries and start_period. ($$ stops Compose from substituting the variables itself.) For MySQL/MariaDB use mysqladmin ping or the image's healthcheck script. 2. Make the app wait for it: depends_on: db: condition: service_healthy. Compose then won't create the app until the healthcheck passes. 3. Add restart: true under that depends_on entry if the app should also restart when Compose restarts the database. 4. For one-off setup steps (migrations, seeding), run them as a separate service and depend on it with condition: service_completed_successfully. 5. Still make the application itself retry database connections with backoff: in production (Kubernetes, cloud) there's no Compose to order startup, and databases can restart at any time. Verify: docker compose ps shows the database as healthy before the app starts, and docker compose up from scratch no longer shows connection-refused errors in the app logs. Sources: https://docs.docker.com/compose/how-tos/startup-order/ https://docs.docker.com/reference/compose-file/services/