How can I make Docker Compose wait for MySQL to be ready before starting my app?

This is a startup-readiness race, not necessarily a mistake in your setup. `depends_on` can control the order containers are started, but the MySQL process may still be initializing when the app tries to connect.

Add a health check to the database and make the app depend on the database becoming healthy:

```yaml
services:
mysql:
image: mysql:8
environment:
MYSQL_ROOT_PASSWORD: secret
MYSQL_DATABASE: app
healthcheck:
test: ["CMD-SHELL", "mysqladmin ping -h 127.0.0.1 -uroot -p$$MYSQL_ROOT_PASSWORD --silent"]
interval: 5s
timeout: 3s
retries: 20
start_period: 20s

app:
build: .
depends_on:
mysql:
condition: service_healthy
restart: on-failure
```

Also make sure the app connects to `mysql:3306`, using the Compose service name, rather than `localhost` or a changing container IP. The health check handles initial startup, while the app should still retry connections if MySQL later restarts or becomes temporarily unavailable.

For troubleshooting, sharing the complete Compose file is useful because connection settings, service names, and environment variables can also cause this error. But based on the logs and the fact that a second startup works, the database initialization race is the most likely explanation.

The workshop's repository version includes a MySQL health check and a `service_healthy` dependency, although the written instructions apparently don't explain that part. You can inspect the database with `docker compose ps`, `docker compose logs mysql`, and `docker inspect` to confirm whether its health check is passing.