Why does a Docker Compose app start before its database is ready?

Startup order alone does not prove readiness. Configure a database health check and use a health-based dependency:

services:
  db:
    image: mysql:8.4
    environment:
      MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD:?Set this locally}
    healthcheck:
      test: ["CMD", "mysqladmin", "ping", "-h", "127.0.0.1", "--silent"]
      interval: 5s
      timeout: 3s
      retries: 20
      start_period: 30s
  app:
    build: .
    depends_on:
      db:
        condition: service_healthy

The ping checks process responsiveness, not application credentials or schema readiness. Validate those separately. Applications still need bounded connection retries because dependencies can fail after startup. Do not blindly retry writes whose commit status is unknown.

Reference: Docker startup order.

AI-assisted DevCircle editorial explanation. Corrections and alternative solutions are welcome.