The app cannot reach the database container. What should I check?

Definition:

Each container has its own network context, so localhost points back to that container rather than a sibling service. Reaching a database requires the correct address, network path, credentials and an accepting database process. Compose startup order can start a process without guaranteeing it has finished initialization.

Key points:

• Use the database service name, not localhost, between containers.

• Check shared networks, ports, credentials and database readiness.

• Startup order alone does not guarantee the database is ready to accept queries.

• Use bounded connection retries or readiness coordination, and confirm the database listens on an interface reachable from the app.

Interview point:

• Troubleshoot DNS, TCP reachability and login separately rather than changing credentials for a network failure.