Why Your Container App Fails After Deployment (And How to Debug It)

It is one of the most frustrating developer experiences: your Docker build completes successfully, the platform reports a deployed status, but visiting the URL returns a 502 Bad Gateway or a timeout error. Tailing the container logs is your first line of defense.

1. The Port Binding Mismatch

The single most common cause of container boot failures is port mismatch. If your container internally binds to port 5000, but your deployment settings expect port 8080, the ingress load balancer will fail to reach your application, resulting in a 502 Gateway error.

Fix: Double check your application startup scripts and ensure the target port option matches your configuration.

2. Missing Environment Variables

If your application requires a database connection string (e.g., DATABASE_URL) to boot, and it is missing or incorrectly formatted, your container process will throw an unhandled exception and exit immediately.

Fix: Verify all database credentials, API keys, and environment variables are mapped in your application settings tab before deploying.

3. CrashLoopBackOff and OOMKilled

If your container starts but exits immediately (CrashLoopBackOff) or consumes more RAM than allocated (OOMKilled), the orchestrator will shut down and restart the pod repeatedly.

Fix: Inspect your runtime logs for uncaught exceptions, and adjust your application resource allocations (CPU/Memory limits) to support the load requirements.