Kubernetes 'Readiness probe failed' (Unhealthy event) and pods never become Ready (0/1 READY). How do I debug it?

Cause: the kubelet runs the readiness probe and it keeps failing, so the pod's Ready condition stays False. The container is not restarted for this (liveness failures cause restarts), but the pod is removed from Service endpoints and rollouts stall. Typical causes: the app needs longer to start than the probe allows, the probe path or port is wrong, the app returns a non-2xx/3xx status (for example 503 while a dependency is down), the probe timeout is too short for a slow endpoint, or the app listens only on localhost so the kubelet cannot reach it. Fix (best first): 1. See the failure message: kubectl describe pod POD_NAME. The Unhealthy event shows the cause, for example an HTTP status code, connection refused or timeout. Also check kubectl logs POD_NAME. 2. Test the same endpoint from inside the pod (kubectl exec) or via kubectl port-forward, using the path and port from the probe. 3. Fix mismatches: probe port must be the one the container listens on, the app must bind to 0.0.0.0 (or the pod IP), and httpGet succeeds only for status 200-399. 4. For slow starters add a startupProbe with a generous failureThreshold times periodSeconds; liveness and readiness probes are held off until it succeeds. This is better than a huge initialDelaySeconds. 5. Adjust timing if the endpoint is just slow: raise timeoutSeconds (default 1 second), periodSeconds or failureThreshold modestly. 6. Make the readiness endpoint reflect only what this pod needs to serve traffic. If it checks a shared downstream dependency, an outage there makes every pod unready at once. 7. Do not remove the readiness probe just to turn the pod green; it would then receive traffic before it can serve it. Verify: kubectl get pods shows READY 1/1, the Ready condition is True (kubectl get pod POD_NAME -o jsonpath='{.status.conditions}'), and the pod appears in kubectl get endpoints for its Service. Sources: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/ https://kubernetes.io/docs/tasks/debug/debug-application/debug-pods/