Shoot-cluster-unreachable state during transient infrastructure API failures

Direct answer

Use a for: clause on the Prometheus alert rule that fires when a shoot enters shoot-cluster-unreachable. Gardener does not provide a native controller flag to delay the state transition based on external infrastructure provider health; the transition is driven by the shoot’s own health-check interval and timeout.

What is confirmed vs. what is likely

Confirmed behavior: When the Gardener controller’s health check for a shoot’s API server fails within the configured timeout, the shoot condition is set to Unreachable with reason shoot-cluster-unreachable. Prometheus can suppress short-lived flaps by requiring the alert expression to remain true for a minimum duration.

Likely explanation for your symptom: Transient provider API throttling or latency spikes cause repeated health-check failures. Because the controller re-evaluates the condition on each reconciliation loop, the shoot can transition into and out of the unreachable state faster than your alerting pipeline can filter it out.

Uncertain / version-dependent: Whether a specific Gardener release has introduced provider-health gating for this transition. Health-check behavior may differ in newer versions; verify against your installed release before assuming a controller-side delay exists.

Recommended Prometheus configuration

Add a for: duration to the existing alert rule. The exact value depends on your provider’s typical recovery window. Start with a conservative value and tune it after observing at least two provider API recovery cycles.

- alert: ShootClusterUnreachable
  expr: <metric reflecting shoot unreachable condition> == 1
  for: 5m
  labels:
    severity: warning
  annotations:
    summary: "Shoot {{ $labels.shoot }} is unreachable"

Avoid an overly short for: value (e.g., 30s) because it will not absorb provider-side timeouts. An overly long value (e.g., 30m) delays genuine incident detection.

Safe verification steps

1. Inspect the shoot’s current condition and last transition time:

   kubectl get shoot <shoot-name> -n <project-namespace> \
     -o jsonpath='{.status.conditions[?(@.type=="Unreachable")]}{"\n"}'

2. Check Gardener controller logs for health-check events and transition timestamps to see whether the unreachable state persists longer than your for: duration.
3. Apply a temporary Alertmanager silence on the shoot-cluster-unreachable label while you adjust the for: value, then observe alert frequency across at least two provider API cycles.
4. Cross-reference the alert timeline with your cloud provider’s status dashboard to confirm that the flap correlates with provider-side latency rather than a real cluster outage.

One missing detail that changes the recommendation

What is the typical duration of your provider’s transient API failures? If recovery usually completes in under two minutes, a for: 5m threshold is appropriate. If failures routinely last ten minutes or more, raise the for: value accordingly, or route the alert to a lower-severity channel until the provider recovers.