Asymmetric network partition in cluster
Your stateful cluster spans AZ-a, AZ-b, AZ-c. At 02:20 you see a confusing 'gray' failure: AZ-b can still reach AZ-a and AZ-c (its outbound is fine and health checks it initiates pass), but AZ-a and AZ-c can no longer reach AZ-b — packets from them to AZ-b time out. Dashboards: AZ-b nodes look healthy from their own vantage and keep reporting themselves up; the cluster keeps a leader in AZ-b but followers in a/c can't replicate from it; clients pinned to AZ-b succeed while others see write stalls and timeouts. No deploy; cloud provider reports a network event in one AZ. How do you triage and mitigate?
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.