An Envoy proxy returning "no healthy upstream" does not mean your backend service has crashed. In many distributed systems, it indicates that healthy backend workers are saturated by slow downstream dependencies, causing health checks to queue and fail.
During one production incident, backend p99 latency formed a sharp cliff that plateaued at the load balancer timeout. The underlying application was neither throttling CPU nor crashing. Instead, slow datastore calls combined with retries tied up every worker thread in the fixed pool. Because health check endpoints shared the same worker queue, incoming probes timed out, prompting the proxy to eject running pods.
Dashboards displaying low CPU utilization and zero crash alerts often mask worker starvation. When deep dependencies stall, synchronous backends fail by dropping their own heartbeats.
Always isolate health check handlers from heavy worker pools to avoid cascade ejections.




