Circuit Breakers Under Stress: Anatomy of a Payment Cascade

A flash sale hit us at 10x baseline RPS. Within four minutes, our Payment Service circuit breaker tripped to OPEN, error rate climbed to 92%, and p99 latency on the payment path went from 200ms to 14.2 seconds. Here's the part nobody tells you on the conference circuit: the circuit breaker didn't fail. It worked exactly as designed. The failure was everywhere else. This is a postmortem of what we saw, why Resilience4j's defaults weren't enough, and the four changes that made the next sale borin...