Idempotency and retry storms — what QA must test in distributed systems
The most expensive class of bugs in payments isn't 'didn't go through' — it's 'went through twice'. A QA-eye view of idempotency: Idempotency-Key, 5 typical retry scenarios, retry storms, tools (WireMock, Toxiproxy, k6), and a 13-point release checklist.