Idempotency = same request run N times ⇒ same effect & same response.
API layer pattern (HTTP)
1) Contract
Require Idempotency-Key on POST that creates resources (optional on GET, safe methods).
For PUT/PATCH, prefer natural keys (resource URI) + concurrency (ETag) over custom keys.
Return Idempotency-Replayed: true on replays.
Network requests fail. Timeouts happen. Clients retry. And without idempotency keys, a single payment...
You hit "Pay" and the browser spins. Network timeout. Did the charge go through? You retry — and get...
You hit "Submit Order" and the network drops mid-request. Did the charge go through? Should you...
Claim, intent, memory and boundary: Four assumptions an idempotency key rests on.
Idempotency is one of the most important concepts in backend engineering, especially for SaaS...
Retries in payment and order APIs are a classic footgun. Your client times out, retries the request,...
