Glossary
Idempotency
Same request twice, same effect once.
An idempotent operation returns the original result when retried with the same key instead of executing twice. Email APIs implement it via an Idempotency-Key header: the first request stores its result durably; repeats within the window replay it.
Without it, every network timeout forces a gamble — retry and risk two receipts, or don't and risk zero. With it, clients retry freely on timeouts and 5xx responses.
In Calder: Idempotency at Calder