Recap: Idempotency Keys

A key turns a write that is not idempotent into one that is, if it is stored before the work starts.

A timeout hides whether a write happened, and a key makes the retry safe either way.

Same write twice comes from timeouts, lost responses, and retries by libraries, proxies and queues.
POST and PATCH need a key when repeating them would cost something; GET, PUT and DELETE do not.
The key is a random value per operation, sent in Idempotency-Key, reused on every retry.
The server records the key before acting and returns the stored response to a repeat.
Scope, expiry and fingerprint: per client, for a stated window, bound to the first request.
Misuse: 422 for a different body, 409 while the first is running, 400 for a missing key.
The race: record the key with an atomic insert, or two copies both act.

A keyed retry, with the atomic record of the key

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