Idempotency key.

also: idempotency keys · Idempotency-Key · idempotency record · idempotency records · idempotency store

A unique ID the client sends with each logical operation; the server stores it with the result and replays that result on retries.

the two-minute refresher

The coat-check ticket: hand in the coat once, show the ticket as often as you like.
First request: store key → 'processing', do the work, store the result. Retry: return the stored result.
Pre-register the key BEFORE calling the bank, so a timeout leaves a record you can reconcile.
Hash the request body too: the same key with a different amount must be rejected.
Only completed records block retries: 'processing' makes the caller wait, 'failed' lets a retry run.

also: idempotency keys · Idempotency-Key · idempotency record · idempotency records · idempotency store

A unique ID the client sends with each logical operation; the server stores it with the result and replays that result on retries.

The coat-check ticket: hand in the coat once, show the ticket as often as you like.
First request: store key → 'processing', do the work, store the result. Retry: return the stored result.
Pre-register the key BEFORE calling the bank, so a timeout leaves a record you can reconcile.
Hash the request body too: the same key with a different amount must be rejected.
Only completed records block retries: 'processing' makes the caller wait, 'failed' lets a retry run.