Idempotency
An operation is idempotent when doing it twice has the same effect as doing it once. Reads are idempotent by nature; a card charge or an order is not. An idempotency key is a unique ID the client sends with a request, so the server can recognize a retry and return the first result instead of acting again.
Reliability
In Quorum#
A checkout sends a charge to the Payment Provider. In the Double charge Incident the provider slows by about 1.5 s, the charge Link times out, and a retry charges a card that the first attempt already charged, so those checkouts fail as duplicate. Research Idempotency keys (it needs Checkout) so a retried charge never charges twice. Without it, set the charge Link's retries to none to end the Incident.
Read next#
Retry with backoff and jitter, At-least-once delivery, Timeout, Transactional outbox