Reliability

Idempotency

Doing the same operation twice has the same effect as doing it once.

An idempotent operation can safely be repeated without causing extra damage — if a payment request is retried after a dropped connection, the customer is still charged only once. The mechanism is usually an idempotency key: a unique ID sent with the request so the server recognizes a duplicate and returns the original result instead of re-running it. This matters everywhere networks are flaky and jobs retry — double-charges, duplicate emails, and doubled inventory are all missing-idempotency bugs. Stripe's API is the canonical example: you pass an `Idempotency-Key` header so a retried charge doesn't bill twice, and you should copy that pattern for your own money- or side-effect-heavy endpoints.