Idempotency: the engineering method for preventing double payments
By Selim Destanci
Founder
Payment APIs live in an unreliable world: timeouts, retries, and impatient users all create duplicate requests. Without protection, a single intended charge can be executed twice.
An idempotency key is a client-supplied identifier attached to a request. The server records the first result for that key and returns the same result for any repeat — so retries are safe by construction.
The hard part is concurrency. Two requests with the same key can arrive simultaneously; a distributed lock plus a persisted result store ensures exactly one execution wins and the rest read the stored outcome.
Done well, idempotency turns a fragile money operation into a safe, replayable one — the foundation every reliable payment system is built on.