The Request Timed Out, But the Payment Succeeded: Building Retry-Safe Mobile APIs

Mobile networks are notoriously unreliable. A common scenario is when a user taps "Pay Now" in an app: the payment request reaches the server and is processed, but the network response never reaches the phone. The client assumes the request failed and retries, leading to the charge running twice. This is precisely the kind of bug that idempotency solves.

Idempotency means that repeating the same operation has no additional effect, and the second attempt should recognize it's a duplicate and do nothing new. In practice, mobile engineers must treat payment or order APIs as idempotent by attaching unique operation identifiers to requests and deduplicating them on the backend. With this approach, even if the network drops a response or the user double-taps a button, the user is charged only once.