Idempotency (멱등성)

한국의 일반적인 경우는 동일한 결제 요청이 두번 들어오면  이중 결제가 발생 하거나 
혹은 기 결제 건으로 실패 응답을 주는 경우가 많은데요. 

알리페이나 구글 같은 외국 회사들은 유일한 requestId  값을 이용해서 이미 처리된 건의 재요청은  
이전과 동일한 응답을 주도록 하고 있네요.

구글의 standard-payments api 문서 중 

https://developers.google.com/standard-payments/

Introduction  |  Standard Payments API  |  Google Developers

Idempotency

All method calls within this API must have idempotent behavior. Simply put, integrators should not attempt to reprocess any request already successfully processed; the response for the successful processing should be reported instead. All methods have a common RequestHeader which contains a requestId. This requestId is the idempotency key for all calls.

For any non terminal answer (a non HTTP 200), it must not be idempotently processed. So a request that previously got a 400, when called a second time must not idempotently return 400, it must be re-evalutated. At re-evaluation, it could return a 400 or be processed successfully.