Idempotency Is Easy Until the Second Request Is Different

People talk about idempotency like it is a solved problem:

Put an Idempotency-Key on the request. Store the response. Replay it on retry.

And yes, that is doable. For the happy path, it is even fairly small.

The client sends:

POST /payments Idempotency-Key : abc-123 Content-Type : application/json

{ "accountId" : "acc_1" , "amount" : "10.00" , "currency" : "EUR" , "merchantReference" : "invoice-7781" }

The server checks whether it has seen abc-123 . If not, it creates the payment. If yes, it returns the previous response.

That version survives the demo.

The part I contest is that this is the hard part. It is not. The hard part starts with the second request, because the second request is not always a clean replay of the first one.

Maybe it is a completed replay. Fine. Return the stored result.

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