[amluto]

I will add: if you have an operation that adds a record to a database (like a payment in the OP example), don’t forget to have some field that the client specifies that can be used to find and query the status of that record later. This field can be the idempotency key itself or another field.

Or you can completely forget this feature and make it really awkward for the client to reconcile their view of the world with yours and/or to check in the request later. cough Mercury cough.

It is, just barely, acceptable to generate the identifier server side and return it to the client.