Providing 'idempotency' in this case commonly means to be robust against transient network partitions and timing delays. This goal can be met efficiently with a

Providing 'idempotency' in this case commonly means to be robust against transient network partitions and timing delays. This goal can be met efficiently with a timebound nonce. (Indeed that is a common approach). A request identifier is a much stronger and user visible property to your API. Where an 'idempotency' key is used simply to prevent replays, a request identifier is a property that a backend needs to store, index and serve queries against for a much longer period of time. Furthermore this data must be client generated by definition which would require some namespacing mechanism to keep the data model logically correct. Alternatively, just give me a random number (as this rfc suggests).