The Idempotency-Key HTTP header field
Googler, opinions are my own.<p>I work in payment and idempotency is always a discussion point.  This draft links to one of our docs (though the draft points to a 404 URL, the correct link being[0]).<p>As the draft calls out, Google Standard Payments uses the "requestId" field as our idempontency key, which is part of the request body, rather than in the HTTP header.  We did this specifically to decouple the protocol from the behavior of the request.  We could take the existing definitions and put that payload into any other protocol and it would work without having to use that protocol's sidechannel data (aka, the HTTP header).<p>Our design makes idempontency part of the application layer, rather than any middlebox or load balancer.<p>[0] <a href="https://developers.google.com/standard-payments/guides/connectivity/protocol-standards#request_idempotency" rel="nofollow">https://developers.google.com/standard-payments/guides/conne...</a>