rest - Idempotent POST methods header in response

Question

I'm implementing an idempotent POST method. So in the request, I have a mandatory header X-Request-Id.
On the backend, I'm checking if there is a recourse with the same X-Request-Id system is returning the existing one and not creating a new resource.

My question is. Is there any standard response header, which should be returned in case of an already created resource?

question from:https://stackoverflow.com/questions/66058939/idempotent-post-methods-header-in-response

Answer 1

I don't think there are any standards around that.

If, from your design POV, this should denote a success (albeit a special one), I'd go with 201 (ie CREATED) for the first invocation and 200 (ie OK) for the subsequent ones.

If you see it as an error (I'd find it odd though) then I'd pick 200 (ie OK) for the first call and 409 (ie CONFLICT) for the subsequent ones.

No matter which one you choose, the most important thing, as you already know, is to be consistent and predictable about that in your API.

MDN on status codes: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status