RFC 5789, "PATCH Method for HTTP", March 2010Source of RFC: IETF - NON WORKING GROUP
Area Assignment: app
Errata ID: 3169
Reported By: Mark Nottingham
Date Reported: 2012-03-28
Verifier Name: Pete Resnick
Date Verified: 2012-03-29
Section 2 says:
If the operation does not modify the resource identified by the Request- URI in a predictable way, POST should be considered instead of PATCH or PUT.
It should say:
If the operation does not modify the resource identified by the Request- URI in a predictable way that's defined by the semantics of the PATCH media type, POST should be considered instead of PATCH or PUT. [Also, I suggest adding this to section two, after the sixth paragraph:] The means of applying a PATCH request to a resource's state is determined by the request's media type. If a server receives a PATCH request with a media type whose specification does not define semantics specific to PATCH, the server SHOULD reject the request by returning the 415 Unsupported Media Type status code, unless a more specific error status code takes priority. In particular, servers SHOULD NOT assume PATCH semantics for generic media types that don't define them, such as "application/xml" or "application/json". Doing so will cause interoperability issues, because the semantics of PATCH become specific to that resource, rather than general.
RFC5789 does not explicitly tie PATCHing semantics to the media type of the request. This was well understood in the discussions around the document, and can be read between the lines in it, but it doesn't come out and say it.