[rasteau]

> what happens if two users modify different fields/properties of the same resource at the same time?

https://tools.ietf.org/html/rfc7231#section-6.5.8

   6.5.8.  409 Conflict

   The 409 (Conflict) status code indicates that the request could not
   be completed due to a conflict with the current state of the target
   resource.  This code is used in situations where the user might be
   able to resolve the conflict and resubmit the request.  The server
   SHOULD generate a payload that includes enough information for a user
   to recognize the source of the conflict.

   Conflicts are most likely to occur in response to a PUT request.  For
   example, if versioning were being used and the representation being
   PUT included changes to a resource that conflict with those made by
   an earlier (third-party) request, the origin server might use a 409
   response to indicate that it can't complete the request.  In this
   case, the response representation would likely contain information
   useful for merging the differences based on the revision history.

Conflict resolution is inherently a consequence of the application-specific semantics of a "conflict". If the notion of "conflict" exists -- i.e., it is not obviated by application design[1] -- then conflict resolution is pushed to the caller.

That said, it may be reasonable to develop reusable conflict resolution mechanisms for reusable definitions of "conflict", allowing for composition of separate concerns as needed. This seems preferable to inventing a new system that binds concerns together with a single, irrevocable notion of "conflict".

[1] Using PATCH does not obviate conflicts that the application design otherwise allows.