[w0utert]

> The core problem in my mind, is not whether one should insert or update on PUT/POST, but whether one should even be considering HTTP-based APIs to be gateways to what amounts to a database.

I think it's widely recognized that you shouldn't consider making your REST API a direct gateway to a database. For one, it's potentially very dangerous, but it is also limiting the design of your API, because typical databases basically have no way to insert anything besides rows of data with fixed columns (relational) or raw untyped data (NoSQL).

I think the 'purest' form of REST API's for creating and updating is that PUT should be idempotent and used for existing resources addressed by an existing URI, while POST can basically create resources any way it sees fit, and return the URI of the new resource. This allows doing a POST followed by a PUT to create and initialize a resource, without having to know how and where the resource will be created, and without requiring to set each and every attribute of the resource with the POST request.