Per my earlier note, I have evaluated my feelings on the units of REST:
1. An architecture is defined by its constraints - Strongly Support
2. Separate client from server - Strongly Support
3. The client / server relationship must remain stateless - Support
4. The client must be able to cache data - Support
5. Uniform Interface - Support
5.1 Identification of Resources - Strongly Support
5.2 Manipulation of Resources through Representations - Strongly Support
5.3 Self Descriptive Messages - Strongly Support
5.4 Hypermedia as the engine of application state - Support
6. Layered System - Support
7. Code on Demand - Don't Support
REST is a single candidate architecture. And an architecture must be evaluated on the whole, rather than the individual parts. Fielding does a great job of stating that every decision in the candidate architecture has trade-offs. He further identifies the applicability of the architecture (distributed systems, system 'feels' stateless, desire to scale, desire to evolve, etc.) Like all candidate architectures, one can only evaluate the architecture against a problem that you are trying to solve. REST by itself can not be evaluated, only evaluated as a candidate solution to a pre-identified problem.
What was interesting to me was the lack of association between REST and HTTP, or more precisely the 'verbs' of HTTP. Instead, Fielding places significant attention on the need to use Resources (think URI), Self Descriptive Messages (think XML), but does imply that a generic interface is required. Somehow... I find myself agreeing with Fielding but disagreeing with many RESTifarians (I'm not sure how this is possible). Fielding suggests a "a uniform interface between components ", however he never mandates what that interface should be - only stating the characteristics. However, I find significant 'folklore' stating that the interface is, "...defined completely and solely by the specified semantics of HTTP, i.e. GET / PUT / POST, etc." (Jeff Bone) He continues to argue, "*there are no applications you can think of which cannot be made to fit into the GET / PUT / POST / resources / representations model of the world!*" The only issue I have is his term "made to fit" - meaning, what was the price of 'making something fit'?
So, with all of this said, where am I landing?
1. REST is a great example of a candidate architecture and should be evaluated based on the problem at hand.
2. REST doesn't dictate HTTP or the HTTP verbs and this is a good thing.
3. REST does state that the interface should be uniform, and this too is a good thing.
4. The base HTTP application semantics can be 'forced' to solve any problem, they just aren't always a good fit.
5. Additional exercises should be performed to document how many RPC style verbs would be translated into a more REST-stlye. Verbs that don't translate shouldn't be forced to fit, rather new verbs should be introduces as part of the first-order vocabulary.