It doesn't solve the problem that RPC leaves the definition of the verbs (ie the procedures) and how they modify state of a common thing undefined. If I call an

It doesn't solve the problem that RPC leaves the definition of the verbs (ie the procedures) and how they modify state of a common thing undefined. If I call an RP twice, what is the effect? How do I know it succeeded? What happens if it fails? etc etc None of these things can be communicated through an IDL definition. HTTP solves this problem by strictly defining the operation of its verbs (HEAD/OPTIONS/GET/PUT/POST/DELETE/PATCH) in terms of idempotency, caching, identification etc. Communicating the structure of the things that you are manipulating in REST over HTTP is done by defining the media types that you expect and respond with. Content identification and headers in content/connection negotiation define the versions and formats of the content of requests and responses.