I'm generally a fan of RESTful API design, but I'm unsure of how to apply REST principles for a validation API.
Suppose we have an API for querying and updating a user's profile info (name, email, username, password). We've deemed that a useful piece of functionality to expose would be validation, e.g. query whether a given username is valid and available.
What are the resource(s) in this case? What HTTP status codes and/or headers should be used?
As a start, I have
The type of thing you've described is certainly more RPC-style in its' semantics, but that doesn't mean you can't reach your goals in a RESTful manner.
VALIDATE HTTP verb, so how much value can you get from structuring an entire API around that? Your story centers around providing users with the ability to determine whether a given user name is available - that sounds to me like a simple resource retrieval check -
GET: /profile/username/... - if the result is a 404, the name is available.
What this highlights is that that client-side validation is just that - client side. It's a UI concern to ensure that data is validated on the client before being sent to the server. A RESTful service doesn't give a whit whether or not a client has performed validation; it will simply accept or reject a request based on its' own validation logic.
REST isn't an all-encompassing paradigm, it only describes a way of structuring client-server communications.