RESTful API: What METHOD/HEADER combo to use for validation-only

Question

I would like my API to have a validation-only request. For example, if I have a URL such as:

http://api.somesite.com/users/12345

and the user is filling out a form of information on a client that I will eventually PATCH/PUT/POST to that resource. As the user is filling out the form, I might want to send over their partially-complete updated representation over to the server periodically so I can display realtime validation of their input (e.g., "That username is already taken", "That password is too short").

There isn't a standard HTTP METHOD or HEADER that seems to allow for this behavior on that same resource. It seems my options are:

1. Create a new subordinate resource for validation
2. Use a custom header (x-somesite-validation-only) and PUT indicating that I want to validate but not save

Answer 1

Some options

1) Use custom header
2) Put something in the query string indicating to validate only
3) Use Action URl e.g. \IndividualClient\123\actions\Validate\Invoke {section 19 here http://restfulobjects.files.wordpress.com/2011/11/restful-objects-spec-052.pdf}
4) Hierarchical URL e.g. \IndividualClient\123\Validation

From this post I find this advice

Do use POST whenever you have to do something that feels RPC-like Do use GET for things like calculations, unless your input is large, in which case use POST

With regard to your specific question, POST should be used for #4 and #5. These operations fall >under the "RPC-like" guideline above. For #5, remember that POST does not necessarily have to >use Content-Type: application/x-www-form-urlencoded. This could just as easily be a JSON or CSV >payload.

Here is what I'm considering:

This is the add of a resource :
user/validation
POST
Request:UserResource
Response:ValidationResult
Response Codes 200, 400. 404. 500

This is the update of a resource
user/204/validation
POST
Request:UserResource,
Response:ValidationResult Response Codes 200, 400. 404. 500