Is returning HTTP 409 appropriate for a validation check?
You've sent a request to the server for it to perform validation. It has successfully performed said validation. From an HTTP perspective, the request was well formed and correctly processed by the server.
So I'd say returning any HTTP error code would be incorrect.
This answer continues to receive downvotes and I'm not entirely sure why (none of the downvoters seem to leave any comments). Through a fair amount of back and forth with the OP, we established that the entire point of this request/response was to perform validation. The server received the request, it performed the validation that it was requested to perform, and it returned the results of that validation process to the caller.
There was absolutely nothing wrong with the client sending this request.
The server understood the request.
The request was valid (from an HTTP perspective).
The server could process the request.
The server performed 100% of the activity it was meant to and is returning the results that are produced having processed the request.
And that is why, as I say, I do not believe that an HTTP error code is appropriate.
I.e. imagine that the server exposes an endpoint that validates email addresses (for whatever particular form you wish to say that validation can be performed). It receives a request saying "validate [email protected]" and it produces a response saying "I took a look at this email address and I'd like you to tell the user that I can't get a valid DNS response for invalid.org". If people don't think a 200 response is correct here, I'd love to understand their reasoning.
While it is defined in a proposed standard still, 422 Unprocessable Entity is an appropriate status.
The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions.
For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions.
References:
- https://www.rfc-editor.org/rfc/rfc4918#section-11.2
- http://developer.github.com/v3/#client-errors
- https://stackoverflow.com/a/2657624/247702
- http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error