Is returning HTTP 409 appropriate for a validation check?

Question

I have a service where some validation rules must be checked before a particular operation should be able to take place.

For instance, the client should not generate

Answer 1

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://tools.ietf.org/html/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