I'm currently returning 401 Unauthorized whenever I encounter a validation failure in my Django/Piston based REST API application. Having had a look at the HTTP Status Code Registry I'm not convinced that this is an appropriate code for a validation failure, what do y'all recommend?
Update: "Validation failure" above means an application level data validation failure, i.e., incorrectly specified datetime, bogus email address etc.
If "validation failure" means that there is some client error in the request, then use HTTP 400 (Bad Request). For instance if the URI is supposed to have an ISO-8601 date and you find that it's in the wrong format or refers to February 31st, then you would return an HTTP 400. Ditto if you expect well-formed XML in an entity body and it fails to parse.
(1/2016): Over the last five years WebDAV's more specific HTTP 422 (Unprocessable Entity) has become a very reasonable alternative to HTTP 400. See for instance its use in JSON API. But do note that HTTP 422 has not made it into HTTP 1.1, RFC-7231.
Richardson and Ruby's RESTful Web Services contains a very helpful appendix on when to use the various HTTP response codes. They say:
400 (“Bad Request”)
Importance: High.
This is the generic client-side error status, used when no other 4xx error code is appropriate. It’s commonly used when the client submits a representation along with a PUT or POST request, and the representation is in the right format, but it doesn’t make any sense. (p. 381)
and:
401 (“Unauthorized”)
Importance: High.
The client tried to operate on a protected resource without providing the proper authentication credentials. It may have provided the wrong credentials, or none at all. The credentials may be a username and password, an API key, or an authentication token—whatever the service in question is expecting. It’s common for a client to make a request for a URI and accept a 401 just so it knows what kind of credentials to send and in what format. [...]