Imagine you have a service which has more than one version. Let's say it's a hotel reservation system, just to pick something familiar. We've picked media types for our versions of the service, and adopted the convention that in the absence of any media types we will use "Version 1" of the service. Assume our media types are "application/hotel-json" and "application/hotel-json-v2".
If you support operations for updating a reservation using PUT, you can run into a situation where you have both a Content-Type header (specifying the content of the entity body being used to update the reservation) and an Accept header specifying the kind or kinds of entities the client can accept in the response. This might be just fine, if the two versions of the service can be intermixed between the sent entity and the returned entity. But the two versions of the service might not mix. For example, an update sent using version one of the updating entity might not contain a value required for version two of the service.
In this situation what the client did is an error, so we clearly need to return a status code in the 400 range. But which is the most appropriate code? A quick look at the 4XX codes shows two promising candidates:
406 - Not Acceptable
415 - Unsupported Media Type
But when we read the description of 415 in RFC 2616 we see it isn't appropriate:
The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method.The Accept header is specifying a media type the service understands for the resource, it's just the combination of provided entity media type and requested return media type that is a problem. Happily, status code 406 fits perfectly:
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.Interestingly, the RFC makes a comment about this very situation, and suggests that returning an entity type different than those specified in the Accept header might be preferable to a 406:
Note: HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable to sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable.In this particular scenario (where a required value isn't provided), one way to return a result would be to "downgrade" to a version 1 response in hopes that works for the client. There are potential issues with this, if the semantics of applying a version 1 update differ from those that would have occurred with a version 2 update. And if the client can't interpret the returned version 1 entity, they may end up in an inconsistent state. But it's a good idea to keep in mind and make work whenever possible.