Mimetypes for a RESTful API

Question

The Sun Cloud API at http://kenai.com/projects/suncloudapis/pages/Home is a good example to follow for a RESTful API. True to RESTful principles, when you GET a resource you get no more nor less than a representation of that resource.

The Content-Type header in the response tells you exactly what the type of that resource is, for example application/vnd.com.sun.cloud.Snapshot+json. Sun has registered these mimetypes with the IANA.

How practical is this in general currently? Most API's I have seen have used the Content-Type of "application/json". This tells you that the response is JSON but nothing more about it. You have to have something in the JSON object, like a "type" property, to know what it is.

I'm designing a RESTful API (which will not be made public, therefore I wouldn't be registering mimetypes). I have been using RESTEasy and I find that even if I specify a complete mimetype, the Content-Type in the response header will be exactly what the Accept request header specified. If the request asks for "application/*+json" by default the response header will have "application/*+json". I can probably fix this by changing the header before the response goes out, but should I try to do that? Or should the response have a wildcard just like the request did?

Or should I just serve up "application/json" like most APIs seem to do?

Additional thoughts added later:

Another way of stating the question is: Should I use HTTP as the protocol, or should I use HTTP only as a transport mechanism to wrap my own protocol?

To use HTTP as the protocol, the entity body of the response contains the representation of the object requested (or the representation of an error message object), the "Content-Type" header contains the exact type of the object, and the "Status" header contains a success or error code.

To use HTTP as merely a transport mechanism, the "Status" header is always set to 200 OK, the "Content-Type" is something generic like "application/json", and the entity body contains something that itself has an object, an object type, an error code and whatever else you want. If your own protocol is RESTful, then the whole scheme is RESTful. (HTTP is a RESTful protocol but not the only possible one.)

Your own protocol will be opaque to all the transport layers. If you use HTTP as the protocol, all the transport layers will understand it and may do things you don't want; for instance a browser will intercept a "401 Unauthorized" response and put up a login dialog, even if you want to handle it yourself.

Answer 1

I use my own vnd.mycompany.mymediatype+xml media types for many of my representations. On the client I dispatch to the appropriate controller class based on the media type of the returned representation. This really allows the server to control the behavior of my client application in response to the user following a link.

Personally, I believe that using application/xml and application/json are one of the worst choices you can make if you hoping to support REST clients. The only exception to this is when the client only uses downloaded code (like Javascript) to interpret the data.