Bear in mind the more things like "Well, you just have to properly use the content negotiation headers" you add to your API, the more the "simplicity" argument of REST goes out the window. I'm not sure I've ever seen really correct use of the content-negotiation headers and claiming it's a solution is dubious... sort of like how I'm still waiting to see proper use of XML Namespaces in the wild. They seem relatively simple to me but the evidence suggests strongly otherwise.

That doesn't make REST "bad", but I think it does start to make the quasi-religious aspect of it less obviously correct.

That's part of my reasoning. Setting proper headers is, yes, more proper... and something essentially nobody does, and many simple API libraries don't support without making using them massively more complicated than prefixing all the URLs with "api/2".

It's a correctness vs usability debate. I'd love to go for correct, but it's extremely painful to enforce in your users in practice.

Using content negotiation, I can--if the server supports it--take the same URI (e.g. http://www.jerf.org/) and pass it to an image viewer to see your face, to my address book to fetch your contact info, etc.

Anything else requires the client knowing how to mangle URIs (e.g. adding /avatar.jpg, /contact.vcf).

I know what content negotiation is; I've never seen it done right. The theory is all beautiful but in practice content negotiation never seems to work right because you never get two people to agree very well on what things are. Another manifestation of the usual semantic classification problem.

It has a dubious track record in X as well. X has been able to do content negotiation for decades for the clipboard but even now it's still spotty and buggy, and that's with everything actually running on the same system. It looks easy. Heck, it looks very easy. It isn't. I can't even tell you exactly why it isn't as easy as it looks (beyond the semantic thing I mentioned above), but the state of the world is pretty clear.

What, realistically, is the difference between knowing how to mangle URIs vs. knowing what content types are available for negotiation?

@awj: if your API does HATEOAS properly, so that your responses inline URLs to other related resources (aiding discoverability of your API through hypertext), then the client can naively take the URLs given to it and cleanly apply Accept headers to them as appropriate.

if you don't want to use content negotiation, then stuff it into query parameters - that is what most API's do (and most that support full content negotiation do anyway)

eg.?v=1&format=xml

but interesting to note is that why is encoding easy to do as part of content negotiation, yet everything else is hard? you never see ?e=UTF-8 or &c=gzip

Many APIs simply specify that the encoding is constant (always UTF-8) so this is not an issue. And many HTTP libraries simply handle the content encoding transparently.

The challenge is some HTTP library APIs commonly used for developing clients offer the following interface: - GET $URL

If you are lucky, then it might also support POST and offer a second parameter for the request body. If are really lucky, there is a way to specify arbitrary methods, but this is fairly rare.

I'm all in favor of supporting fallbacks for more limited clients. But query params are not a great idea since they can prevent caching from happening.

Add a random number to the end as a param to force no-caching.

My point was that it's not a good thing for requests for cacheable content, that might not be cached since you used a query param to access it. Some caches ignore requests with query params since "queries" are often not static.