Wednesday, November 28, 2007

Meta-data and RESTful web services

I'd like to think out loud about REST and URIs and ROA. The concepts behind REST are ideas I am really thinking about lately, but there some issues I find in reticifying it with my current software needs. Let's say I have a URL/URI:

which represents the addressable location of the item I am interested in (uniquely, and cleverly, identified by the identifier "123"). What happens when my service undergoes many updates and versioning needs to be introduced? I could add a version tag into the URI like this:

Although the question remains: is this a version of the resource, or of the mechnism via which I access that resource? Let's leave that question alone and move on to more practical matters.

Let's say I now need to indicate a return (mime) type. The resource may be available as a JSON string or an XML document. Naturally, or at least what we've been trained to do, is to stick a well-known extension to the end of the resource name, as in

and if I combine that with my versioning:

OK, this seems relatively reasonable, but what happens when I introduce another piece of meta-data, say locale? Where, on my last URI, do I "RESTfully" add this meta-data? After the mime extension?

in the URI itself?

This version seems a little weird, so let's skip that one and just consider locale at the end of the URI.

If we choose a convention for location of these points of meta-data and stick with it, we could be OK. But then what happens when doesn't care or assumes a default resonse type and drops the mime extension? The URI could becomes

So now the service needs to figure out if the "en_US" is a document/mime type or a locale definition. Yeah, you could create a bas-ass regex engine to derive all the possible variants, but would you really want to?

I think I could go on and on in this vein, adding in more meta-data points (although I don't really have any more at this time), but suffice it to say that if your needs are simple (you don't care about versioning or locale or any of that other accoutrement), you could probably go through life quite happy with our original URI

But when you need more expressiveness, I'm just not sure if making the URI bend over backwards (huge caveat) without creating your own, custom URI rules/syntax is going to get the job done in a reasonable, RESTful manner. I'm just not sure...

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