Can you show me where this definition of a "URL" vs. a "URI" is made in any RFC or standard-like document?
Sure, we have a _sense_ of how the connotation is different, but I don't think that sense is actually formalized anywhere. And that's part of what makes it confusing, yeah. I think the sem web crowd actually embraces this confusingness, they want to have it both ways: Oh, a URI doesn't need to resolve, it's just an opaque identifier; but you really should use http URIs for all URIs; why? because it's important that they resolve.
In general, combining two functions in one mechanism is a dangerous and confusing thing to do in data design, in my opinion. By analogy, it's what gets a lot of MARC/AACR2 into trouble. It's also often a very convenient thing to do, and convenience matters. Although ironically, my problem with some of those TAG documents is actually that they privilege pure theory over practical convenience.
Over in: http://www.w3.org/2001/tag/doc/URNsAndRegistries-50-2006-08-17.html
They suggest: "URI opacity 'Agents making use of URIs SHOULD NOT attempt to infer properties of the referenced resource.'"
I understand why that makes sense in theory, but it's entirely impractical for me, as I discovered with the SuDoc experiment (which turned out to be a useful experiment at least in understanding my own requirements). If I get a URI representing (eg) a Sudoc (or an ISSN, or an LCCN), I need to be able to tell from the URI alone that it IS a Sudoc, AND I need to be able to extract the actual SuDoc identifier from it. That completely violates their Opacity requirement, but it's entirely infeasible to require me to make an individual HTTP request for every URI I find, to figure out what it IS. Infeasible for performance and cost reasons, and infeasible because it requires a lot more development effort at BOTH ends -- it means that every single URI _would_ have to de-reference to an RDF representation capable of telling me it identifies a SuDoc and what the acutal bare SuDoc is. Contrary to the protestations that a URI is different than a URL and does not need to resolve, following the "opacity" recommendation/requirement would mean that resolution would be absolutely required in order for me to use it. Meaning that someone minting the URI would have to provide that infrastructure, and I as a client would have to write code to use it.
But I just want a darn SuDoc in a URI -- and there are advantages to putting a SuDoc in a URI _precisely_ so it can be used in URI-using infrastructures like RDF, and these advantages hold _even if_ it's not resolvable and we ignore the 'opacity' reccommendation. There are trade-offs. I think a lot of that TAG stuff privileges the theoretically pure over the on the ground practicalities. They've got a great fantasy in their heads of what the semantic web _could_ be, and I agree it's theoretically sound and _could_ be; but you've got to make it convenient and cheap if you actually want it to happen for real, sometimes sacrificing theoretical purity. And THAT'S one important lesson of the success of the WWW.
From: Code for Libraries [[log in to unmask]] On Behalf Of Alexander Johannesen [[log in to unmask]]
Sent: Tuesday, April 14, 2009 9:48 AM
To: [log in to unmask]
Subject: Re: [CODE4LIB] resolution and identification (was Re: [CODE4LIB] registering info: uris?)
On Tue, Apr 14, 2009 at 23:34, Jonathan Rochkind <[log in to unmask]> wrote:
> The difference between URIs and URLs? I don't believe that "URL" is something that exists any more in any standard, it's all URIs. Correct me if I'm wrong.
Sure it exists: URLs are a subset of URIs. URLs are locators as
opposed to "just" identifiers (which is an important distinction, much
used in SemWeb lingo), where URLs are closer to the "protocol like"
things Ray describe (or so I think).
> I don't entirely agree with either dogmatic side here, but I do think that we've arrived at an
> awfully confusing (for developers) environment.
But what about it is confusing (apart from us having this discussion
:) ? Is it that we have IDs that happens to *also* resolve? And why is
> Re-reading the various semantic web TAG position papers people keep
> referencing, I actually don't entirely agree with all of their principles in practice.
Well, let me just say that there's more to SemWeb than what comes out of W3C. :)
Project Wrangler, SOA, Information Alchemist, UX, RESTafarian, Topic Maps
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