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Re: Clarifying what a URL identifies (Four Uses of a URL)

From: Tim Bray <tbray@textuality.com>
Date: Tue, 21 Jan 2003 13:01:37 -0800
Message-ID: <3E2DB531.1020104@textuality.com>
To: Sandro Hawke <sandro@w3.org>
Cc: Michael Mealling <michael@neonym.net>, David Booth <dbooth@w3.org>, www-tag@w3.org, "Roy T. Fielding" <fielding@apache.org>, Dan Connolly <connolly@w3.org>

Sandro Hawke wrote:

> URIs are strings which are used for different things in different
> situations, in a manner controlled by the semantics of the situation.

On the other hand, using the same URI to mean different things is a Bad 
Thing and leads to confusion and misbehavior not only at the Semantic 
Web level but in terms of general human utility.  When you say that 
http://example.com/moby represents Moby Dick you need to be clear 
whether you mean Melville's novel in the abstract, some particular copy 
on a shelf, the online Gutenberg text, a record in a particular library 
catalog, or a fictional cetacean.

The Web Architecture defines a Resource to be anything that is 
identified by a URI.  Clearly, a resource whose identification is 
muddified or inconsistent is less useful, and makes the Web less useful.

> There is no single thing identified by a URI in all situations.

There is in the Web formalism; it's called a Resource.  One of the holes 
in Web Architecture is that it doesn't include anything to tell you what 
a Resource "is", just to deliver representations.  Which is why we need 
RDF.  If your application depends on semantic properties of particular 
resources, you'd better write them down and publish them and agree on a 
way to find the published properties.

> notion that there is or should be exactly one conceptual thing
> corresponding in all situations in some standand way to each URI (that
> the URI is a logical symbol with a single denotation) is a fallacy
> which causes the httpRange-14 rat-hole.  

It's a formalism.  The Web Architecture has a formalism called a 
"Resource" which is the one thing that corresponds to each URI.  This is 
fact of life and isn't going away.  Deal with it.  The formalism is 
plenty complete and consistent enough to support the construction and 
efficient operation of the Web.  -Tim
Received on Tuesday, 21 January 2003 16:01:42 UTC

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