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

From: David Booth <dbooth@w3.org>
Date: Mon, 20 Jan 2003 20:39:57 -0500
Message-Id: <>
To: www-tag@w3.org
Cc: Sandro Hawke <sandro@w3.org>, "Roy T. Fielding" <fielding@apache.org>, Dan Connolly <connolly@w3.org>

Sandro, Roy and others have recently been discussing what a URI identifies 
(related to the open TAG issue httpRange-14 [1]).  I have written a new 
document, "Four Uses of a URL: Name, Concept, Web Location and Document 
Instance", that attempts to help clarify and illustrate the issue:

         http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm .

It attempts to be intuitive and understandable while also being reasonably 
precise.  It also offers a slightly different perspective than I have 
previously seen.  I have specifically avoided using the terms "resource", 
"document" and "representation" because I find those terms unclear or 
unintuitive.  Quoting from the abstract:

URLs can be used to identify abstract concepts or other things that do not 
exist directly on the Web. This is sensible, but it means that the same URL 
might be used in conjunction with four different (but related) things: a 
name, a concept, a Web location or a document instance. Somehow, we need 
conventions for denoting these four different uses. Two approaches are 
available: different names or different context. The "different names" 
approach requires new URI schemes; the "different context" approach 
requires syntactic conventions for indicating the intended context.

and the conclusions:
In using URLs to identify concepts . . ., we need syntactic conventions for 
denoting each of these four things . . . .

If we wish to create a Semantic Web in which statements are unambiguous and 
machine processable, then any machine-processable language that uses URLs 
must clearly specify which of these four things is intended when a URL is 
written in that language.  But for sanity across languages, it would be 
nice to have some common conventions.

As a result of this analysis, I find myself leaning toward Sandro's 
conclusion[2], that we should use context indicators to distinguish these 
different uses of a URL.  However, I'd like to know if others think my 
analysis is incorrect and I should reach a different conclusion.  Comments?

[1] http://www.w3.org/2001/tag/ilist#httpRange-14
[2] http://lists.w3.org/Archives/Public/www-tag/2003Jan/0149.html

David Booth
W3C Fellow / Hewlett-Packard
Telephone: +1.617.253.1273
Received on Monday, 20 January 2003 20:40:34 UTC

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