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Re: [ontolog-forum] {Disarmed} Re: OWL and lack of identifiers

To: Ken Laskey <klaskey@xxxxxxxxx>
Cc: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Ed Barkmeyer <edbark@xxxxxxxx>
Date: Thu, 12 Apr 2007 18:00:52 -0400
Message-id: <461EAC14.2030009@xxxxxxxx>
Ken Laskey wrote:    (01)

> I often questioned when talking about a URI that dereferenced to a Web 
> page whether I was talking about
> - the Web page (say, a collection of information on King Arthur),
> - the subject of the Web page (i.e. King Arthur), or
> - some particular piece of information (say, King Arthur had a sword 
> named Excalibur)    (02)

The URI is by definition a designator for the "resource".  So perhaps the 
question is: "What is the resource?"    (03)

When the URI is a reference to a Web page (full stop), the resource is the web 
page, and by extension, the information content of the web page.  This latter 
may be what Ken means by "the subject of the Web page".  The problem is that 
the information content of the same Web page can change, and that is not true 
of any other resource.  On the Web, the content doesn't really have 
identification when the page is volatile.    (04)

I would go so far as to say that stable web pages correspond to the concept 
"resource" and volatile web pages don't.  There is probably a useful 
distinction between a web page that changes to contain the "current" 
information on a fixed topic, like a weather site, or a wiki, and a web page 
that is the blog of the day.  The former can be viewed as "nearly stable" from 
the point of view of information content, while the latter is just a location 
at which somewhat random information is posted.  And the latter is a place, 
not a resource.    (05)

I have argued with TBL before that URIs that are URLs confuse WHAT something 
is with WHERE it is.  And it is only an acceptable idea when that relationship 
is required to be 1-to-1.  The idea of identifiers is that you can test for 
equal.  When the same thing can be in multiple places, unequal doesn't tell me 
anything, which is annoying, especially when tools think unequal to the 
expected value means unusable.  And when the same place can hold different 
things, equal doesn't tell me anything, which defeats the purpose.    (06)

Sir Tim's view is that URLs can be useful URIs and it is up to the provider to 
manage it properly, as W3C does.  Unfortunately, much of the Internet and much 
of the tooling is built by people who don't know about, don't care about, or 
don't understand that they should care about, responsible management.
(I wonder how many XML tools would break if the namespace URL for XML Schema 
pointed to a local copy of the specification...  Is the W3C URI THE name or A 
name for the XML Schema specification?)    (07)

The webhead idea is that you will always go to the URL, fetch the resource, 
and use it.  The idea that a tool has been pre-programmed to support that 
*content*, and, in conducting a web-based transaction, this might require the 
tool to fetch and compare two 10MB files to determine whether they are 
*versions of* the same specification, is beyond their hobbyist view of the 
Internet.    (08)

Now, if the resource is a "particular piece of information", that should be 
different.  The URI for a particular piece of information should extend the 
URL by a "fragment identifier".  HTML fragment identifiers are bookmarks that 
tell you where the piece of information starts but unfortunately don't bound 
the other end.  XPath fragment identifiers refer to bounded XML elements that 
should represent the "particular piece of information" exactly.  RDF fragment 
identifiers are typically XML ids, which refer to a particular XML element 
that should include the definition of the concept.  The XML technology doesn't 
extend to finding all the references to that id (rdf:about) that extend the 
concept by adding properties.  RDF technology does, but to be useful, that 
technology has to be what is addressed by the URL -- the RDF query service, 
which is yet another kind of "resource".    (09)

-Ed    (010)

Edward J. Barkmeyer                        Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263                Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263                FAX: +1 301-975-4694    (011)

"The opinions expressed above do not reflect consensus of NIST,
  and have not been reviewed by any Government authority."    (012)

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