W3C home > Mailing lists > Public > uri@w3.org > March 2002

Re: draft-masinter-dated-uri-01.txt

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Fri, 08 Mar 2002 11:07:23 +0200
To: ext Brian McBride <bwm@hplb.hpl.hp.com>, URI <uri@w3.org>
Message-ID: <B8AE4BED.103EF%patrick.stickler@nokia.com>
On 2002-03-04 17:22, "ext Brian McBride" <bwm@hplb.hpl.hp.com> wrote:

> Comments on:
> 
>> INTERNET-DRAFT                                         Larry Masinter
>> draft-masinter-dated-uri-01.txt                         March 1, 2002
>> Expires September 2002
>> 
>>         "duri" and "tdb" URN namespaces based on dated URIs
>
> ... I'm trying to ask whether the two URI's
> 
>  urn:duri:2001:http://www.ietf.org
>  urn:duri:2000:http://www.ietf.org

My intuitions say that they denote the same resource,
namely the web location, but at different points in time.

Thus:

 http://www.ietf.org
   = A web location

 urn:duri:2000:http://www.ietf.org
   = The state of http://www.ietf.org at 2000-01-01T00:00:00Z

 urn:duri:2001:http://www.ietf.org
   = The state of http://www.ietf.org at 2001-01-01T00:00:00Z

Thus, one could say things that are time dependent about
the web location named by the URL such as the owner, security
settings, redirections, etc.

It doesn't, however, seem to achieve the intended purpose
of capturing a specific resource residing at that location
at a given point in time. To do so, one would need a variant
of the tdb: URI scheme, e.g. tla: "thing located at" and
use that in conjunction with the duri: scheme. E.g.

 http://www.ietf.org
   = A web location, where some resource may be accessible.

 urn:tla:http://www.ietf.org
   = A resource located at http://www.ietf.org

 urn:duri:2000:urn:tla:http://www.ietf.org
   = The resource located at http://www.ietf.org at 2000-01-01T00:00:00Z

 urn:duri:2001:urn:tla:http://www.ietf.org
   = The resource located at http://www.ietf.org at 2001-01-01T00:00:00Z

A URL (sorry for using an obsolete, classical term ;-) is the name of
a location. That location is itself a resource, and some other non-location
resource may reside at that location, and one may use the name of the
location as an indirect alias for the name of the resource residing
at the location, but the name of the location is not the same as
the name of the resource accessible from that location.

Consider the attached RDF/N3 statements, which describe some resources,
some of which are locations, some of which are resources accessible
from a named location, and some of which are abstract.

Note especially that each location resource and non-location resource
have a diffferent creator and title.

--

BTW: I don't see tdb: as being semantically distinct from duri:.

E.g. in the I-D it is stated:

  "For example, "urn:tdb:2001:http://www.ietf.org" can be used to
   designate the Internet Engineering Task Force organization, at least
   as it was described by or referenced by its home page at the first
   instant of 2001."

But the 'thing denoted by' the URL http://www.ietf.org is not
the Internet Engineering Task Force organization, but a web
location accessible by the HTTP protocol and thus, the above
tdb: URN is semantically equivalent to the duri: URN
"urn:duri:2001:http://www.ietf.org", both of which denote the
state of the web location "http://www.ietf.org" at
2001-01-01T00:00:00Z.

If one wishes to denote the Internet Engineering Task Force
organization itself, then they should use a non-location specific
URI to do so, such as a voc: URT or similar non-dereferencable
name, or some anonymous resource; e.g.:

   [ a x:Organization ;
     x:name "Internet Engineering Task Force" ;
     x:homePage <http://www.ietf.org> ]

There is no need for any tdb: URI scheme, because the 'thing
denoted by' a URI is the thing denoted by the URI, nothing else.

A URI does not denote two things.

> Is it possible to argue that:
> 
>  http://mycollege.edu/students/Mary
> 
> does indeed name Mary, and what you get when you do an HTTP GET
> on that URL is a representation of Mary?

Well, aside from Star Trek technology, I don't see how you would
GET a representation of Mary, per the semantics of HTTP. You might
GET a representation of Mary's home page, or of her CV, or of a photo
of Mary, etc. but you wouldn't GET a representation of *Mary* herself.

To some extent, the use of the term 'representation' by HTTP is
unfortunate, because in non-HTTP terms, a photo is a representation of
Mary -- but in the HTTP world, there is an additional level
of indirection between the non-HTTP representation of Mary (the photo)
and the HTTP representation of the non-HTTP representation of
Mary (the byte stream returned by GET), and these two levels of
indirection often get confused such that folks think that what is
returned by GET is a representation of Mary herself, rather than
the actual case, which is that what is returned is a representation
of the representation of Mary. (ouch, my head hurts... ;-)

> Leaving aside, however sorting out what the properties of resources are,
> I wouldn't write the RDF example above that way, as it is at best,
> likely to confuse.  Better would be:
> 
> <rdf:RDF>
>  <rdf:Description about="http://mycollege.edu/courses/6.001">
>    <s:students>
>      <rdf:Bag>
>        <rdf:li rdf:parseType="Resource">
>          <foo:homepage resource="http://mycollege.edu/students/Amy"/>
>        </rdf:li>
>        <rdf:li rdf:parseType="Resource">
>          <foo:homepage resource="http://mycollege.edu/students/Tim"/>
>        </rdf:li>
>        <rdf:li rdf:parseType="Resource">
>          <foo:homepage resource="http://mycollege.edu/students/Mary"/>
>        </rdf:li>
>      </rdf:Bag>
>    </s:students>
>  </rdf:Description>
> </rdf:RDF>
> 
> which inserts extra resources to 'represent' the students as different
> resources from their home pages.

Yup. Exactly.

The students are not their home pages.

Cheers,

Patrick

--
               
Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com





Received on Friday, 8 March 2002 04:05:41 UTC

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