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Re: URIs / URLs

From: Pierre-Antoine Champin <champin@bat710.univ-lyon1.fr>
Date: 08 Apr 2001 18:24:44 +0200
To: Dan Connolly <connolly@w3.org>
Cc: www-rdf-interest@w3.org, www-rdf-logic@w3.org
Message-Id: <20010408162245.KBFC13088.cuyuni.paris.none.net@dyn-212-129-7-87.ppp.libertysurfpro.net>
Dan,

one key idea of the not was : *locating* and *identifying* are not the
same.
Though, URLs are frequently used to identify resources, either the one
they locate or another one.
This makes them tricky and unreliable as identifiers.

On 06 Apr 2001 08:54:05 -0500, Dan Connolly wrote:
> I don't see any justification for the claim
> that namespaces are disjoint from HTTP resources.

Because it can not be transported over that protocol (bor any protocol:
it is an abstract thing).
A specification of it can, indeed.

> the system works best when you can use such
> an identifier to GET a specification/description
> of the vocabulary.

The system works better when you know what exactly is identified by an
identifier.
Here, is it the decscriton or the described thing ??
There is often no mean to know.

Note that the description and the described thing are *distinct*, hence
they need disctinct identifier.


>   "For example, the URI of the type property of RDF is
>   http://www.w3.org/1999/02/22-rdf-syntax-ns#type. As a
>   matter of fact, the property itself is not located by that
>   URL: its description is. "
> 
> Again, I see no justification for the claim that this
> identifier doesn't identify a property.

I did not write that !
I wrote that the URL does not *locate* the property, but its
description.
See how the confusion between location and identification is easy...

>   "URLs are transient 
>   That means they may become invalid after a certain period
>   of time [9]. "
> 
> That's a fact of life in a distributed system. URNs may
> become invalid after a period of time too.

There is no reason why an URN would becom invalid once defined.
It may becom obsolete, unused, but one thing is for sure :
it will never be re-defined (according to URNs requirements).
This is an interesting property for identifiers. 

> Naming is a social contract, and the http: contract
> works and the urn: contract doesn't.

Once again, HTTP work as locators, not as names.


> 
>   "In the immediate interpretation, a URL identifies the
>   resource retrieved through it."
> 
> to be precise: it identifies the resource accessed
> thru it. In the general case, you can't retrieve
> a resource, but only a representation of one.

Right, this is what we call an "instance" of the resource in the note.
Note that this is why we immediately discarded the immediate
interpretation.


One thing might not have been clear in our note :
we do not reject URLs for what they are good at. The allowed the web to
become what it is now. I even believe that many application can them to
identify namespaces, persons, etc.

The point we wanted to raise is that locating is not the same as
identifying, and identifying a description is not the same as
identifying the described thing. And doing thos approximations must be
done, if ever, with awareness.

  Pierre-Antoine Champin
Received on Sunday, 8 April 2001 12:22:57 GMT

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