W3C home > Mailing lists > Public > www-rdf-logic@w3.org > April 2001

Re: URIs / URLs

From: Aaron Swartz <aswartz@swartzfam.com>
Date: Wed, 11 Apr 2001 18:27:41 -0500
To: Lee Jonas <lee.jonas@cakehouse.co.uk>, RDF Interest <www-rdf-interest@w3.org>, RDF Logic <www-rdf-logic@w3.org>
Message-ID: <B6FA5298.8F73%aswartz@swartzfam.com>
Lee Jonas <lee.jonas@cakehouse.co.uk> wrote:

> The point is that "retrieval" is not an endemic aspect of URLs.  URLs merely
> identify by location.  Software agents do something with that identifier
> (which just happens to be retrieval of the identified resource, mostly).

Yup! That's why partly URIs are "identifiers", I think.

> In terms of resources changing, two examples you cited sprang out at me:
> 1) Two people with the name Champin at the same university at different
> points in time.
> 2) Different versions of a W3C Working Draft.
> 
> It seems that 1) is clear cut: two different resources identified by the
> same URL because they occupy the same location at different points in time.

Actually, if we start out being clear that's not true.

http://example.edu/~champin/ could be said to represent the person named
Champin currently enrolled at Example University. Thus, the resource would
stay the same, although the entity (and more specific resource) it referred
to would change.

> This is a problem in general in terms of the transient nature of the
> Internet.

But yes, changing the meaning of URIs is bad. However, not specifying the
exact meaning of URIs is also bad (as the above example shows).

> However, 2) is not so straight forward.  The W3C use URLs that identify
> different versions of a document as different resources (by incorporating
> the publish date).  Yet the latest version is also identified by a URL that
> does not contain any distinguishing date information.  The resource
> retrieved by this URL changes to always retrieve the "latest version".

This is very similar to the example above. Just as our hypothetical homepage
represented the current Champin, the W3C URL represents the current version
of the document. You'll probably be interested in TimBL's writing on Generic
resources:

http://www.w3.org/DesignIssues/Generic

> I would suggest that a new URN scheme could directly represent the notion of

Why do we need a URN scheme for this? Can't we just use RDF? Although a URI
scheme might be nice, but I can't see where it would be useful. And how
would we represent versions? With numbers? dates?

> 1) Firstly, make the services processes (i.e. daemons).
> The extra constraints of making the URNs in documents conform to the http
> protocol for N2L mappings disappear (hooray!).  You also don't have to
> specify L2N mappings within documents, avoiding unnecessary clutter.

I don't see what you mean by this.

> (i.e. just do a DNS lookup on a URN /
> URL to identify the server with the relevent N2L / L2N daemons for that
> domain).

How can you do a DNS lookup on a URN or a URL? And what type of DNS record
would we use?

> Anyone wanting N2L & L2N capabilities for mapping urns within
> their own domain space simply run these services alongside DNS.

URNs have domain spaces?

-- 
[ Aaron Swartz | me@aaronsw.com | http://www.aaronsw.com ]
Received on Wednesday, 11 April 2001 19:27:54 GMT

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