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

RE: URIs / URLs

From: Charles McCathieNevile <charles@w3.org>
Date: Thu, 12 Apr 2001 09:26:16 -0400 (EDT)
To: Lee Jonas <lee.jonas@cakehouse.co.uk>
cc: RDF Interest <www-rdf-interest@w3.org>
Message-ID: <Pine.LNX.4.30.0104120914440.18863-100000@tux.w3.org>
On Thu, 12 Apr 2001, Lee Jonas wrote:

  My interpretation is: resources are things that can evolve, representations
  are distinct "snapshots" of a particular resource state, conceptually taken
  at the point of access (this then includes representations of resources
  provided by CGI scripts, etc).  A W3C Working Draft evolves, the html doc
  retrieved from its "latest version" URL gets a representation of the latest
  version of the Working Draft.

I think that we agree on this idea.

  >
  >> The first issue could have been addressed more formally (and hence
  >> consistently) with a simple versioning scheme.
  >
  >What about ETags?

  I am not familiar with these.  Can you give me some pointers?

ETags are an HTTP feature that allows you to identify a particular version of
something retrieved as the result of a GET

[snip]
  Although it would sometimes be unavoidable, wouldn't it be nice to find out
  the type of a representation without having to negotiate every time?

Sure. I thought that one of the useful features of RDF is that it can be used
to describe what representation types there are for a given resource, and
perhaps how to get a given type (for example many W3C resources have an
identifier for the resource, and an identifier for particular versions of teh
representation).
[snip]
  Reserving URLs to identify things that you can access representations of has
  certain advantages.  Not least is keeping at least 94% of them vancable.  It
  seems like a simple distinction to me.  In an ideal world, URLs are always
  vancable, URNs may be so, but not necessarily.

In my ideal world you can always get something from a URI, which might be a
statement that this URI is an identifier for something you can't get on the
wire (although I  am intrigued by the possiblity of serialising aaron and
getting him to my meetings through http instead of Delta Airlines, for
economic reasons... <grin/>. Just don't try it on me yet... <bigGrin/>)

  It is a fundamental aspect of the way URLs are defined to be used.  They
  *locate* (note I did not say *identify*) representations (snapshots of
  state) of underlying resources, not the resources themselves.  When
  resources change, new representations may appear at the same and/or
  different locations.  The only way RDF could satisfactorily deal with this
  is if it described the resources directly by using URN identifiers, which
  could be subsequently mapped to a URL locating an appropriate
  representation.

I think this is where we get to the real nub of the problem. I do not agree
that a UR* locates something. It identifies it. The Web provides a way of
getting something where the identifier is a URI (and of putting something at
a location that can be found, if it has a URI identifier). That something can
either be the thing itself, or some information about the thing. And that
depends on the semantics of the identifier, not the syntax. A new syntax
doesn't change that, and a syntax that says "use a URI to work out how to get
another kind of identifier" doesn't seem to add anything except a layer of
complexity.

cheers

Charles
Received on Thursday, 12 April 2001 09:26:22 GMT

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