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


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

  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
  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

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


Received on Thursday, 12 April 2001 09:26:22 UTC

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