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RE: Valid representations, canonical representations, and what the SW needs from the Web...

From: <Patrick.Stickler@nokia.com>
Date: Sun, 2 Feb 2003 18:50:53 +0200
Message-ID: <A03E60B17132A84F9B4BB5EEDE57957B5FBAFA@trebe006.europe.nokia.com>
To: <julian.reschke@gmx.de>, <www-tag@w3.org>



> -----Original Message-----
> From: ext Julian Reschke [mailto:julian.reschke@gmx.de]
> Sent: 02 February, 2003 15:17
> To: www-tag@w3.org
> Subject: RE: Valid representations, canonical 
> representations, and what
> the SW needs from the Web...
> 
> 
> 
> > From: www-tag-request@w3.org 
> [mailto:www-tag-request@w3.org]On Behalf Of
> > Patrick.Stickler@nokia.com
> > Sent: Sunday, February 02, 2003 1:44 PM
> > To: masinter@adobe.com; dehora@eircom.net
> > Cc: www-tag@w3.org
> > Subject: RE: Valid representations, canonical 
> representations, and what
> > the SW needs from the Web...
> >
> > ...
> >
> > Though it does raise the obvious question: since this standardized
> > functionality exists, why do we need RDDL? Or rather, why does a
> > RDDL instance need to be treated as a *representation* of an XML
> > Namespace, rather than just a body of metadata describing various
> > characteristics of the namespace resource and relations to other
> > resources?
> 
> I'm still not sure that I follow you that a RDDL document can't be a
> representation of a namespace. If a textual description of a 
> namespace can't
> be used as a representation, what else? 

But the content of a RDDL document is largely information not
inherent in the XML namespace of which it is supposedly a
"representation".

> Or are you saying that there
> actually *isn't* a valid representation for a namespace?

No. I would accept an enumeration of names grounded in that namespace
to be a valid representation of the namespace (and if folks consider
a namespace to be infinite, which technically, I guess it is, then
an enumeration of the names in-use or explicitly identified as
usable/significant/whatever by the namespace owner).

But most/all of a RDDL document says nothing about the namespace,
but about other resources in some way related to the namespace.
It's like a representation of Paris describing all the cities of
Europe because Paris is in Europe. 

The vocabularies, models, stylesheets, etc. identified and described
in a RDDL document are not part of the namespace. They are merely
related to the namespace because the utilize names grounded in that
namespace, and hence I consider such verbose descriptions of 
*primarily other* resources in a "representation" of an XML namespace
to be a deviation/violation of the Web/REST architecture.

> > Why not just do a PROPFIND on the XML Namespace to get the same
> > knowledge that would be defined in a RDDL instance, which would
> > then not blurr and distort the conceptual relationship between
> > resource and representation, since I'm presuming that whatever is
> > returned by PROPFIND is not considered to (necessarily) be a
> > representation of the resource.
> 
> It's definitively metadata about the resource. However, 
> WebDAV currently is
> weak in distinguishing between resource metadata and 
> representation metadata
> (a similar problem exists with HTTP headers, so this is by 
> inheritance).

I've been looking at WebDAV in more detail and I also noted a
practical problem in that it restricts metadata to "simple"
name/value pairs and only one value per name which would make
it rather inconvenient to use with RDF since although one might
use RDF collections to group multiple values, the usual practice
is to simply express multiple statements with the same property.
Clearly some mapping (and alot of forethought) would have to be
done to use PROPFIND in the manner I was thinking.

Ideally, one could get the WebDAV server to accept/return RDF...

> > Granted, typical browser users are not used to thinking about
> > metadata, but that doesn't mean they would not understand and
> > welcome a means to ask a server "Tell me about this thing"
> > rather than "Show me this thing".
> 
> Well, they can do that right now.
> 
> What's not really possible right now is to have metadata for 
> a resource
> (PROPFIND succeeds) with no representation (GET/HEAD fail), 
> because that's
> not really compatible with the underlying model (PROPFIND for 
> non-collection
> resources basically being an extended HEAD method with XML 
> marshalling).

Well, if that's true, then WebDAV definitely fails as a solution
to standardized access to resource metadata, since one would expect
to be able to use the same solution for all resources, whether or
not any representation is available.

Pity...

Patrick

--
Patrick Stickler, Nokia/Finland, (+358 40) 801 9690, patrick.stickler@nokia.com
 
Received on Sunday, 2 February 2003 11:50:56 GMT

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