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RE: Resolving references against base URIs

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Mon, 15 Apr 2002 13:17:58 -0400 (EDT)
Message-Id: <5.1.0.14.0.20020415170527.03294b90@0-mail-1.hpl.hp.com>
To: Paul Grosso <pgrosso@arbortext.com>, Al Gilman <asgilman@iamdigex.net>, "Jeremy Carroll" <jjc@hplb.hpl.hp.com>, "Williams, Stuart" <skw@hplb.hpl.hp.com>
Cc: <www-xml-linking-comments@w3.org>, <uri@w3.org>
At 09:51 15/04/2002 -0500, Paul Grosso wrote:
>At this point, it looks like we are all using different phrases
>(not even sentences) in various specs to support different views.
>
>My view is that an RDF resource should be something of MIME type XML,
>and anything of MIME type XML that does anything with XML Base has to
>interpret things the same way.  I would consider this an architectural
>issue, so maybe it's time this issue should be sent to the TAG.  (Two
>of the three authors of RFC 2396 are on the TAG.)

Hi Paul,

It may be that this needs to go the tag, but I'd like to make sure we do 
our best to clarify the issue first.  I hope Jeremy will forgive me butting in.

I'll try to keep this short, but it may be a little long.

First: the problem RDF is trying to solve.  The current RDF specs have 
encouraged the use of the following idiom:

   <rdf:Description rdf:about="#foo">
     ...

The value of the rdf:about attribute is turned into an absolute URI 
reference by concatenating the '#foo' with the URI of the containing document.

This causes problems.  Folks copy the file from the web to their hard drive 
so they can work on it in a plane, and the uri changes to something like 
file:c:\temp\....rdf and this is really useless for rdf users.  Or folks 
wish to include RDF in say a message protocol where  there is no base uri 
of the document.
This is the cause of one of, if not the, most frequent newbie problem with 
DAML that we see on jena-dev.

So we are looking for a way to retain this convenient syntax, but have the 
uri's produced not change when the file is copied or mirrored.

To appreciate what is happening here, we need to look at a semi-fictional 
RDF processing pipeline:


input xml document -> xml parser -> rfc2396 processor -> rdf parser -> rdf 
graph

We start with an xml document and end up with a datastructure.  The 
datastructure is not a DOM; its not a representation of an xml 
document.  It is as far as xml is concerned, an application data structure.

For each value of an rdf:about attribute, the rfc2396 processor outputs 
either an absolute URI or a same document reference.  The absolute URI is 
processed according to RFC2396.  Same document references are recognised 
according to RFC 2396.

All is in conformance with rfc 2396 at this point.

Now the RDF parser comes in to play and it is required to transform the 
value of each rdf:about attribute into an absolute uri reference.  If the 
RFC 2396 processor has produced an absolute uri reference, it need do 
nothing.  If however, it is a same document reference, then, just as a 
browser will handle same document references specially, so does RDF.  It 
transforms the same document reference into an absolute URI according to an 
algorithm defined by the RDF specs.  The mimetype of an rdf document will 
be text/xml+rdf.  As far as xml base and rfc 2396 are concerned, this is 
application code over which they have no say.

What I have tried to do here is to position RDF as an application built on 
top of XML and to suggest that XML should not be allowed to express 
constraints on how applications process it.

There is a deal of sophistry in this argument :( but RFC 2396 doesn't 
really meet our needs.  Are there any plans to update/refine it in the near 
future?

Brian
Received on Monday, 15 April 2002 13:39:55 GMT

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