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Re: Common Metadata (was:RE: RDF in XHTML)

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Fri, 20 Apr 2001 16:44:33 +0100
Message-ID: <3AE05961.201ECFF8@hplb.hpl.hp.com>
To: Danny Ayers <danny@panlanka.net>
CC: Lee Jonas <lee.jonas@cakehouse.co.uk>, Murray Altheim <altheim@eng.sun.com>, RDFInterest <www-rdf-interest@w3.org>
Hi Danny,

So the problem is, "How is common metadata to be represented in RDF
XML?" and you give a good example.

Two ideas have been suggested: using aboutEachPrefix and using seeAlso.

There is an issue listed regarding aboutEachPrefix:

  http://www.w3.org/2000/03/rdf-tracking/#rdfms-abouteachprefix

One of the problems is that aboutEachPrefix is described in the syntax
but is not described in the model.  This is pretty dodgy ground at the
moment because a lot of systems don't implement it.

Even if one were to use aboutEachPrefix, where would the XML for those
statements be?  In a separate document?  How would a system find that
document, given one of your 1000 pages?

Regarding seeAlso, the definition of seeAlso states:

  The property rdfs:seeAlso specifies a resource that might provide
  additional information about the subject resource.

So an RDF processor is not required to follow the link, but could do so.
However, what do you put in the see also document?

  <rdf:RDF>
    <rdf:Description about="WHAT">
      <dc:creator>foo bar</dc:creator>
    </rdf:Description>
  </rdf:RDF>

The problem is that the value of the about attribute is absolute, it
does not change its meaning depending on what link you followed to get
it.  So
that doesn't seem to do what you need.

A combination of seeAlso and aboutEachPrefix might do it.  Put the
seeAlso in each document and use aboutEachPrefix in the 'see also'
document.  But as I said, aboutEachPrefix is not widely implemented.

A syntactic include mechanism might do what you are looking for.  It
might then include say:

  <rdf:RDF>
    <rdf:Description rdf:about="">
      <dc:creator>foo bar</dc:creator>
    </rdf:Description>
  </rdf:RDF>

Note the rdf:about.  There are problems here.  First, a robot finding
the file to be included would see that it was created by foo bar, which
might well not be true.  Secondly, XML isn't too good at being nested in
other XML at the syntactic level because of issues with things like
uniqueness of ID's and namespace prefix usage.

However, for the range of things under your control, you could arrange
to
do this with say, an XSLT transform, to insert the meta data
automatically.  This is not really an RDF solution.  If I had an
immediate problem, I think this is the way I'd go.

In terms of the issue list, I suggest you have highlighted an aspect
of the rdfms-abouteachprefix issue, and I'd propose to add a reference
to this thread under a 'see also' link under that issue.

Brian



Danny Ayers wrote:
> 
> I can't see how RDF could be practical without some mechanism for
> inherited/shared metadata (you've got a 1000 documents with the same
> author - do you need to specify this a 1000 times), but I'm thinking that
> because this is so significant it must have been dealt with already -
> seeAlso doesn't really seem adequate, I'm not sure of the history of
> aboutEachPrefix, but surely there is already some mechanism in place? Lee's
> xsl:include/import suggestion sounds possible. If this angle is well
> covered, an idiot's guide is needed (for this idiot at least), otherwise I
> think it could well be an issue.
> 
> ---
> Danny Ayers
> http://www.isacat.net
> 
> <- -----Original Message-----
> <- From: www-rdf-interest-request@w3.org
> <- [mailto:www-rdf-interest-request@w3.org]On Behalf Of Lee Jonas
> <- Sent: 20 April 2001 19:47
> <- To: 'Danny Ayers'; Murray Altheim
> <- Cc: RDFInterest
> <- Subject: RE: RDF in XHTML
> <-
> <-
> <-
> <-
> <- Danny Ayers [mailto:danny@panlanka.net] wrote:
> <-
> <- ><- The RDF Schema spec hints at 'rdfs:seeAlso' for cascading purposes,
> <- e.g.:
> <- ><-
> <- ><- <rdf:RDF ...(namespaces)...>
> <- ><-  <rdf:Description rdf:about="" rdfs:seeAlso="furtherinfo.rdf"/>
> <- ><- </rdf:RDF>
> <- >
> <- >Looks viable - I wonder if there's anything more than hints
> <- >this is rather important, probably well known (but I can't
> <- think of it) -
> <- >what is the best way to use inheritance of metadata between documents?
> <- >How do I avoid adding the same author information to every one of the
> <- >million pages I've written (with the aid of some monkeys)?
> <- >
> <-
> <- Well, this is an entirely different prospect, and one I think that the
> <- 'rdf:aboutEachPrefix' predicate was intended to address.
> <- However, AFAIK it
> <- has a lot of opposition due to the fact that it is difficult to
> <- implement in
> <- practise, e.g.:
> <-
> <- 1) Consider starting with a resource and trying to determine who
> <- the author
> <- was.  If this info is in a 'rdf:aboutEachPrefix' statement in
> <- some other rdf
> <- doc, not even referenced from the resource you are currently
> <- processing, it
> <- is nigh on impossible to determine.
> <-
> <- 2) It relies upon the hierarchical location of resource representations -
> <- the granularity of what these kinds of statements apply to is
> <- too course -
> <- i.e. all resources whose URIs 'startWith' a common substring. It
> <- might have
> <- been better to do something akin to what XPointer does for XML.
> <-
> <- Suggestion:
> <- IMHO rdf:seeAlso is equivalent to xsl:include semantics.  What
> <- is lacking is
> <- xsl:import semantics.  The latter might allow you to define a set of
> <- statements that apply to their current doc, then 'importing'
> <- that doc from
> <- another would make those same statements apply to the doc doing the
> <- importing.  Hence, importing a handful of rdf docs containing common
> <- statements (e.g. author) from a million XHTML web pages would
> <- save a lot of
> <- typing!
> <-
> <- This would solve both 1) and 2) above to some degree of
> <- satisfaction - the
> <- degree of satisfaction of 2) depends on how you structure your imported
> <- docs.  Is this worthy of the issues list?
> <-
> <- >
> <- >
> <- ><- >On another line, forget XHTML for a moment, how do we embed
> <- metadata in
> <- ><- >other XML markups?
> <- ><-
> <- ><- I thought this is what XML Namespaces are for!  Just embed your
> <- elements,
> <- ><- any processor that doesn't recognise the namespace (within its
> <- ><- context) can
> <- ><- ignore it.  Note that to validate such documents properly (and allow
> <- such
> <- ><- open mixing-and-matching of different 'XML mini-languages') will
> <- ><- require XML
> <- ><- Schema.
> <- >
> <- >Quite. (to avoid this going in circles, please ignore) so why
> <- should XHTML
> <- >be treated any different?
> <-
> <- One reason is that it is currently validated using a DTD.  Whether XML
> <- Schema would fare better, I don't altogether know for sure.
> <-
> <- (Note that my current understanding of XML Schema is not perfect, I am
> <- hoping that XML Schema allows you to freely mix-and-match elements in
> <- different XML namespaces.)
> <-
> <- regards
> <-
> <- Lee
> <-
Received on Friday, 20 April 2001 11:44:57 GMT

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