Expressing XLink concepts in XML Schema

Dan Brickley Daniel.Brickley at
Sat Jul 31 17:48:06 BST 1999

On Sat, 31 Jul 1999, Didier PH Martin wrote:
> This is because some are stretching too much the utility of RDF. At its
> origin, RDF has been designed as a metadata encoding tool. 

And XML was designed for structured document interchange, but that's not
stopped people having all sorts of unanticipated fun with it...
(and can anyone remember what the Internet itself was originally for? ;-)

It may well be that RDF is in some quarters being stretched beyond
current capabilities. It certainly hasn't suffered the dangerous hype
currently afflicting XML, and I personally hope it never does. Appeals
to origins don't help much here: is better to argue on technical
capabilities. There are certainly many things now missing from RDF which
hinder our use of it for Web database-oriented applications.
Data aggregation for example turns out to be difficult (even though
URI-based graphs provide a solid foundation...) given the modest
constraint mechanisms in the proposed Schema 1.0 system. Because we have
no notion of cardinality/occurance in 1.0, aggregating overlapping data
is hard without having hard-coded additional knowledge of the
vocabualries/applicatiosn concerned. This has nothing to do with the
origins of RDF, of XML, or of the Internet. It's just an interesting
fact about what's currently feasible given the specs on the table.

> a) rdf for metadata - to add information to existing resources
> b) xlink for resource linkage - to link resources
> This said, we'll have more and more similarities because:
> a) all these languages are XML based,
> b) we start to see now some common sub languages used across different tools

No - the similarity it because RDF describes resources in terms of their
relationships with other resources (conceptual and concrete) as well as
ascribing simpler flat attributes to those resources. Xlink allows for
links to be expressed between document-like resources, and for those
knowledge of those relationships to be stored outside the resources
concerned. These are clearly overlapping pieces of the same larger
puzzle. For my money the XArc proposal goes some way towards addressing
potential overlaps here.

> like for instance XPath and XPointer (re-united) and the string based
> pointer language used in other XML based languages.
> James said:
> Also between XML Schemas and RDF Schemas.
> Didier says:
> Yes and this is good news because having a single schema language is more
> intelligent than having to still live in the Babel tower.

I'd rather have a single data model and a family of schema languages
built to a common approach...


Daniel.Brickley at                  
Institute for Learning and Research Technology
University of Bristol,  Bristol BS8 1TN, UK.   phone:+44(0)117-9287096

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