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Re: Short Essay: Squeezing RDF into a Java Object Model

  • From: David Megginson <david@m...>
  • To: "XML Developers' List" <xml-dev@i...>
  • Date: Mon, 3 May 1999 14:48:37 -0400 (EDT)

java isxml
W. E. Perry writes:

 > David Megginson wrote:
 > 
 > > Obviously, there's a much more complex model underlying RDF than the
 > > spec lets on, and that model affects not only the ease or difficulty
 > > of implementing an object model, but also the difficult of many
 > > standard operations like queries against a collection of RDF
 > > statements and storage in a relational database.

 > May I respectfully submit that the problem is not the complexity of
 > the model underlying RDF, but its simplicity and relative freedom
 > from restriction, permitting the very sort of extension that leads
 > to the implementational problems David Megginson illustrates.

I'm not certain that I follow -- I did not consider extensions at all
in my essay.

Certainly, it is possible to build complex layers on top of a simple
data model, and much of what is in the RDF-syntax spec (such as the
containers, rdf:type, rdf:value, rdf:Statement, etc.)  together with
all of the RDF-schema spec can be taken in that light; as a result, I
considered none of that in my model.

The problem is that the bottom-level, primitive model required for RDF
support is itself rather complicated.  It is certainly fair, in the
model, to include

Statement
---------
- Subject
- Predicate
- Object

but you also have to specify what a Subject, Predicate, and Object
are.  They are not simply resources/URIs, but have properties of their 
own:

Subject
-------
- value
- isPrefix?

Predicate
---------
- value

Object
------
- value
- language
- isResource?
- isXML?

This is the bare-bones RDF data model, according to my reading of the
spec.  Now, I will certainly admit that I have not been involved in
the RDF design process, and may have misunderstood something: I
encourage all members of XML Dev to feel free to correct me, no matter
how experienced or inexperienced they may be with RDF.

 > Effectively it is just such a cumulative body of metadata which
 > David Megginson is seeking. It would provide the ability to refer
 > to components larger than simple subjects, predicates and objects,
 > such as resources as objects or prefixes as subjects. Such a
 > framework would comprehend and permit reference to any such objects
 > either top-down, from the perspectives of their larger containers,
 > or bottom-up from the perspective of their sub-components.

I don't think that that's what I was looking for, though it certainly
sounds interesting.  Right now, I'm just considering the smallest
possible data model required for implementing the RDF-syntax
specification.


All the best,


David

-- 
David Megginson                 david@m...
           http://www.megginson.com/

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