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Hi Dan, 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. Dan said: 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. Didier says: Thanks to call for order ;-) a) When you say data aggregation, so that no confusion about what you mean is inserted in the message, what do you mean exactly? There is several ways to aggregate so I want to be sure I understand which one you are talking about. b) In rdf, because its intent is not for data base data exchange, there is a lack of relation notation between entities. In fact, there is no entity relationship in rdf except arcs to relate tuples (subject, property, value). So, it is more a property/value kind of stuff. A set of properties could be aggregated into classes. From these classes we can obtain instances. There is no actual capabilities to relate classes. Didier said: > 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 Dan said: 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. Didier says: It is because you have the capability to create a "record" or "group of properties". The problem then is to use the <rdf:description about=""> construct which semantically does not make too much sense in this context (db data exchange). So, what we need, is a schema language allowing us to define classes and properties (rdf schema language made some marks here), relations between classes (nothing yet). The rdf schema language has some interesting constructs to allow us to define classes and properties but nothing to define relation between classes. So, really what is interesting here for data exchange is not the relationship (i.e. the tuple) per se but the capability to create classes and then have instances of these classes and also to more formally describe properties contained in these classes. Now, what's missing is class relationship. To translate the object notation "class" into the relational DB world, we can translate "class" by table, property by "column" and finally instance by "row". By the way, it is possible to use the rdf schema language without using the rdf constructs like <rdf:description>. The schema language could be used to define any domain specific language or more simply a DB exchange language. With such a schema, a good interpreter can do the job to understand the data encoded in the document. Dan said: 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. Didier says: xlink is about linkage, period. Yes with some imagination I can use it for an other usage like, I would be using a pencil to write, as a pendulum, as a piece of wood, as a fire starter, etc... But its best usage is to be used to write :-) Didier said: > 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. Dan said: I'd rather have a single data model and a family of schema languages built to a common approach... Didier says: What do you mean here? That having a lot of schema language is better than having one? This is actually the case anyway and this has been like that before xml. I don't see your point, we are talking here of the xml world. I would, for myself prefer to have a set of coherent specs. And for instance, a strong xml based schema language. To reduce complexity, probably a language divided into several modules so that we can use the module we need. This would be a more useful architecture and also, easier to learn for newbies. regards Didier PH Martin mailto:martind@n... http://www.netfolder.com xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ and on CD-ROM/ISBN 981-02-3594-1 To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...)
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