[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RDF and Dereferencing
Sean B. Palmer writes: > BTW, Paul: You cannot say that namespaces shouldn't be dereferenced: that > would make RDF useless overnight. But on the othr hand, you cannot say that > everything shoyuld be dereferenced: that would be absurd! Actually, it would cause problems only for RDF schemas. RDF model and syntax without schemas does not need Namespace dereferencing, though it does mention it. Some RDF people want to do something similar with the URIs that act as resource identifiers (they look and act a lot like Namespace-qualified names, but technically, they are not), but I believe that is very bad practice. Consider the following: ACME News wants to publish some information about XML, so they publish it like this: <rdf:Description> <dc:title>XML</dc:title> <dc:description> The Extensible Markup Language, a specification created by the World Wide Web Consortium, based on SGML. </dc:description> <dc:category rdf:resource="http://acmenews.com/categories/computing"/> <dc:category rdf:resource="http://acmenews.com/categories/markup"/> <dc:category rdf:resource="http://acmenews.com/categories/web"/> </rdf:Description> Now, the easiest way to identify this (a way used in some of the RDF spec examples) is to use a local identifier, like this <rdf:Description rdf:ID="xml"> ... </rdf:Description> If the RDF document containing the information were located at http://www.acmenews.com/topics then the full identifier would be http://www.acmenews.com/topics#xml It seems to have a nice symmetry at first -- if any RDF document refers to the resource http://www.acmenews.com/topics#xml, a processor can simply dereference the identifier, parse the RDF document, and get some information about it. This approach is entirely unsuitable, however, because we want a Web where *lots* of people can say things about XML, not just ACME News, and where everyone talking about XML can (if they wish) use the *same* identifier, which plays a role equivalent to that of a primary key in databases. Now, imagine a different approach. ACME news creates an identifier for XML http://acmenews.com/topics/xml and because ACME news did it first and is fairly well-known (and published their identifiers), other people find it convenient to use the same identifier so that users and search engines can cross-reference the information. Now, even though ACME News was the first to create an identifier for XML, everyone can talk about it without assuming that ACME news's information has any kind of primacy. For example, xmlrules.com might have <rdf:Description rdf:about="http://acmenews.com/topics/xml"> <dc:title>XML</dc:title> <dc:description> The answer to war, disease, and world hunger, XML enables instant communication among all peoples, bringing about a siblinghood of man. </dc:description> </rdf:Description> and xmlsucks.com might have <rdf:Description rdf:about="http://acmenews.com/topics/xml"> <dc:title>XML</dc:title> <dc:description> Another pathetic attempt to impose order and authority on the free market of ideas that is the Web, probably backed by Microsoft or the Rand Corporation. </dc:description> </rdf:Description> Someone buying news stories from ACMENews.com (or other providers who use ACME's identifiers) can use the topic identifier in the story to look up background information on XML from a variety of sources, not just from ACME itself. After all, who's to say that ACME News's apparently neutral description is any better than the apprently biased ones at xmlrules.com or xmlsucks.com? Should each have been forced to use a different identifier for the same thing? Should the party that invents the identifier automatically have precedence in the definition? If RDF were intended only for providing metadata about Web pages, then an approach like this would probably work; but in that case, we wouldn't really need RDF to begin with (a little clever meta-tag manipulation would be as much as most users could handle). All the best, David -- David Megginson david@m... http://www.megginson.com/
|
PURCHASE STYLUS STUDIO ONLINE TODAY!Purchasing Stylus Studio from our online shop is Easy, Secure and Value Priced! Download The World's Best XML IDE!Accelerate XML development with our award-winning XML IDE - Download a free trial today! Subscribe in XML format
|