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the semantic web a gide to the future of XML, Web Services and Knowledge Management - Daconta M,C.

Daconta M,C. the semantic web a gide to the future of XML, Web Services and Knowledge Management - Wiley publishing , 2003. - 304 p.
ISBN 0-471-43257-1
Download (direct link): thesemanticwebguideto2003.pdf
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<rdf:RDF
xmlns:dc=’http://purl.org/dc/elements/1.1/’
xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
xmlns:earl='http://www.w3.org/2001/03/earl/0.95#’>
<rdf:Description rdf:about='http://example.org/onto#Jane'> <earl:asserts rdf:parseType='Resource'>
<rdf:subject>
<earl:WebContent
rdf:about='http://example.org/onto#MyPage'>
<dc:creator
rdf:resource='http://example.org/onto/person/Mary/'/>
</earl:WebContent>
</rdf:subject>
<rdf:predicate> rdf:resource='http://www.w3.org/2001/03/earl/0.95#passes'/> <rdf:object>Accessibility Tests</rdf:object>
</earl:asserts>
<earl:email rdf:resource='mailto:Jane@example.org'/> <earl:name>Jane Jones</earl:name>
</rdf:Description>
</rdf:RDF>
Listing 5.5 Generated RDF example of reification.
The method for reifying statements in RDF is to model the statement as a resource via explicitly specifying the subject, predicate, object, and type of the statement. Once the statement is modeled, you can make statements about the modeled statement. The reification is akin to statements as argument instead of statements as fact, which is useful in cases where the trustworthiness of the source is carefully tracked (for example, human intelligence collection). This is important to understand, as reification is not applicable to all data modeling tasks. It is easier to treat statements as facts.
Figure 5.6 displays a graph of the reified statement. Note that the statement is treated as a single entity via an anonymous node. The anonymous node is akin to a Description element without an rdf:about attribute. The rdf:parseType attribute in Listing 5.5 means that the content of the element is parsed similar to a new Description element.
96
Chapter 5
JOjxJ
http://*xjmplt.or0/onto#Jine
Figure 5.6 Graph of reification.
IsaViz is copyrighted by the W3C. All Rights Reserved. http://www.w3.org/Consortium/Legal/.
Admittedly, reification is not simple. Many people come to RDF understanding the basics of the triple but missing the utility of reification. Most databases treat data as facts, so it is a stretch to think about data as assertions. One com-monsense application of reification is annotations of other people's work. Annotations, by nature, are statements about someone else's statements. So, clearly, reification fits there. At the same time, it will take training for developers and modelers to understand where to use reification and what rules apply to reified statements. In fact, some current Semantic Web applications explicitly eliminate reification from their knowledge bases to reduce the complexity. Complexity hurts adoption, and the adoption of RDF by mainstream developers has been significantly slower than other W3C technologies. The next section examines the reasons why.
Why Is RDF Not in the Mainstream?
The Resource Description Framework has been a W3C Recommendation (synonymous with standard) since February 22, 1999, only slightly more than a year after the XML 1.0 Recommendation. When giving briefings on the future of XML, we are thus surprised to learn that many people have never heard of RDF. Outside of the digital library and artificial intelligence communities, RDF
& Graph
Understanding the Resource Description Framework
97
has not achieved mindshare with developers or corporate management. A demonstration of this mindshare gap is to compare the adoption of XML to the adoption of RDF. One simple measure is to compare, at this writing, the number of technical books on RDF versus XML and the number of commercial products supporting RDF versus XML as shown in Figure 5.7.
Figure 5.7 is based on simple Web queries of online retailers, and though not scientific, the disparity is so stark that you can easily confirm the results yourself. The Books column is a count of technical books available on Amazon.com, Software Products is a count of software available from Programmer's Paradise (www.programmersparadise.com), and Vendor Site Hits is a count of Web results returned by a search of Microsoft.com. Similar ratios were found when examining other sites or even the Web at large via a Google.com search.
Why has RDF adoption been so weak? There are multiple reasons:
RDF doesn't yet play well with XML documents. At this writing, you cannot validate RDF embedded in other XML or XHTML documents because of RDF's open grammar. In other words, RDF allows you to mix in any namespace-qualified elements you want (as demonstrated in the preceding examples). Additionally, there is a fairly esoteric issue regarding a difference between how XML Schema and RDF process namespaces. This has led many people to view RDF and XML documents as two separate paths for meta data. Therefore, the businesses traveling along the XML/XHTML path assume that their direction is incompatible with the RDF path. This is not true. The fact that RDF is serialized as XML means that both XML Schema and RDF share a common syntax. In the debates on this subject, it is clear that the intent of the W3C RDF Working Group is to resolve these differences so that RDF can be successfully embedded in XHTML and XML documents. Additionally, several tools mix RDF and HTML (including SMORE, demonstrated in the next section), so it is clear that bridges are being built to resolve this issue. Lastly, several solutions to this issue were proposed by Sean Palmer in the document "RDF in HTML: Approaches," available at http://infomesh.net/2002/rdfinhtml/.
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