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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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Understanding the Resource Description Framework
Noncontextual modeling is a continuum and not a single point. In fact, markup languages have been following the trend toward noncontextual modeling over the last several years via namespaces and modularization. Namespaces divide a set of terms (used as elements or attributes) into domain-specific vocabularies with fixed definitions. Modularization allows namespaces to be mixed and matched to assemble a document (sometimes on the fly) that conveys the desired meaning. Two examples of such modularization are XHTML and XBRL. XHTML is described in detail in the next chapter. XHTML modularization allows you to mix and match vocabularies inside of HTML documents. The extensible business reporting language (XBRL) uses both modularization and taxonomies (discussed in Chapter 7) for the description of financial statements for public and private companies. The XBRL specifications are available at http://www.xbrl.org.
RDF takes this trend toward composeable context to its logical conclusion. How does RDF implement noncontextual modeling? RDF creates a collection of statements and not a document. Therefore, the context of a set of RDF statements cannot be determined beforehand; instead, it is wholly dependent on the statements themselves and the relationships between the sentences. In a sense, this disconnect between a list of statements and a hierarchical tree is the root cause of the difficulty in encoding RDF in RDF/XML syntax, because it attempts to marry a list of statements with a hierarchical tree structure. Following are two key aspects of this noncontextual modeling:
Non-contextual modeling uses explicit versus implicit relationships.
XML documents create a hierarchy of name/value pairs. As demonstrated in Chapter 3, both elements and attributes revolve around a name and a typed value. However, XML does not state the relationship between the name and the value. The relationship between them is implicit. On the contrary, RDF uses an explicit relationship between the name and the value with the triple structure: subject, predicate, and object.
A graph is less brittle than a tree. A collection of RDF statements can be added to dynamically without regard to order or even previous statements. In fact, a previous statement can be reified and deprecated by another statement. This allows the RDF graph to be robust in the face of change and suffer less from the brittle data problem and need for versioning and compatibility issues that can plague XML documents. Why is this? Part of the reason is the basic difference between a document and a collection of RDF statements. Tim Berners-Lee highlighted several of these differences in his document entitled "Why RDF Model is Different from the XML Model," available at http://www.w3.org/DesignIssues/RDF-XML.html. He stresses several differences between the XML document
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model and an RDF graph. First is that there are many possible XML documents that can express a set of semantic assertions. Therefore, RDF simplifies this via a semantic model also known as the triple model. In other words, RDF makes you explicitly define the semantics of your data and thus avoid confusion and alternate syntaxes.
Another obvious difference he highlights is that order is often very important in a document but not important to an RDF graph. Many times the order reflects implicit context not expressed in the name/value pairs. By forcing explicit relationships between subjects and objects, RDF avoids this. Of course, if order is important and it changes, you have an incompatible change to the document structure; hence, this is another example where an RDF list of statements is less affected by change and therefore less brittle.
One application (among many) that is bridging the gap between contextual and noncontextual modeling is called SMORE, developed by Aditya Kalyan-pur of the University of Maryland, College Park. SMORE stands for Semantic Markup, Ontology, and RDF Editor. It allows you to embed RDF markup inside of HTML documents during the HTML authoring process. Figure 5.11 displays embedding an RDF triple in a simple HTML document by highlighting some text in the HTML editor.
Chapter 5
Figure 5.11 Semantic Markup, Ontology, and RDF Editor (SMORE).
Understanding the Resource Description Framework
Figure 5.11 is a simplified view of the SMORE desktop, which starts out with four windows: an HTML editor (shown), semantic data representation (shown), Web browser (not shown), and an ontology manager (not shown). SMORE allows you to select an ontology and easily add triples about the information in your Web pages to your HTML document. Listing 5.9 displays the generated document with the RDF embedded in the head of the HTML document.
<html>
<head>
<script type="application/rdf+xml">
<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:general1.0="http://www.cs.umd.edu/projects/plus/DAML/onts/gen-
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