The World Wide Web - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 14

< ? xml version =”1.0”? >< ! DOCTYPE people SYSTEM
”http://www.wsmo.org/workinggroup.dtd” >< ! −− This XML document gives
information about working group members of the WSMO working group
−−>< people xmlns=”http://www.wsmo.org/namespace” >
< title > WSMO working group members < /title >
< member chair=”yes” >
< firstname > Dieter < /firstname >< lastname > Fensel < /lastname >
< a liation > DERI International < / a liation >
< /member >
< member chair=”yes” >
< firstname > John < /firstname >< lastname > Domingue < /lastname >
< a liation > Open University < / a liation >
< /member >
< member >
< firstname > Axel < /firstname >< lastname > Polleres < /lastname >
< a liation > Univ. Rey Juan Carlos < / a liation >
< /member >
.
< /people >
Listing 2.1. An XML document
parsing and have made XML a widely accepted interchange format for various
(semi-)structured data over the last few years. A possible XML version of the
example in Fig. 2.4 is shown in Listing 2.1
Obviously, designated tag names and attributes in XML facilitate easier
processing of the content by a machine. XML allows the application designer to
design his/her own language with specific tags and attributes, leaving almost
all the freedom of SGML. The specific elements and attributes used are, just
as in SGML, declared in a DTD.
However, how can a Web browser, for instance, know how to display the
content in an XML document? More generally, how can machines using dif-
ferent tags and attributes still exchange data? To solve this, XML comes
with several accompanying standards for querying XML documents (XPath
and XQuery) and translating between different XML formats (XSLT). Using
these mechanisms, the document in Listing 2.1 can, for instance, be trans-
formed to XHTML, the XML version of HTML, which can be displayed by
Web browsers.
In the following, we shall briefly outline some of the essentials of XML,
such as namespaces and DTDs, and come to the conclusion that DTDs are
sometimes insu cient for describing the structure of the data in an XML
document. To overcome this, another standard for defining the structure of
XML documents, XML Schema, has been defined. After taking a look at XML
Schema, we shall also briefly introduce XPath and XSLT to familiarize the
reader with the fundamental ideas. We shall close this section and chapter
with a discussion of the outlook for XML applications and some bridging
remarks concerning the Semantic Web technologies described in more detail
in the next chapter.