XML, Web Services, and the Data Revolution

By Frank  P.  Coyle

XML is not just a technology for defining data vocabularies. Surrounding XML is a wide variety of XML standards and initiatives that act in combination with XML to address many of the issues associated with bringing XML into mainstream computing, namely presentation, structure, and transformation. As Figure 2.1 shows, the XML core includes XML itself, based on the XML 1.0 specification, and namespaces, the specification that allows XML documents from different sources to be combined and yet be able to disam biguate elements with the same name from different sources. Following is a list of other categories in the XML technology:

• Structure and data types: When using XML to exchange data among clients , partners , and suppliers, it's important to be able to define how XML documents should be structured. DTDs and XML Schema provide this capability. DTDs come out of the world of Standard Generalized Markup Language (SGML), focusing primarily on structure by specifying what elements and attributes are considered valid for a particular XML instance document. DTDs have limited capability to specify data types, a circumstance that can be explained historically by considering their origins in the document culture (see Figure 1.9 in Chapter 1). XML Schema is a more recent initiative of the World Wide Web Consortium (W3C) that puts a more conventional data processing spin on describing XML data with more precision than with DTDs.

• XML presentation technologies: In keeping with an important design pattern for robust systems, XML intentionally separates data content from presentation through the introduction of supporting technologies that focus on delivering content to users via a variety of devices and presentation media. Among the technologies for presentation are XHTML, a modular XML-conformant replacement for HTML; CSS for controlling the display properties of HTML or XML in Web browsers; XSL and XSL Formatting Objects (XSL-FO) for formatting XML for various output media; XForms for collecting data from Web forms and returning XML; VoiceXML, for delivering content to voice-enabled devices; and Wireless Markup Language (WML), for delivery to wireless devices enabled for Wireless Application Protocol (WAP).

• XML manipulation technologies: Manipulation technologies provide the capability to extract and transform XML in different ways. These technologies play an important role in server-based XML processing for business-to-business (B2B) data manipulation and exchange. XSLT is widely used to transform XML from one format to another; XPath is a technology used by other XML technologies to navigate through an XML tree structure and zero in on particular elements or subtrees; XLink is a technology for creating and describing links between resources and for enabling links that go beyond the simple unidirectional links of the current Web; and XQuery is an evolving technology for extracting and querying XML repositories.

• Other related technologies: The XML family of technologies also includes initiatives for working with metainformation, which is literally information about the information contained in an XML document. Technologies in this space include RDF and InfoSet.