Core XML

Extensible Markup Language (XML) is a specification by the Worldwide Web Consortium (W3C) for creating markup languages such as XHTML. The XML specification is being used in an increasingly wide range of applications for exchanging data between different environments, platforms and media. One of the main reasons for its rapidly growing popularity is its flexibility. It allows developers to define their own hierarchy of tags which reflect the structure of their data. In addition to the core XML specification, a number of XML related technologies have been developed for working with XML projects.

XML Schema

XML schema documents and schema languages are used to validate and document XML markup languages. They allow developers to explicitly lay down the syntactical and structural ground rules for particular classes of XML documents. A number of schema languages have been developed but perhaps the two most important are those defined by the W3C: Document Type Definition (DTD) which was built into the original XML specification and the more XML schema language developed a few years later.

XSL, XSLT AND XSL-FO

Extensible Stylesheet language (XSL) comprises two parts: XSL Transformations (XSLT) and XSL Formatting Objects (XSL-FO). XSLT stylesheets are essentially XML documents which transform a given class of XML document (referred to as the source tree) into a different class of XML document (referred to as the output tree). XSLT documents can be compared to CSS documents but are much more powerful. XSL-FO documents are written in XML and control the formatting layout in an output document in a very detailed manner. It allows the definition of layout masters from which are derived page sequences for generating the final document pages. Flow objects are then used to bind data to the various regions of pages.

XPath, XLink, XPointer and XQuery

XPath, XML Linking Language (XLink), and XML Pointer Language (XPointer) are all provide solutions for linking and addressing when working with XML documents. XPath is a declarative language which uses expressions that resemble the directory paths used to describe locations in computer file systems. XPath expressions are used in XSLT documents to address specific nodes within the hierarchy of the XML source document. XLink is essentially a method of creating hyperlinks within XML documents. It overcomes some of the limitations of the hyperlinks used in XHTML, such as the fact that each hyperlink has only one source and one destination. XPointer allows you to create links which point to very specific points within the hierarchy of an XML document by including XPath expressions in its syntax. XQuery is designed to query XML data in much the same way that SQL is used to query databases. It uses XPath expressions to extract elements and attributes within the structure of an XML document.

The author is a trainer and developer with Macresource Computer Training, a UK IT training company offering XML and XSLT training courses in London and throughout the UK.