3.5 XML

XML...is so simple and elegant as a concept, that one now wonders how is possible that this has not been invented years and years ago. In fact, it is very hard to think any computer data that cannot be described by some XML structure. W3C created XML to allow web documents to be easily interpreted by humans and computers as well, and is now widely used for the representation of arbitrary data structures, i.e. in web services, as well as the underlying model of several data formats for different desktop applications (Microsoft Office 2007 uses docx format, or Sun's Open Office).

XML is not a programming language as per se. Is a descendant of SGML, with user created mark-up and tags which provide the documents with the required semantics necessary to be successfully interpreted by the applications which uses them, thus greatly enhance the interoperability of different computer systems. In order to do so, Document Type Definition(DTD) and XSD schemas define the structures based on which XML documents are created and validated against, with the latter providing additional support for data definition and datatypes.

On the web front, XHTML, an extended HTML version, added well-formed and case-sensitive restrictions and as a consequence of this, XHTML documents can be processed using standard XML tools, like XPath (used to traverse the logical structure of an XML document in order to query and extract the encapsulated data), as well as the XSL Stylesheet Language which allows the transformation of an XML document into another.

Finally, modern databases (like MS Sql Server 2005 or Oracle), have incorporated the XML technology, by supporting native XML datatypes and integrated versions of tools like the XQuery which allows access and navigation of xml documents based on XPath 2.0.