XML Concepts and Technologies

Extensible Markup Language (XML). XML is a set of rules for creating structured text documents using tags similar to those in HTML. An XML-formatted purchase order might include elements such as “customer name,” “item quantity,” or “price.” Tags (e.g., <CustomerName>) specify where elements begin and end within the document. XML 1.0 is a World Wide Web Consortium (W3C) Recommendation, the highest level of standardization granted by that body.

Schemas. XML allows developers to define new element types and tags using schemas. An XML schema also spells out which elements must or may appear in a particular type of XML document. With a schema, an XML document processor (such as Internet Explorer 5) can automatically validate documents. For example, it can verify that an invoice document includes a required “vendor name” element. There are several languages for writing schemas; XML Document Type Definitions (DTD) is the only standard one and the most widely used today. Microsoft’s XML tools use a proprietary schema language called XML Data Reduced (XDR), which makes a number of improvements over DTD. For example, XDR schemas can specify data types (e.g., decimal number, date) and allowable values of elements, enabling parsers to do much more extensive validation. Microsoft says it will eventually replace XDR with the XML Schema language, a schema language based in part on XDR that is currently a Candidate Recommendation of the W3C (which puts it roughly a year away from full Recommendation status). Microsoft also says it will provide automated tools for translating XDR schemas to XML schemas.