Chapter 3. The Extensible Markup Language

This chapter provides a general overview of XML. If you are familiar with XML, feel free to skip ahead to Chapter 4.

James Clark invented the name Extensible Markup Language and its abbreviation XML. He has been quoted as saying, "XML isn't going to win any prizes for technical elegance. But it was just simple enough that it could get broad acceptance, and it has just enough Standard Generalized Markup Language (SGML) stuff in it that the SGML community felt they could embrace it." (SGML is a standard indicating how to specify a document language or tag set.)

A markup language is a mechanism for identifying structure in a document. The Extensible Markup Language Recommendation [XML] specifies a way to structure, store, and send information. It does not say anything about what information should be presented to the user, however. Rather, this meta-markup language is used for creating other languages (each called an XML "application"). As such, XML is really a grammatical system that enables the creation of customized markup languages for particular documents and domains [Harold].

It is a quirk of XML documentation nomenclature that it uses the term "application" very liberally. In particular, computer code that reads or prints XML, generates or verifies XML signatures, or the like, is frequently called an "application," such as an "XML signature application" in the XML literature. This terminology is employed even when, in other computer contexts, the code would be referred to as a "subroutine," "library," or other term clearly indicating that it is merely one building block in a complete application or system.

An XML markup language defines tags for labeling content, and the relationship between such tags. Consider the following example:

 <foo>. . . content of tag foo . . .</foo> 

The XML Recommendation defines a methodology for tag creation. It specifies neither tag semantics nor a specific tag set. That is, XML specifies structure, not meaning. You can define an infinite number of markup languages based on the XML Recommendation standards.

Once defined, tags are mixed with character data to form an "XML document." An XML document can take numerous forms. For example, it can be a logical structure within a computer program or an external file in the traditional sense. Likewise, an XML document can be sent as a data stream, reported as a database result set, or dynamically generated by one application and sent to another.