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What is it?
Extensible Markup Language is a document format that is both human- and machine-readable, as it uses plain text for data representation. XML documents contain both data and metadata tags describing what the data is.
XML has been widely adopted because it is easy to create new industry-specific languages. Tag sets can be quickly agreed between business partners.
Some have argued that the language is becoming too confusing. For example, the World Wide Web Consortium's (W3C) technical committee is trying to develop a standard XML format for musical notation. Sixteen existing musical XMLs are being taken into account, including Standard Music Description Language, Extensible Score Language, Music and Lyrics Markup Language and two with dangerously similar names: MusicML and MusiXML.
Where did it originate?
Five years ago, when the W3C set out to make the richly productive but dauntingly difficult Standard Generalised Markup Language as easy to use as HTML.
What is it for?
XML found its earliest uses in publishing, but it is increasingly used as a business data exchange format. Once transformed into XML, documents can be easily extended to include new data fields, but can still be understood by people who used the original human-readable document. However, in practice, the tag sets can prove a stumbling block
What makes it special?
It is quick and easy to integrate into existing business processes. XML is universally recognised, quick to develop and flexible enough to be used almost anywhere.
XML parsers and other tools can be downloaded for free, and are being built into development tools and end-user products. From the employer's point of view, XML skills are widely available, whereas for XML developers, there is a great variety of work on offer.
How difficult is it to master?
HTML developers should make the move easily enough, but they will also find XML requires far more precision and discipline. Unless developers stick to the rules, the code will not work.
Where is it used?
By publishers, the chemical and oil industries, banks, life and pensions firms, electrical retailers, the Ordnance Survey, agricultural suppliers, meat and poultry companies, students of ancient Sumerian literature and space exploration researchers.
XML is taking over from Electronic Data Interchange as a cheaper way of exchanging business documents.
What systems does it run on?
Most software suppliers support it. Microsoft's new information-gathering tool, Office Infopath 2003, uses industry-standard XML.
Not many people know that...
There is an XML for chess (ChessML), for weather (WeatherML) and for historical events (HEML).
What is coming up?
The W3C's query working group, which includes IBM, Oracle and Microsoft, is developing standards that will enable all XML documents to be used as one enormous database.