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Overview of XML

XML (eXtensible Markup Language) is a method for putting structured data in a text file.

Although an XML document superficially resembles an HTML document in that they are both the product of a markup language that uses tags, that is where the similarity ends. XML overcomes the common pitfalls of unstructured markup languages, of which HTML is the most widely used.

XML produces files that overcome ambiguity, lack of extensibility, lack of support for internationalization/localization, and platform-dependency.

XML is often called a meta-markup language, and that is because XML tags are used differently than those in convential markup languages. Whereas <p> means paragraph in HTML, <p> in XML could mean anything depending on the stylesheet that is used to translate the XML. Therefore, tags delimit pieces of data in order to structure it appropriately, but leave the interpretation of the data completely to the application that reads it.

The rules of XML syntax are much stricter than for HTML. In HTML, forgotten tags or attributes are tolerated. But the official XML specification prohibits second-guessing the meaning of a broken XML file; if the file is broken, an application has to stop and issue an error.

XML files are text files. As such, they are larger than equivalent binary files, but they can be easily compressed. XML files are not intended to be human- parsable the way HTML file are, but they are readable for purposes of debugging, for example.

The XML family of technologies

A family of technologies has developed around the XML 1.0, specification that defines what tags and attributes are. These include:

» Xlink describes a standard way to add hyperlinks to an XML file. 

» CSS, the stylesheet language, is applicable to XML and to HTML.

» XSL is the advanced language for expressing stylesheets. It is based on XSLT, a transformation language for rearranging, adding or deleting tags and attributes.

» DOM is a standard set of function calls for manipulating XML (and HTML) files from a programming language.

» XML Namespaces is a specification that describes how to associate a URL with every tag and attribute in an XML document.

» XML Schemas 1 and 2 help developers to precisely define their own XML-based formats.

What is the origin and future of XML?

Development of XML started in 1996, and it became a World Wide Web Consortium (W3C) standard in February 1998. It evolved from SGML, developed in the early '80s and an ISO standard since 1986, and from HTML, whose development started in 1990.

The designers of XML took the best parts of SGML, guided by the experience with HTML, and set out to produce something that is simpler to use. Growth of XML has outpaced the most optimistic projections.

Choosing XML for data interchange is analogous to choosing SQL for databases. It's universally used, but you still have to build your own database and your own programs/procedures to manipulate it. There are already many tools available, and many software developers who are proficient with XML.


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