Extensible Markup Language (XML)

As the popularity of the Web exploded, HTML's limitations became apparent. HTML's lack of extensibility (the ability to change or add features) frustrated developers, and its ambiguous definition allowed erroneous HTML to proliferate. The need for a standardized, fully extensible and structurally strict language was apparent. As a result, XML was developed by the W3C.

Data independence, the separation of content from its presentation, is the essential characteristic of XML. Because XML documents describe data in a machine independent manner, any application conceivably can process them. Software developers are integrating XML into their applications to improve Web functionality and interoperability.

XML is not limited to Web applications. For example, it is increasingly being employed in databasesthe structure of an XML document enables it to be integrated easily with database applications. As applications become more Web enabled, it is likely that XML will become the universal technology for data representation. All applications employing XML would be able to communicate with one another, provided they can understand their respective XML markup schemes, called vocabularies.

The Simple Object Access Protocol (SOAP) is a technology for the transmission of objects (marked up as XML) over the Internet. Microsoft's .NET technologies (discussed in the next two sections) use XML and SOAP to mark up and transfer data over the Internet. XML and SOAP are at the core of .NETthey allow software components to interoperate (i.e., communicate easily with one another). Since SOAP's foundations are in XML and HTTP (Hypertext Transfer Protocolthe key communication protocol of the Web), it is supported on most types of computer systems. We discuss XML in Chapter 19, Extensible Markup Language (XML), and SOAP in Chapter 22, Web Services.