Extensible Markup Language (XML) is a generic markup language primarily used to store / transfer data from Unicode text fields in structured branching. This language is termed extensible because it allows the user to define the tags from items. The user can increase the namespace tags and use the definitions of other users.
The World Wide Web Consortium (W3C), developer of standards for the sharing of information on the Internet, recommends the XML syntax to express specific markup languages.
Many languages follow the XML syntax: SVG vectorized graphic data, XHTML web page data, data feeds for content syndication, XSLT for data transformation of an XML document, etc. This syntax is recognizable by its use of angle brackets () for element tags and the string for code comments.
The initial objective is to facilitate the automated exchange of content between heterogeneous information systems (interoperability). XML is a simplification of the Standard Generalized Markup Language (SGML) which retains the essential principles as:
– The structure of an XML document can be defined and validated by a schema,
– An XML document is completely transformed into another XML document.
To promote exchange with other word processors, Microsoft proposed RTF, “rich text format” (1987). This is not a binary format, the commands are entered as readable text.
The documents referred to as well formed are those that meet all the basic specifications of format and can therefore be successfully parsed by any parser. The documents have to follow a strictly hierarchical structure with regard to labels that define its elements. A label must be properly included in another, ie, the labels must be properly nested, and content elements properly closed.
XML documents allow only a root element from which all others are parties, ie can only have one initial element. The XML attribute values must always be enclosed in single or double quotes.
A few months after its release, XML was used to encode data, program changes, represent an object printer, and define the schema of an XML document. Few years later, the uses had ballooned to include:
– Markup language documents,
– Data format
– Description language document format (DSDL)
– Representation language (text, image …)
– Programming language,
– Communication protocol.
– Data interchange over the Internet.
These categories provide a rough classification of languages based on XML (or accepting an XML expression). The list of languages mark some outstanding specifications.
XML has become the reference format for the exchange of data, including metadata. The transfer of information between relational database illustrates the benefits and limitations of this form for this purpose.
XML version 1.0 was released February 10, 1998. Version 1.1 released February 4, 2004 made improvements in the support of different versions of Unicode. The W3C recommends XML processors recognize both versions, although the first version is much more prevalent than the latter.