XML

Extensible_Markup_Language_(XML)_logo

XML (Extensible Markup Language) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. It is defined by the W3C's XML 1.0 Specification and by several other related specifications, all of which are free open standards.

History

XML was developed by the World Wide Web Consortium (W3C) and became a W3C Recommendation on February 10, 1998. It was designed to be a simplified subset of SGML (Standard Generalized Markup Language), which is a system for defining generalized markup languages for documents.

Design Goals

The design goals for XML are:

• XML shall be straightforwardly usable over the Internet.
• XML shall support a wide variety of applications.
• XML shall be compatible with SGML.
• It shall be easy to write programs that process XML documents.
• The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
• XML documents should be human-legible and reasonably clear.
• The XML design should be prepared quickly.
• The design of XML shall be formal and concise.
• XML documents shall be easy to create.
• Terseness in XML markup is of minimal importance.

Structure

XML documents are composed of units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form markup. Markup encodes a description of the document's storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.

Elements

Elements are the primary building blocks of XML. They are delimited by tags, with a start tag <element> and an end tag </element>. Elements can contain other elements, text, or a mixture of both.

Attributes

Attributes provide additional information about elements. They are placed inside the start tag of an element and are usually in name/value pairs.

Prolog

The prolog of an XML document can contain an XML declaration, which specifies the XML version and the character encoding used in the document.

Applications

XML is used in many aspects of web development, often to simplify data storage and sharing. Some common applications include:

• RSS feeds
• SOAP (Simple Object Access Protocol)
• AJAX (Asynchronous JavaScript and XML)
• SVG (Scalable Vector Graphics)
• XHTML (Extensible Hypertext Markup Language)

Related Technologies

• XSLT (Extensible Stylesheet Language Transformations)
• XPath (XML Path Language)
• XQuery (XML Query Language)
• XML Schema (XSD)
• DTD (Document Type Definition)

See Also

• JSON
• HTML
• YAML
• W3C
• SGML