Extensible Markup Language (XML) is a flexible, text-based markup language designed to store, transport, and structure data in a format that is both human-readable and machine-parseable. Developed by the World Wide Web Consortium (W3C) and first published as a recommendation in 1998, XML uses a hierarchical, tag-based structure where users define their own custom tags to describe the data they contain. This self-describing nature makes XML a versatile format for data exchange between different systems and platforms.
XML's design follows several key principles that contributed to its widespread adoption:
- Extensibility – Unlike HTML, XML has no predefined tags; developers create tags that match their data model.
- Strict syntax – XML documents must be well-formed, with properly nested and closed tags, ensuring reliable parsing.
- Platform independence – As plain text, XML can be processed by any programming language on any operating system.
- Schema validation – XML Schema (XSD) and DTD allow documents to be validated against formal structure definitions.
- Unicode support – Full internationalization support for content in any language.
XML has been used heavily across software development. It's the basis for configuration files in frameworks like Spring and Maven, powers SOAP-based web services, defines user interfaces in Android development, and underpins document formats like Microsoft Office's OOXML and the open-source ODF. RSS and Atom feeds, SVG graphics, and XHTML are all XML-based formats still in widespread use.
While JSON has largely replaced XML for lightweight data interchange in modern web APIs due to its simpler syntax, XML is still the go-to choice in enterprise environments where strict schema validation, complex document structures, namespace management, and transformation tools like XSLT are needed.