XMLs

XMLs are XML documents—files or data streams that encode information using elements and attributes defined by the author. XML, or Extensible Markup Language, is a text-based format designed to be both human readable and machine readable, intended for flexible data interchange between diverse systems.

A typical XML document begins with an optional XML declaration and, optionally, a Document Type Definition declaration. It must contain a single root element, with all other elements properly nested. XML is case sensitive, and attributes must be quoted. Document well-formedness requires correct start and end tags, proper nesting, and valid syntax for entities and character data. In addition to well-formedness, documents can be validated against a schema or a DTD to enforce structure and data types.

Common mechanisms for defining the structure of XMLs include XML Schema (XSD), RELAX NG, and DTDs. Namespaces are used to prevent name collisions when combining XML data from different vocabularies.

XMLs are processed by parsers in many programming languages. They are commonly transformed or queried using XSLT, XPath, or XQuery, and serialized in various encodings. Widespread XML-based formats include RSS feeds, SOAP messages, SVG graphics, and XHTML documents.

Advantages of XMLs include platform independence, self-description, and strong support for validation and transformation. Limitations include verbosity and processing overhead relative to binary formats. XMLs remain a foundational technology for data interchange and configuration across many industries.