XMLSchema

XML Schema is a language for defining the structure, content, and semantics of XML documents. The standard, developed by the World Wide Web Consortium (W3C) and often referred to as XML Schema Definition (XSD), specifies how to declare elements, attributes, data types, and the allowable document structure, enabling validation of XML instances against a schema. The schema language is expressed as XML itself and uses the namespace http://www.w3.org/2001/XMLSchema.

XML Schema originated in the early 2000s as a comprehensive replacement for DTDs, offering a typed data model and better support for modularization, namespaces, and extensibility. The core specifications were published as W3C Recommendations for XML Schema 1.0, with XML Schema 1.1 released later. The language is designed to describe the allowable content of elements and attributes, enforce data types, and express constraints beyond what DTDs provide, including complex type definitions and formal relationships between elements.

Key features include a rich type system with built-in and user-defined simpleType and complexType definitions, inheritance through extension and restriction, and facets that constrain values. It supports lists and unions, identity constraints such as key, unique, and keyref, and structuring constructs like sequence, choice, and all for content models. Schemas can import or include other schemas, and support namespaces to enable reuse and composition.

In practice, XML Schema files are used to validate XML documents, guide data binding in programming languages, and define data interchange contracts. While it remains the most widely adopted XML schema language, it coexists with other schema approaches such as Relax NG and Schematron, and with older DTD-based validation.