XSD

XSD, short for XML Schema Definition, is a W3C standard for describing the structure, content, and semantics of XML documents. It provides a vocabulary of elements and attributes, data types, and constraints that can be used to validate an XML instance document. By specifying the allowable structure and data types, XSD helps ensure that documents are well-formed and conform to a defined data model, enabling reliable data exchange between systems.

An XSD is itself an XML document. It defines complex types (combinations of elements and attributes) and simple types (single data values), and supports features such as type derivation, namespaces, element order, and occurrence constraints (minOccurs, maxOccurs). Schemas can import or include other schemas and can reuse named types across documents. Built-in datatypes include string, boolean, decimal, integer, dateTime, and many others, while users can restrict types via facets like length, pattern, enumerations, or numeric ranges.

XSD 1.0 established the core standard in the early 2000s; XSD 1.1 extends it with more constraints and assertions. Schema-aware validation is performed by XML parsers and validators, which can validate an XML document against a schema identified by xsi:noNamespaceSchemaLocation or xsi:schemaLocation or embedded inline. Real-world use cases include data interchange, configuration files, and document validation in industries such as finance, publishing, and government.

Compared with DTDs and RELAX NG, XSD offers stronger data typing and namespace support, at the cost of greater complexity. W3C standards and multiple open-source and commercial tools support XSD validation across platforms and languages.