XSDs

XSD stands for XML Schema Definition, a vocabulary for defining the structure and semantics of XML documents. It specifies which elements and attributes may appear, their data types, and how they relate to one another.

XSD schemas are written in XML themselves. They define schema components such as element declarations, attribute declarations, and type definitions (simpleType and complexType). They support structural models using sequences, choices, and all, and they permit grouping through named groups.

XML Schema Datatypes provide built-in types such as string, boolean, decimal, integer, date, and time, with user-defined types possible via simpleType and complexType. Constraints use facets to restrict values—for example, minLength, maxLength, pattern, and enumerations—and occurrence constraints control how many times an element may appear (minOccurs, maxOccurs). Identity constraints such as unique, key, and keyref further constrain data relationships.

Namespaces and scope are managed with targetNamespace, which defines the namespace the schema applies to, and elementFormDefault and attributeFormDefault, which determine whether local elements and attributes must be qualified in documents. Schemas can import or include other schemas to share types and definitions.

Validation and usage: XML processors validate documents against a given XSD, using validators such as Xerces, Saxon, or similar tools. The W3C XML Schema standards include XSD 1.0 and 1.1, with 1.1 adding assertions for more expressive constraints. XSDs are widely used in data exchange, web services, and document formats to ensure data integrity.