XSDL - XML Schema Definition Language

XML Schemas represent the objects, and their properties and relationships for an XML application. The schema identifies the components and provides the rules by which XML documents are created, validated and processed. Consider XML schemas as the grammar for a customized Internet ready vocabulary.
Recently, the W3C's Schema Working Group submitted a working draft detailing a new XML schema standard, the XML Schema Definition Language, (XSDL). XSDL promises to bridge schema requirements for both the structured content and data processing worlds. Note, in the meantime there are three other choices for advanced XML schema formats; XML-Data, SOX and DCD. Microsoft's current validating XML parser supports XML-Data. We find many people turning to XML-Data today as they await the completion of the XSDL specification.

A database-inspired method for specifying constraints on XML documents using an XML-based language. Schemas address deficiencies in DTDs, such as the inability to put constraints on the kinds of data that can occur in a particular field (for example, all numeric). Since schemas are founded on XML, they are hierarchical, so it is easier to create an unambiguous specification, and possible to determine the scope over which a comment is meant to apply.

An XML Schema is a set of 'schema components'. There are 13 kinds of component in all, falling into three groups.

• The primary components : which may (type definitions) or must (element and attribute declarations) have names are as follows:
• Simple type definitions
• Complex type definitions
• Attribute declarations
• Element declarations
• The secondary components : which must have names, are as follows:
• Attribute group definitions
• Identity-constraint definitions
• Model group definitions
• Notation declarations
• The "helper" components : provide small parts of other components; they are not independent of their context:
• Annotations
• Model groups
• Particles
• Wildcards
• Attribute Uses