Design requirements

From the W3C Note "XML Schema Requirements" (Feb. 1999):

Design principles: The XML schema language shall be

1. more expressive than XML DTDs;
2. expressed in XML;
3. self-describing;
4. usable by a wide variety of applications that employ XML;
5. straightforwardly usable on the Internet;
6. optimized for interoperability;
7. simple enough to be implemented with modest design and runtime resources;
8. coordinated with relevant W3C specs.

Structural requirements: The XML schema language must define:

1. mechanisms for constraining document structure (namespaces, elements, attributes) and content (datatypes, entities, notations);
2. mechanisms to enable inheritance for element, attribute, and datatype definitions;
3. mechanism for URI reference to standard semantic understanding of a construct;
4. mechanism for embedded documentation;
5. mechanism for application-specific constraints and descriptions;
6. mechanisms for addressing the evolution of schemata;
7. mechanisms to enable integration of structural schemas with primitive data types.

W3C's schema language, XML Schema, has been defined with these criteria in mind. In our opinion, XML Schema

• is not expressive enough,
• is not self-describing,
• definitely not simple,
• does not properly address schema evolution.