XML Schema

Appendix A. XML Schema Languages

What Is a XML Schema Language?

Roughly speaking, XML schema languages describe XML documents. Different approaches to that task, however, provide a wide range of functionality.

XML Schema Languages Are Not Schemas

The first thing we can say about XML schema languages is they are not schemas. At least they do not match the definition of a schema as given by Webster’s dictionary, which states: “an outline or image universally applicable to a general conception, under which it is likely to be presented to the mind; as, five dots in a line are a schema of the number five; a preceding and succeeding event are a schema of cause and effect.”

This definition does not apply to the languages known as “XML schema languages”; most of these are more complex than the documents they describe and are too difficult to “be presented to the mind.” They focus on defining validation rules more than on representing or modeling a class of documents. When they do model a class of documents, they often want to add information to the documents they model.

Looking past the formal label of schemas, how can we classify so-called “XML schema languages”? Looking at all XML schema languages (DTDs, W3C XML Schema, RELAX NG, and also languages such as Schematron), the one thing they have in common is being transformations, which take a “schema” and an instance document as an input and transform them into a validation report, and optionally, into a PSVI (Post Schema Validation Infoset), ...