XML Hacks

Use RELAX NG and Schematron Together to Validate Business Rules

There are few issues regarding XML validation that cause as many headaches as validation of business rules (constraints on relations between element and attribute content in an XML document). This hack helps relieve that headache.

Even after the release of the new, grammar-based schema languages XML Schema and RELAX NG, it remains difficult to express restrictions on relations between the contents of various elements and attributes. This hack introduces a method that makes it possible to validate these kinds of rules by combining two XML Schema languages, RELAX NG (http://www.relaxng.org/) and Schematron (http://www.ascc.net/xml/resource/schematron/).

W3C XML Schema (http://www.w3.org/XML/Schema) lacks much support for co-occurrence constraints, and RELAX NG supports them only to the extent that the presence or absence of a particular element or attribute value changes the validation rules. On the other hand, Schematron provides good support for these types of constraints. Schematron is a rule-based language that uses path expressions instead of grammars to define what is allowed in an XML document. This means that instead of creating a grammar for an XML document, a Schematron schema makes assertions applied to a specific context within the document. If the assertion fails, a diagnostic message that is supplied by the author of the schema is displayed.

One drawback of Schematron is that, although the definition of ...