Metaschema Layering for XML

The Extensible Markup Language (XML) is based on the concept of schema languages, which are used for validation of XML documents. In most cases, the metamodeling view of XML-based application is rather simple, with XML documents being instances of some schema, which in turn is based on some schema language. In this paper, a metaschema layering approach for XML is presented, which is demonstrated in the context of various application scenarios. This approach is based on two generalizations of the standard XML schema language usage scenario: (1) it is assumed that one or more schema languages are acceptable as foundations for an XML scenario, but these schema languages should be customized by restricting, extending, or combining them; (2) for applications requiring application-specific schema languages, these schema languages can be implemented by reusing existing schema languages, thus introducing an additional metaschema layer. Metaschema layering can be used in a variety of application areas, and this paper shows some possible applications and mentions some more possibilities. XML is increasingly entering the modeling domain, since it is gradually moving from an exchange format for structured data into the applications as their inherent model. XML modeling still is in its infancy, and the metaschema layering approach presented in this paper is one contribution how to leverage the most important of XML feature’s, which is the reuse of existing concepts and implementations.