16.7 Summary
In this chapter, we have reconsidered schema languages. A schema is useful as a mechanism for checking documents and can also be used as a data model for accessing valid documents.
A number of mechanisms have been introduced to W3C XML Schema to provide powerful data models and help interworking with Java and RDBMS. However, one could argue that such mechanisms have complicated W3C XML Schema and have actually made it difficult to use schemas as data models and to map W3C XML Schema to Java, RDBMS, and other programming languages.
On the other hand, RELAX NG is a simple schema language. The reason is that RELAX NG validators are not intended to solve all problems. Rather, data binding tools for RELAX NG provide easy access to valid documents and mapping from RELAX NG to Java or RDBMS.
Special-purpose schema languages such as RDF Schema are useful for particular types of XML documents. They provide higher abstraction and make data binding easier. RELAX Namespace allows special-purpose and general-purpose schema languages to be combined to collectively form a single schema.
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