RELAX NG by Eric van der Vlist will be published by O'Reilly & Associates (ISBN: 0596004214)
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Table of Contents
Although RELAX NG started as a standalone project under the auspice of the Organization for the Advancement of Structured Information Standards (OASIS), RELAX NG is now being standardized at ISO (ISO/IEC JTC1 SC34 WG1 to be precise) as a part of a multi-part standard named DSDL (see http://dsdl.org).
Standing for "Document Schema Definition Languages", DSDL is a recognition that the validation of XML documents is a subject too wide and complex to be covered by a single language. It also acknowledges that the industry needs a set of simple and dedicated languages to perform different validation tasks - as well as a framework in which these languages may be used together.
There are many different aspects in validating (or schematizing) XML documents which can be categorized into:
Validating the structure of the document, i.e. checking the containment of elements and attributes (this is the domain in which RELAX NG is very good).
Validating the content of each text node and attribute independently of each other (this is where datatype libraries are needed).
Validating integrity constraints between different elements and attributes.
Validating any other rules (often called business rules).
Throughout this book, we've seen how RELAX NG can help us to cover an important part of this issue. We've also seen that RELAX NG is simple and efficient because it has stayed focused on solving one, and only one, problem. There are huge gaps which cannot be covered by RELAX NG. For instance, if an XML vocabulary includes mixed content models, you can't restrict the content of your documents to be ASCII only, neither can you define that the content of your "modeling" element must be spell checked. The goal of DSDL is to provide means to fill out these gaps and to cover the whole domain of document validation.
DSDL can be seen as a framework and set of languages to check the quality of XML documents and this issue appears to be crucial for any XML based application. Recent works such as the presentation given by Simon Riggs at XML Europe 2003 or the work of Isabelle Boydens ("Informatique, normes et temps." Bruxelles, �ditions E. Bruylant, 1999.) about the quality of large databases have shown that about 10% of XML documents (or data records) contain at least one error. This level of quality is unacceptable for many applications. DSDL could thus be a technology which is absolutely indispensable for most of XML applications.
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All text is copyright Eric van der Vlist, Dyomedea. During development, I give permission for non-commercial copying for educational and review purposes. After publication, all text will be released under the Free Software Foundation GFDL.