Other Approaches to Schemas for XML

As described in the February 2005 issue of CPU - Computer Power User magazine.

The following links provide information on the historical antecendents to the XML Schema - and information about alternatives developed outside the auspices of the World Wide Web Consortium.

A) XML-Schema Related

The following proposals were submitted to the W3C as proposals for an official schema language (the links go to W3C notes documenting the proposals).

• XML-Data (obsolete)
• XML-Data Reduced (XDR) (see also an updated version at http://www.ltg.ed.ac.uk/~ht/XMLData-Reduced.htm: obsolete, but still supported by Microsoft)
• Document Content Description for XML (DCD) (obsolete)
• Document Definition Markup Language (DDML) (obsolete)
• Schema for Object-Oriented XML (SOX) (obsolete but still supported by DataChannel)

B) Independent Schema Approaches

The following schema approaches were developed independent of the W3C:

• Document Structure Definition (DSD) (obsolete)
• RELAX (obsolete: merged with TREX to form RELAX-NG)
• Schematron. See also http://www.schematron.com/ for information on the ISO standard.
• TREX (obsolete: merged with RELAX to form RELAX-NG). TREX was based in part on the model introduced in the XDuce programming language.
• RELAX-NG
• Examplotron

C) Articles Comparing/Contrasting Different Approaches

• Comparing XML Schema Languages
• Comparing XML Schema with DTDs, Perl regular expressions, and SGML
• Chapter on XML schemas, with comparisons
• Taxonomy of XML Schema Languages Using Formal Language Theory (very mathematical!)

D) Discussion of Problems with XML Schema

• Problems with XML Schema
• Problems with content models