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Understanding RELAX NG

XML validation that's just right

Nicholas Chase (nicholas@nicholaschase.com), Author, Studio B

Nicholas Chase, a Studio B author, has been involved in Web site development for companies such as Lucent Technologies, Sun Microsystems, Oracle, and the Tampa Bay Buccaneers. Nick has been a high school physics teacher, a low-level radioactive waste facility manager, an online science fiction magazine editor, a multimedia engineer, and an Oracle instructor. More recently, he was the Chief Technology Officer of Site Dynamics Interactive Communications in Clearwater, Florida, USA, and is the author of several books on Web development, including XML Primer Plus (Sams). He loves to hear from readers and can be reached at nicholas@nicholaschase.com.

Summary: Many developers find DTD validation too simple (it doesn't use XML syntax, and doesn't enable developers to specify many of the rules needed in the real world) and XML Schema too complicated. RELAX NG represents a compromise: It uses XML syntax, and it enables developers to create most of the same rules as the W3C XML Schema language, but with a greatly simplified syntax. This tutorial explains the concepts behind RELAX NG in both its XML and compact forms.

Date: 04 Dec 2003
Level: Introductory
PDF: A4 and Letter (97 KB | 35 pages)Get Adobe® Reader®

Activity: 13005 views
Comments:

Introduction

What is this tutorial about?

As far back as the original XML 1.0 Recommendation, XML has included the ability to validate it, or compare it to a predefined structure. The first version of validation was called the Document Type Definition (DTD), but many developers thought this was too simple: It doesn't use XML syntax, and doesn't enable developers to specify many of the rules needed in the real world. The W3C then developed XML Schema, but many developers thought this was too complicated: It uses XML syntax and enables users to create just about any rule under the sun, but at the cost of adding complexity. Others combined the TREX and RELAX proposals into RELAX NG and many developers think it's just right. It uses XML syntax, and it enables developers to create most of the same rules as the W3C XML Schema language, but with a greatly simplified syntax.

This tutorial explains the concepts behind RELAX NG in both its XML and compact forms. It covers:

Creating a RELAX NG schema
Elements
Attributes
Types
Optional items
Mixed content
Restrictions
Modularizing a schema
Creating these rules using the compact syntax

The tutorial doesn't actually build an application; it uses a command-line application to simply validate the document against the schema document.

Tools

This tutorial doesn't involve any actual programming, but to verify the schema files you create, you'll need a validator. RELAX NG validators exist for a number of different languages, including:

Java technology -- Jing (http://www.thaiopensource.com/relaxng/jing.html) and MSV (http://wwws.sun.com/software/xml/developers/multischema/)
C -- Libxml2 (http://www.xmlsoft.org/)
C# -- Tenuto (http://sourceforge.net/projects/relaxng)
Python -- XVIF (http://freshmeat.net/projects/xvif/?topic_id=868)
ActiveX -- a DLL, VBRELAXNG (http://www.geocities.co.jp/SiliconValley-Bay/4639/vbrelaxng/vbrelaxng.html)