Date: Mar. 2, 2001
Link: http://xml.oreilly.com/news/xmlnut2_0201.html
Source Author or Organization: Elliotte Rusty Harold, O'Reilly & Associates

This article by Elliotte Rusty Harold (co-author of O'Reilly & Associates' XML In A Nutshell) provides an explanation of the Resource Directory Description Language (RDDL).

Harold discusses the most troublesome of the problems arising from a controversy surrounding the Namespaces in XML specification and the question "What goes at the end of an XML Namespace URI?" The correct answer (nothing: it's a URI, not a URL) has been a thorn in the side of many developers who were confused by the existence of XML Namespace URIs (Uniform Resource Identifiers) that didn't actually map to actual URLs (Uniform Resource Locators).

RDDL documents provide a solution to what has become the riddle of XML Namespaces. RDDL documents reside at the end of an XML Namespace URI, providing lists of available machine-processable documents that may be associated with the XML Namespace itself. Supported documents include (but are not limited to): specification documents, Cascading Style Sheets (CSS) , XSLT style sheets, Document Type Definitions (DTDs), as well as Schematron, REgular LAnguage description for XML (RELAX) and Tree Regular Expressions for XML (TREX) schemas.

Harold briefly explains RDDL syntax and how the rddl:resource element is used to provide simple XLinks to connect an XML Namespace to its associated documentation as well as how to embed RDDL elements within XHTML documents using the XHTML Modularization and XHTML Basic specifications.