Want to Transition from DTDs to XML Schemas?  
Just RELAX!

One of the papers to be presented at XTech at the end of February focuses on RELAX (REgular LAnguage for XML)..  RELAX is a language for representing regular sets of XML documents as grammars. According to the presentor, Murata Makoto, A RELAX grammar generates a set of XML documents. Conversely, XML documents can be validated against a RELAX grammar. RELAX consists of RELAX core and RELAX Namespace. RELAX core provides modules, which declare and constrain elements and attributes in a single namespace."   The XTech presentation will focus on RELAX core. 

RELAX is being positioned as a transition path from traditional XML DTDs to XML Schemas.  When RELAX is compared with a traditional DTD (Document Type Definition), we see that RELAX has new features that DTDs do not have:

• A RELAX grammar can be written as an XML document.
• RELAX borrows rich datatypes of XML Schema Part 2.
• RELAX is namespace-aware.

RELAX is more expressive than DTD in representing structural constraints on elements and attributes. RELAX, however, does not provide mechanisms for declaring entities, notations, and default values, which have been captured by DTDs. Rather, RELAX is intended to be used in conjunction with DTD.  A RELAX grammer is made up of one or more modules.  A RELAX module consists of rules (corresponding to element type declarations) and patterns (corresponding to attribute list declarations).

Perhaps the biggest question about RELAX is why do we need it.  Doesn't RELAX provide some of what XML Schema Definition Language is supposed to provide?  When is RELAX useful?  Why not just wait for the W3C XML Schema Definition Language?  These questions are answered in the FAQ page of the RELAX Web site:

• The scope of XML Schema is very broad. Thus, its standarization, implementation and adoption require a relatively long time. Furthermore, to take full advantage of XML Schema, extensions to DOM and SAX are required. 
• On the other hand, RELAX is much more modest. Thus, it is easier to standardize, implement, and adopt. RELAX is a combination of (1) features of DTD, and (2) rich datatypes of XML Schema represented in the XML syntax. RELAX also has some other mechanisms, but they have been eliminated from the conformance level "classic".
• RELAX helps migration from DTD to XML Schema. You can assume that RELAX is DTD combined with datatype information in the XML instance syntax and start to use RELAX right now. When XML Schema is available, migration from RELAX to XML Schema will be possible without loss of datatype information.

Will RELAX be used?  Only time will tell.  It certainly seems that the upward compatibility with DTDs will be helpful from those of us already using either XML or SGML DTDs.  Keep an eye on RELAX and visit the RELAX Web site for more information.