XML Standards Update;
XML Schema Updated One Last Time!

On March 30, a second, and hopefully final "Proposed Recommendation" for the 3 documents makal' to one datatype."  In reading the abstract of this new version of XML Schema we find that "The only change from that draft is that the type there called 'number' iing up XML Schema was posted.  This latest version of XML Schema contained only one minor change from the first "Proposed Recommendation" version of XML Schema that was published on March 16, 2001.  According to the press release from W3C, the purpose of this version is to restore "the name 'decims here renamed 'decimal'. This type was called 'decimal' up until the draft of 16 March 2001, so this change simply restores the original name of this type."

XML Schema is perhaps the most significant specification to come out of W3C in the past 2 years.  This specification contains 3 parts:

• XML Schema Part 0 is also known as the Primer.  It is a document intended to provide an easily readable description of the XML Schema and is designed to help the reader quickly understand how to create schemas using the XML Schema language. The Primer is the only non-normative portion of the schema specification.  It contains numerous examples which are complemented by extensive references to the normative texts.
• XML Schema Part 1 is also known as Structures.  It specifies the XML Schema definition language.  The schema definition language, which is itself represented in XML 1.0, uses namespaces, substantially reconstructs and considerably extends the capabilities found in XML 1.0 document type definitions (DTDs). 
• XML Schema Part 2 is also known as Datatypes.  It specifies facilities for defining datatypes to be used in XML Schemas as well as other XML specifications. Like the schema definition language, the datatype language is represented in XML 1.0 and provides a superset of the capabilities found in XML 1.0 document type definitions (DTDs) for specifying datatypes on elements and attributes.

The XML Schema activity within the W3C was launched just prior to XML 98 in Chicago.  It has taken about 3 years of work to complete this very important cornerstone web technology.  In September 2000, the last Working Draft of XML Schema was published.  This document presented significant changes  from earlier working drafts.  Some of the notable changes in the September 22 WD included:

• Equivalence classes have been renamed substitution groups, to reflect the fact that their semantics is not symmetrical.
• The content model of the complexType element has been significantly changed, allowing for tighter content models and a better fit between the abstract component and its XML Representation
• A new form of schema composition operation, similar to that provided by include but allowing constrained redefinition of the included components has been added, using a redefineelement.
• The defaulting for minOccurs and maxOccurs attributes of elementhas been simplified: it is now 1 in both cases, with no interdependencies.
• The content model for the group element when it occurs at the top level has been tightened, to allow only a single all, choice, group, or sequencechild.
• To avoid potential confusion the DTD for schemas is no longer normative, but its use is still encouraged.

In October, 2000, XML Schema moved to "Candidate Recommendation."   The first "Proposed Recommendation" followed on March 16, 2001.  Even though a second version for Proposed Recommendation was published on March 30, the  final date for review comments remains Monday, April 16.  Perhaps we will have an announcement of final Recommendation status for XML Schema at XML Europe in Berlin in May!

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