XML News from Saturday, February 26, 2005

The W3C XML Schema Working Group has posted the second public working drafts of XML Schema 1.1 Part 1: Structures and XML Schema 1.1 Part 2: Datatypes. According to the introduction to the structures spec,

The Working Group has two main goals for this version of W3C XML Schema:

• Significant improvements in simplicity of design and clarity of exposition without loss of backward or forward compatibility;
• Provision of support for versioning of XML languages defined using the XML Schema specification, including the XML transfer syntax for schemas itself.

These goals are in tension with one anotherThe Working Group's strategic guidelines for changes between versions 1.0 and 1.1 can be summarized as follows:

1. Support for versioning (acknowledging that this may be slightly disruptive to the XML transfer syntax at the margins)
2. Bug fixes (unless in specific cases we decide that the fix is too disruptive for a point release)
3. Editorial changes
4. Design cleanup will possibly change behavior in edge cases
5. Non-disruptive changes to type hierarchy (to better support current and forthcoming international standards and W3C recommendations)
6. Design cleanup will possibly change component structure (changes to functionality restricted to edge cases)
7. No significant changes in functionality
8. No changes to XML transfer syntax except those required by version control hooks and bug fixes

The aim with regard to compatibility is that

• All schema documents conformant to version 1.0 of this specification should also conform to version 1.1, and should have the same validation behaviour across 1.0 and 1.1 implementations (except possibly in edge cases and in the details of the resulting PSVI);
• The vast majority of schema documents conformant to version 1.1 of this specification should also conform to version 1.0, leaving aside any incompatibilities arising from support for versioning, and when they are conformant to version 1.0 (or are made conformant by the removal of versioning information), should have the same validation behaviour across 1.0 and 1.1 implementations (again except possibly in edge cases and in the details of the resulting PSVI);

Changes in the data type spec include:

• "0000" is a legal year and values with negative years map onto the timeline such that "the year 0000 is 1 B.C.E., the year –0001 is 2 B.C.E., etc."
• Distinction between identity and equality; for instance positive and negative zero would be equal but not identical. Think of the difference between == and equals() in Java.
• New yearMonthDuration and dayTimeDuration types
• A precisionDecimal type
• An anyAtomicType data type

There are also open issues including how to align the W3C XML schema language with XML 1.1.