The W3C XML Schema Working Group has posted the last call working draft of XML Schema 1.1 Part 1: Structures. According to the introduction,
The Working Group has three 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 this specification, including the XML transfer syntax for schemas itself.
- Provision of support for co-occurrence constraints, that is constraints which make the presence of an attribute or element, or the values allowable for it, depend on the value or presence of other attributes or elements.
These goals are in tension with one another. The Working Group's strategic guidelines for changes between versions 1.0 and 1.1 can be summarized as follows:
- Support for versioning (acknowledging that this may be slightly disruptive to the XML transfer syntax at the margins)
- Support for co-occurrence constraints (which will certainly involve additions to the XML transfer syntax, which will not be understood by 1.0 processors)
- Bug fixes (unless in specific cases we decide that the fix is too disruptive for a point release)
- Editorial changes
- Design cleanup will possibly change behavior in edge cases
- Non-disruptive changes to type hierarchy (to better support current and forthcoming international standards and W3C recommendations)
- Design cleanup will possibly change component structure (changes to functionality restricted to edge cases)
- No significant changes in existing functionality
- No changes to XML transfer syntax except those required by version control hooks, co-occurrence constraints 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 behavior 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 or co-occurrence constraints, and when they are conformant to version 1.0 (or are made conformant by the removal of versioning information), should have the same validation behavior across 1.0 and 1.1 implementations (again except possibly in edge cases and in the details of the resulting PSVI);
Comments are due by November 8.
