xmlhack: W3C XML Schema still has big problems

In last week's W3C Technical Plenary session in Boston, some attendees were far from happy about the state of W3C XML Schema, including James Clark.

Clark, the technical lead for XML 1.0 and author of influential XML open source software, noted that he was reluctant to hurt the feelings of the XML Schema Working Group, but then expressed his view that the XML Schema effort was "little short of a disaster."

Echoing concerns expressed commonly in the XML community, Clark commented on the length of time the specification had taken to create, and noted that few people seemed to like it. He drew especial attention to XML Schemas Part 1 as being overly complex, yet lacking in expressive power.

Clark recently created TREX, an alternative schema language, due to his dissatisfaction with the W3C's XML Schema.

It is clear that W3C XML Schema in its current form will not be acceptable to a large part of the XML developer world (as well as several members of its own Working Group). Difficulties with XML Schema's lack of an underlying mathematical model recently led XML Query WG members Jonathan Robie, Phillip Wadler and colleagues to develop Model Schema Language (PDF), essentially an attempt to retrofit a mathematical model to W3C XML Schema.

Many developers would be prepared to ignore W3C XML Schema and use an alternative, were it not for the proposals to drive Schemas into specifications such as XSLT and XPath, accompanied by the prospect of increasingly W3C XML Schema-centric programming.

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There's a lot of pressure inside the W3C to utilize XML Schemas in other standards. Look at the XForms effort. Originally, unhappy with XML Schemas we proposed using a different but simpler syntax which would be compatable with XML Schemas. This would allow us some indepedence in case XML Schemas did not find acceptance. The chair was pressured into mandating the more complex XML Schemas syntax.

No would argue that reusing existing (proven) specs is not a good thing, but reusing specs that are as yet unproven is extremely risky. The W3C is building a house of cards that will fall if one card fails. Many think that XML Schemas may be the card that brings the rest down.