W3C Publishes an Update to Guide to Versioning XML Schema 1.1

Posted by Jean-Jacques Dubray on Aug 21, 2007

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It is fair to say that XML Schema 1.0 has experienced a broad adoption despite all the heat it received by the data, web and document communities during its debut. People have stopped using DTDs and even light-weight alternatives such as Relax NG seem to have lost momentum. The W3C started a few years ago the XML Schema 1.1 working group which will publish a last call possibly next month, which means  "it should become a W3C recommendation [the highest status of a specification] within 12 months or so", says David Orchard, a Standards Architect at BEA and member of the working group.

This means that if you have a large body of XML Schemas, this is probably a good time to start planning for migration of both your schemas and data binding strategies.

Modularity, extensibility and versioning are critical aspects of message format design within a Service Oriented Architecture, and there has been long debates in the early years of XML Schema about the best usage patterns since the specification allows for many feature combinations (global vs local types or elements, namespace qualified or not,...). Roger Costello has written an extensive summary of these patterns. Each of them offers various degrees of modularity and extensibility.

In the new XML Schema 1.1 version, the working group has spent a lot of time looking at XML Schema versioning. They used as an input a large number of use cases and in July, the W3C published an intermediary update to the "Guide to Versioning XML Languages Using new XML Schema 1.1 features" (which will become a W3C note once final) as well as three TAG documents the "Extending and Versioning Languages: Terminology,  XML Languages and Strategies".

XML Schema 1.1 contains a number of new extensibility mechanisms.

1. Weak wildcards - permits wildcards adjacent to optional elements
2. Updated All Group - wildcards within All Group
3. Negative wildcard - exclude specific namespaces and names
4. Fallback To Declared Type - use declared type if xsi:type is unknown
5. Auto-insertion of wildcards
6. Not in Schema wildcard - a wildcard that allows anything not defined in the current schema

One of the key innovations, is the ability to weave extensibility within element sequences, while in 1.0 additional elements had to be tail-gated at the end of complex type definitions. The Guide to Versioning details a number techniques that will undoubtedly generate more design patterns and produce more flexible service interfaces, achieving looser coupling between service consumers and producers.

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