Impact of XML Schema Evolution

We consider the problem of XML Schema evolution. In the ever-changing context of the web, XML schemas continuously change in order to cope with the natural evolution of the entities they describe. Schema changes have important consequences. First, existing documents valid with respect to the original schema are no longer guaranteed to fulfill the constraints described by the evolved schema. Second, the evolution also impacts programs, manipulating documents whose structure is described by the original schema. We propose a unifying framework for determining the effects of XML Schema evolution both on the validity of documents and on queries. The system is very powerful in analyzing various scenarios in which forward/backward compatibility of schemas is broken, and in which the result of a query may no longer be what was expected. Specifically, the system offers a predicate language that allows one to formulate properties related to schema evolution. The system then relies on exact reasoning techniques to perform a fine-grained analysis. This yields either a formal proof of the property or a counter-example that can be used for debugging purposes. The system has been fully implemented and tested with real-world use cases, in particular with the main standard document formats used on the web, as defined by W3C. The system precisely identifies compatibility relations between document formats. In case these relations do not hold, the system can identify queries that must be reformulated in order to produce the expected results across successive schema versions.