4.4. Metadata Change Management

Most data sources are not static, and just as the structure of data may change so may the meaning of the data. For instance, the currency of book price required in a query result may change from German Marks to European Currency Units (ECUs). Therefore, it is important that the methods for relevance reasoning during the query routing process allow for changes in data semantics of producers' data sources and consumers' query profiles.

The methods presented for semantic relevance reasoning during the query routing process can be used in such a dynamic environment.

• When changes are made in a source, they will be captured by the wrapper in the content and capability profile of the source. The metadata manager must reevaluate the corresponding source against those queries that use the source. The reevaluation process is performed by comparing the query profiles with the updated portions of the source capability profile.
• When changes are made in a user query profile, the metadata manager must reevaluate the selected sources to check if they may continue to contribute to the answer of the query. If new sources are available online, the metadata manager will extend the reevaluation process to perform the two levels of relevance pruning over these new sources.