Figure 3.

A schematic of the BIRN Mediator. Users are presented with a single database view that is achieved through a uniform semantically-consistent domain model of all of the data. No data are stored at the mediator; the data remain at the sources. The mediator reconciles the semantic discrepancies among the sources by using a set of declarative logical descriptions of the contents of the sources. The user poses queries to the mediator using terms from the domain model. The mediator then uses the source descriptions to identify the sources relevant to the user query and to rewrite the domain-level user query, expressed in terms of the domain model, into a source-level query, expressed in terms of the source schemas. The Mediator architecture is designed in a modular fashion, so that any mediation approach that produces source queries in a language supported by the query evaluation engine can be used by the system.