Optimizing queries in distributed and composable mediators

The mediator-wrapper approach to integrate data from heterogeneous data sources has usually been centralized in the sense that a single mediator system is placed between a number of data sources and the applications. As the number of data sources increases, the centralized mediator architecture becomes a bottleneck. This paper presents an architecture for composable and distributed mediator servers, defined in terms of other mediator servers. The modularity of composable mediators allows to build larger systems of distributed mediators integrating many data sources, without the need to maintain a global schema. Composable mediators furthermore provide data independence by allowing locality of changes in both submediators and data sources. However a problem with a distributed and composable mediator architecture is that the query performance may degrade as the number of mediators increases. We describe some challenges for processing queries in this type of environment, and propose a distributed query decomposition algorithm that eliminates some of the overhead of logical mediator composition. For certain mediator compositions it produces distributed query plans whose inter-mediator data flow is optimal with respect to the query but is different from the logical interdependencies between the involved mediators. Experimental results show that this strategy improves the query performance and allows an increase of the number of mediators without query performance degradation.