Query processing in the WASSIT mediation framework

As more and more data is available through the Web, mediation of information from multiple autonomous and heterogeneous data sources becomes a crucial task for future Web information systems. We describe the features of our mediation framework WASSIT (frameWork d´integrAtion de reSSources de donnees par la medIaTion), which provides the user with an integrated view of the underlying data sources. In particular, we describe the high level architecture of WASSIT and present how user´s queries are processed focusing on query rewriting and query optimization. For query rewriting, we use OWL ontology to describe the mappings between global schemas and local schemas and propose an approach to generate query execution plans for XQuery queries. To optimize queries over sources with limited query capabilities, we adopt a formalism that describes data sources capabilities and propose an algorithm for producing equivalent query plans. We present also our cost model for choosing the optimal plan.