Mediators over ontology-based information sources

We propose a model for providing integrated and unified access to multiple information sources of the kind of Web catalogs. We assume that each source comprises two parts: (a) an ontology i.e. a set of terms structured by a subsumption relation, and (b) a database that stores objects of interest under the terms of the ontology. We assume that these objects belong to an underlying domain that is common to all sources (e.g. a set of Web pages of interest), and that different sources may use different ontologies with terms that correspond to different natural languages or to different levels of granularity. Information integration is obtained through a mediator comprising two parts: (a) an ontology, and (b) a set of articulations to the sources. Here, by articulation to a source we mean a set of relationships between terms of the mediator and terms of that source. Information requests (queries) are addressed to the mediator whose task is to analyze each query into sub-queries, translate them into queries to the appropriate sources, then merge the results to answer the original query. We study the querying and answering process in such a model and present algorithms for handling the main tasks of the mediator, namely, query translation between the mediator and the sources, source selection and result merging to produce the final answer.