Integrating and Exchanging XML Data Using Ontologies

While providing a uniform syntax and a semistructured data model, XML does not express semantics but only structure such as nesting information. In this paper, we consider the problem of data integration and interoperation of heterogeneous XML sources and use an ontology-based framework to address this problem at a semantic level. Ontologies are extensively used for domain knowledge representation, by virtue of their conceptualization of the domain, which carries explicit semantics. In our approach, the global ontology is expressed in RDF Schema (RDFS) and constructed using the global-as-view approach by merging individual local ontologies, which represent XML source schemas. We provide a formal model for the mappings between XML schemas and local RDFS ontologies and those between local ontologies and the global RDFS ontology. We consider two cases of query processing, specifically for data integration and for data interoperation. In the first case, the user poses an RDF query on the global ontology, which is answered using all the mapped XML sources. In the second case, a query is posed on a single source and then is mapped to the XML sources that are connected to that source. For each case, we discuss the problem of query containment and present an equivalent query rewriting algorithm for queries expressed in two languages: conjunctive RDQL and conjunctive XQuery.