A Two Layered Approach for Querying Integrated XML Sources

The problem of data integration (query decomposition, data fragmentation) has been widely studied in literature, but the inherent hierarchical nature of XML data presents problems that are specific to this data model. Each many-to-many conceptual relationship must be mapped to a specific hierarchical structure in XML. Different XML sources may implement the same many-to-many conceptual relationship in different ways. In our approach the problem of integration of XML data sources is decomposed in two problems: (1) that of fragmentation of a global graph-like model (e.g., an ER model) into several local graph-like models conceptually representing data sources and (2) that of mapping the local graph-like model into an XML tree-like schema. This paper presents a set of fragmentation operators specifically designed for our approach, as well as a query decomposition mechanism that allows a query stated at the conceptual level to be decomposed into an XQuery statement at the XML level. As the query language at the conceptual level, we adopt CXPath (conceptual XPath) a query language we have defined in previous work.