Achieving Adaptivity For OLAP-XML Federations

Motivated by the need for more flexible OLAP systems, this paper presents the results of work on logical integration of external data in OLAP databases, carried out in cooperation between the Danish OLAP client vendor \targit and Aalborg University. Flexibility is ensured by supporting XML as the external data format, since almost all data sources can be efficiently wrapped in XML\@. Earlier work has resulted in an extension of the \targit system, allowing external XML data to be used as dimensions and measures in OLAP databases. This work has led to a number of new ideas for improving the system's ability to adapt to changes in its surroundings. This paper describes the potential problems that may interrupt the operation of the integration system, in particular those caused by the often autonomous and unreliable nature of external XML data sources, and methods for handling these problems. Specifically, we describe techniques for handling {\it changes} in external XML data sources. We also describe techniques for improving the reliability of external XML sources, e.g.\, when these are found on the Internet, by dynamically trying to locate alternative sources during the evaluation of a query. Finally, we discuss solutions to a number of other possible problems, and show how the techniques can be integrated in the \targit architecture. Experiments performed with a prototype implementation of central functionality shows the viability of the proposed solutions.