Scalable Semantic Overlay Routing in MANETs

MANETs are an interesting domain for P2P technologies, in particular in the form of content-based routing and semantic overlays. In such an overlay, XML routers match XPATH queries against data in the form of XML documents. Due to the matching complexity, the scalability of such overlays becomes a key concern. Typically, researchers propose to aggregate the queries to reduce the filtering burden on the XML routers. As new XPath queries are added, existing queries deleted, or the topology between routers changes, these aggregate data structures need to be updated in some or all intermediate XML routers. Such updates can be fairly complex, to ensure that previously aggregated queries are now properly matching the new set of queries. We review a number of aggregation approaches and evaluate their scalability in light of a dynamically changing set of queries. Our survey shows that very few aggregation schemes support efficient dynamic updates of the subscription set, in particular in the case of deleting queries. We then show, through a small implementation, how one particular aggregation approach allows updating the aggregate data structure efficiently, while maintaining a high routing performance.