Location-aware query processing in mobile database systems

Query processing in mobile database systems is a special challenge because we have to consider the limited resources of mobile systems as well as the changing location of mobile users. In this paper, we concentrate on the optimisation of queries including location information. We present a cost model for query optimisation incorporating mobility specific factors like energy and connectivity. Furthermore, we examine different localisation strategies for mobile users. This innovative connection of both factors is studied using two typical classes of queries. Using a newly developed simulation model, we describe that no single localisation strategy performs acceptable under every condition. We therefore identify the critical factors for adapting a query processing subsystem to the employed location management strategy. 1 I n t r o d u c t i o n The development of wireless communication systems and lightweight computers has led to the availability of a research area called mobile computing. The subject of research is the effect of mobility on systems, users, data and computing. In the area of database systems, mobility offers access to information anywhere and anytime. In order to access information, we have to consider the query processing in mobile database systems as one major task. Queries in mobile systems are different from stationary systems. There, location awareness adds power to the mobile user but also adds complexity to the query processPermission to make digital/hard copy of all or part of this work for personal or classroom use is granted without l~e provided that copies are not made or distributed lbr profit or commercial advantage, the copyright notice, the title of the publication and its date appear, and notice is given that copying is by permission of ACM. Inc. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. ,: 1q98 ACM 0-89791-969-6/98/0002 3.50 ing. Examples of location aware queries in mobile database systems are traffic information systems that give the user information with respect to his current location ("where is the next pub serving Becks beer") or queries retrieving data from mobile users whose location needs to be looked up. Apart from this new kind of queries, mobile systems pose even more new requirements on the query processing subsystem of a mobile database because of the limited resources available on typical mobile computers. Therefore, query subsystems of mobile databases have to be modified compared to stationary query processors [4]. Additionally, location information about mobile users must be categorised as only temporarily valid since mobile users may move and data on their actual location has to be updated in order to address them properly. The location management component is responsible for the storage of current location information. Ad hoc queries that use location information about mobile users have to access the location management component. Depending on the used localisation strategy, we have different costs associated with processing this kind of query. Therefore, an optimal query processing subsystem of a mobile database has to take the strategy used by the location management component into account for answering queries. In this paper, we present a new approach to the integration of these two components. We pay special attention to the usage of fuzzy information as provided by many real-world location management components. Starting by an introduction to existing location management and mobile query processing strategies in the next section, we afterwards present a new abstract model for representing mobile systems and location management components. In section four, we discuss our approach to location aware query optimisation in mobile database systems. For this, we have developed a mobility aware cost model for processing location dependent queries. To validate the model,