A SpatioTemporal Placement Model for Caching Location Dependent Queries
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Client side caching of location dependent queries is an important technique for improving performance of location-based services. Most of the existing research in this area has focused on cache replacement and invalidation through incorporating some aspects of the spatial and temporal semantics embedded in the location queries, while assuming an ad hoc cache placement. Very few have studied the impact of spatial and temporal validity semantics and the motion behavior of mobile clients on the effectiveness of cache placement and ultimately the performance of the client cache. This paper proposes an adaptive spatio-temporal placement scheme for caching location dependent queries. The cache placement decision is made according to the potential cache benefit of the query results based on the spatio-temporal properties of query results and the movement patterns of the mobile client, aiming at increasing the cache hit ratio. We introduce the concept of ‘Overlapping Cache Benefit’ as a measure of the hit rate of a cached item, and present three spatio-temporal cache placement schemes, which provide a step-by-step in-depth analysis of various factors that may affect the performance of a client cache in mobile environments. We implemented the spatio-temporal placement model in the first prototype of the MOBICACHE system. Our experimental evaluation shows that the spatial locality and the movement patterns of mobile clients are critical factors that impact the effectiveness of cache placement and the performance of client cache, and the proposed adaptive spatio-temporal cache placement approach yields higher hit ratio and better response time compared to existing mobile cache solutions.
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