Achieving flexible cache consistency for pervasive internet access

Caching is an important technique to support pervasive Internet access. Cache consistency measures the deviation between the cached data and the source data. In mobile computing environments, especially with ad hoc networks, users are in great need of the flexibility in tuning their consistency requirements, in order to make tradeoffs between the specified cache consistency and the cost incurred. Existing works have used Delta Consistency (DC) and Probabilistic Consistency (PC) which, to some extent, provide the users with such flexibility. In this paper, we propose a general consistency model called Probabilistic Delta Consistency (PDC). PDC covers all existing consistency models including DC and PC, and integrates the flexibility granted by both DC and PC. Thus, PDC enables the users to flexibly specify their consistency requirements in two orthogonal dimensions, namely the deviation in time/value and the ratio of queries gaining the specified consistency. We also propose a consistency maintenance algorithm, called Flexible Combination of Push and Pull (FCPP), which can meet users' consistency requirements specified under the PDC model. An analytical model is derived to achieve the optimized combination of push and pull, so as to ensure the user-specified consistency requirements, while minimizing the consistency maintenance overhead. Extensive simulations are conducted to evaluate the performance of the FCPP algorithm. Evaluation results show that, compared with the widely used Dynamic TTR algorithm, FCPP can save up to 68% of the traffic overhead and reduce the query delay by up to 84%.