A Strategy to Manage Cache Consistency in a Distributed Mobile Wireless Environment

Mobile computing environments are characterized by slow wireless links and relatively underprivileged hosts with limited battery powers, predisposed to frequent disconnections. Caching data at the mobile hosts (MHs) in a wireless network helps alleviate problems associated with slow, limited bandwidth wireless links, by reducing latency and conserving bandwidth. Battery power is conserved by reducing the number of up-link requests. A mobile computing environment is a distributed system, thus when data at the server changes, the client hosts must be made aware of this fact in order for them to invalidate their cache otherwise the host would continue to answer queries with the cached values returning incorrect data. The nature of the physical medium coupled with the fact that disconnections from the network are very frequent in mobile computing environments demand a cache invalidation strategy with minimum possible overheads. In this paper, we present a new cache maintenance scheme, called AS. The objective of the proposed scheme is to minimize the overhead for the MHs to validate their cache upon reconnection, to allow stateless servers, and to minimize the bandwidth requirement. The general approach is i) to use asynchronous invalidation messages, and ii) to buffer invalidation messages from servers at the MH’s Home Location Cache (HLC) while the MH is disconnected from the network and redeliver these invalidation messages to the MH when it gets reconnected to the network. Use of asynchronous invalidation messages minimizes access latency, buffering of invalidation messages minimizes the overhead of validating MH’s cache after each disconnection and use of HLC off-loads the overhead of maintaining state of MH’s cache from the servers. The MH can be disconnected from the server either voluntarily (e.g. switching off the laptop) or involuntarily (e.g. wireless link failure, handoff delay); we capture the effects of both by using a single parameter s: the percentage of time a mobile host is disconnected from the network. We demonstrate the efficiency of our scheme through simulation and performance modeling. In particular, we show that the average data access latency and the number of up-link requests by a MH decrease by using the proposed strategy at the cost of using buffer space at the HLC. We provide analytical comparison between our proposed scheme and the existing scheme for cache management in a mobile environment [BI95]. Extensive experimental results are provided to compare the schemes in terms of performance metrics like latency, number of up-link requests etc. under both high and low rate of change of data at servers for various values of the parameter s. A mathematical model for the scheme is developed which matches closely with the simulation results.