A transaction model to improve data availability in mobile computing

We incorporate a prewrite operation before a write operation in a mobile tr ansaction to improve data availability. A prewrite operation does not updat e the state of a data object but only makes visible the future value that t he data object will have after the final commit of the transaction. Once a transaction reads all the values and declares all the prewrites, it can pre -commit at mobile host (MH) (computer connected to unreliable mobile commun ication network). The remaining transaction's execution (writes on database ) is shifted to the mobile service station (MSS) (computer connected to thr reliable fixed network). Writes on database consume time and resources and are therefore shifted to MSS and delayed. This reduces wireless network tr affic congestion. Since the responsibility of expensive parr of the transac tion's execution is shifted to the MSS, it also reduces the computing expen ses at mobile host. A pre-committed transaction's prewrite values are made visible both at mobile and at fixed database servers before the final commi t of the transaction. Thus, it increases data availability during frequent disconnection common in mobile computing. Since a pre-committed transaction does not abort, no undo recovery needs to be performed in our model. A mob ile host needs to cache only prewrite values of the data objects which take less memory, transmission time, energy and can be transmitted over low ban dwidth. We have analysed various possible schedules of running transactions concurrently both at mobile and fixed database servers. We have discussed the concurrency control algorithm for our transaction model and proved that the concurrent execution of our transaction processing model produces only serializable schedules. Our performance study shows that our model increas es throughput and decreases transaction-abort-ratio in comparison to other lock based schemes. We have briefly discussed the recovery issues and imple mentation of our model.