Failure Recovery based on Quasi-Synchronous Checkpointing in Mobile Computing Systems

Mobile computing systems are expected to revolutionize the way computers are used. Mobile hosts have small memory, a relatively slow processor and low power batteries, and communicate over low bandwidth wireless communication links. In this paper, we address the problem of failure recovery in mobile computing systems. Any recovery method for mobile computing systems should take into consideration energy and communication bandwidth constraints under which mobile computers have to operate. Synchronous checkpointing is not suitable for mobile systems since it involves high communication cost over a low bandwidth network. Asynchronous checkpointing is not suitable because multiple checkpoints need to be stored in the stable storage and also some or all of the checkpoints taken may be useless for constructing consistent global checkpoints. In this paper, we propose a low-overhead recovery algorithm based on a quasi-synchronous checkpointing algorithm for mobile computing systems. The checkpointing algorithm preserves process autonomy by allowing them to take checkpoints asynchronously and uses communication-induced checkpoint coordination for the progression of the recovery line which helps bound rollback propagation during a recovery. Thus, it has the easeness and low overhead of asynchronous checkpointing and the recovery time advantages of synchronous checkpointing. The checkpointing algorithm ensures the existence of a recovery line consistent with any checkpoint of any process all the time. The recovery algorithm exploits this feature to restore the system to a state consistent with the latest checkpoint of a failed process, in the event of a failure. It uses selective pessimistic message logging at the receiver end to handle the messages lost due to rollback.