On the Optimality of Rollback-Recovery Protocol Preserving Session Guarantees

The rVsAll rollback-recovery protocol assures that consistency models, called session guarantees, are provided for mobile clients and unreliable servers. The protocol is optimized according to the required consistency model by taking into account properties of session guarantees and integrating their consistency management with known rollback-recovery techniques: message-logging and checkpointing. We have proved the correctness of rVsAll protocol in terms of safety and liveness properties. These properties assert that clients access object replicas maintained by servers according to the required session guarantee regardless of server’s failures, and state that each access operation issued by clients will eventually be performed (in a finite time), even in the presence of server’s failures.

In this paper, we show that the proposed protocol is also optimal, in the sense that, the consistent global checkpoint taken in rVsAll protocol contains the minimal number of operations indispensable to fulfill the required session guarantees after server’s failure and its recovery. The paper presents the proof of the optimality property of the proposed protocol.