Message Logging: Pessimistic, Optimistic, and Causal

Message logging protocols are an integral part of a technique for implementing processes that can recover from crash failures. All message logging protocols require that, when recovery is complete, there be no orphan processes, which are surviving processes whose states are inconsistent with the recovered state of a crashed process. We give a precise speciication of the consistency property \no orphan processes". From this speciication, we describe how diierent existing classes of message logging protocols (namely optimistic, pessimistic, and a class that we call causal) implement this property. We then propose a set of metrics to evaluate the performance of message logging protocols, and characterize the protocols that are optimal with respect to these metrics. Finally, starting from a protocol that relies on causal delivery order, we show how to derive optimal causal protocols that tolerate f overlapping failures and recoveries for a parameter f : 1 f n.