Why Optimistic Message Logging Has Not Been UsedIn

Much of the literature on message logging and checkpointing in the past decade has been based on a so-called optimistic approach 1] that places more emphasis on failure-free overhead than recovery ee-ciency. Our experience has shown that most telecommunications systems use a pessimistic approach because the main purpose of using message logging and checkpointing is to achieve fast and localized recovery , and the failure-free overhead of a pessimistic approach can often be made reasonably low by exploiting application-speciic information. Much of the existing work on message logging and checkpointing assumes a piecewise deterministic (PWD) execution model 2]. Under the PWD assumption , each process execution is viewed as a number of state intervals bounded by nondeterministic message receiving events 1. Execution within each state interval is completely deterministic, and hence replayable. This allows the use of message logging as a form of checkpointing because the state of a process can be de-terministically reconstructed by replaying the logged messages in their original order. There are primarily three factors that need to be considered for trade-oos when designing a message logging protocol: failure-free overhead, the number of rolled-back surviving processes, and recovery time. A protocol is called optimistic if it optimistically assumes that failures are rare events, so optimizing failure-free performance is more important than 1 Or more general nondeterministic events 3]. achieving good recovery performance. In contrast, a pessimistic protocol always pessimistically prepares for failures, so it is willing to pay higher failure-free overhead in order to recover faster should a failure occur. A pessimistic logging protocol always logs a message before processing (or delivering) it so that every message is retrievable, and every process state is recreatable through message replaying under the PWD assumption. A signiicant advantage is that any failed process can then locally recover by itself without requiring the cooperative rollbacks of its correspondents. Since a failure between message receipt and message logging may result in a lost message unknown to both the sender and the receiver, the issue of atomic message receipt-logging needs to be addressed. Traditional pessimistic logging protocols guarantee the atomicity either by implementing atomic three-way message delivery (to the receiver, receiver's backup and sender's backup) 4, 5] or atomic two-way transmission (to the receiver and a centralized recorder) 6]. The optimistic approach 1] was proposed based on the assumption that synchronously logging every message upon its receipt can result in an unacceptably high …