Exploiting Virtual Synchrony in Distributed Systems

We describe applications of a new software abstraction called the virtually synchronous process group. Such a group consists of a set of processes that cooperate to implement some distributed behavior in an environment where events like broadcasts to the group as an entity, process failures, and process recoveries appear to occur synchronously. The utility of this approach is illustrated by solving a number of classical problems using our methods. Many are problems that are quite difficult in the absence of some sort of support, and all are easily solved in the context of the mechanisms we propose here. We then describe a new version of the ISIS system, which is based on this abstraction. $ISIS_{2}$ provides a number of high level mechanisms that facilitate the use of process groups in application software design, including addressing support for atomic communication with the members of a single or several groups (even when their membership is changing), group RPC constructs, a package of distributed programming tools, a fault-tolerant asynchronous bulletin board mechanism, and resilient objects