Abstract

Abstract: We describe applications of a virtually synchro-nous environment for distributed programming, which underlies a collection of distributed programming tools in the 1SIS2 system. A virtually synchronous environment allows processes to be structured into process groups, and makes events like broadcasts to the group as an entity, group membership changes, and even migration of an activity from one place to another appear to occur instan-taneously-- in other words, synchronously. A major advantage to this approach is that many aspects of a dis-tributed application can be treated independently without compromising correctness. Moreover, user code that is designed as if the system were synchronous can often be executed concurrently. We argue that this approach to building distributed and fault-tolerant software is more straightforward, more flexible, and more likely to yield correct solutions than alternative approaches. 1. A toolkit for distributed systems Consider the design of a distributed system for factory automation, say for VLSI chip fabrication. Such a system would need to group control 'processes into services responsible for different aspects of the fabrication procedure. One service might accept batches of chips needing photographic emulsions, another oversee transport of chips from station to sta-tion, etc. Within a service, algorithms would be needed for scheduling work, replicating data, coordi-

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