Group communication systems are powerful building blocks that facilitate the development of fault-tolerant distributed applications. Such systems generally run in an asynchronous fault-prone environment, and provide semantics (called Virtual Synchrony) that mask the asynchrony and unreliability of the environment. In order to implement Virtual Synchrony semantics, group communication systems typically impose blocking periods during which applications are not allowed to send messages. This paper presents a novel form of group communication, Optimistic Virtual Synchrony (OVS). OVS allows applications to send messages during periods in which existing group communication services block, by making optimistic assumptions on the network connectivity. Optimistic Virtual Synchrony allows applications to determine the policy as to when messages sent optimistically should be delivered and when they should be discarded. Thus, OVS gives applications fine-grain control over the specific semantics they require, and does not impose costs for enforcing any semantics that they do not require. At the same time, OVS provides a single easy-to-use interface for all applications. The paper presents several examples of applications that may exploit OVS and empirical results that show the performance benefits of using OVS.
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