Optimistic virtual synchrony

We present Optimistic Virtual Synchrony (OVS), a new form of group communication which provides the same capabilities as Virtual Synchrony at better performance. It does so by allowing applications to send messages during periods in which services implementing Virtual Synchrony block. OVS also 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.