Virtual Synchrony

Virtual synchrony is an interprocess message passing (sometimes called event queue management) technology. Virtual synchrony systems allow programs running in a network to organize themselves into process groups, and to send messages to groups (as opposed to sending them to specific processes). Each message is delivered to all the group members, in the identical order, and this is true even when two messages are transmitted simultaneously by different senders. Application design and implementation is greatly simplified by this property: every group member sees the same events (group membership changes and incoming messages) in the same order.

This form of synchronization is virtual because the actual situation is more complex than seems to be the case from a programmer's perspective: the ordering property (a so-called synchronous event notification model) can sometimes be violated. For example, the messaging system might sometimes report the same events in different orders at different processes, but only in situations where the processes won't notice. If they are replicating data, the most common use of virtual synchrony, the replicas remain in identical states. The flexibility associated with this limited form of event reordering permits virtual synchrony platforms to achieve extremely high data rates while still preserving very strong fault-tolerance and consistency guarantees.