The term " virtual synchrony " refers to the fact that applications see the shared data evolve in what seems to be a synchronous manner.
32.
These multicasts are delivered to all the non-failed members listed in the view during which delivery is scheduled, a property referred to as virtual synchrony.
33.
Virtual synchrony implementations employ a primitive called " flush " to force the service back into a consistent state before interactions with external clients or storage systems.
34.
:Contrast this with the same protocol, but in a system lacking virtual synchrony ( for example, using UDP multicast, which provides no guarantees at all ).
35.
Andrew S . Tanenbaum, Maarten van Steen ( 2002 ) . " Textbook, covers a broad spectrum of distributed computing concepts, including virtual synchrony ."
36.
In contrast, virtual synchrony implementations with in-memory data replication can generally update a replicated variable as soon as a message describing the update reaches the relevant group members.
37.
Virtual synchrony is distinguished from classical state machine replication because the model includes features whereby a programmer can request early ( optimistic ) delivery of messages, or relaxed forms of ordering.
38.
Inside a cloud computing platform one finds services such as lock management systems ( Google's is called Zookeeper ), and these are implemented using virtual synchrony or closely related models.
39.
So there is a case in which virtual synchrony seems to lie : it behaves as if no message was sent, and yet the crashed processes might actually have exchanged a message.
40.
We mentioned that there is a sense in which virtual synchrony is a " weaker " model than transactional one-copy serializability or state machine consensus in the style of Paxos.
