Virtual Synchrony - Performance

Performance

Among the three models, virtual synchrony achieves the highest levels of performance, but this comes at a cost: virtual synchrony's fault-tolerance is weaker than other models.

The Paxos and transactional models guarantee a higher degree of durability in the presence of crashes. Both models need to first ensure that an update is recorded in a write-ahead log before any process can actually perform the update. This introduces a form of two-phase commit into the protocol, and hence slows things down: first the update is sent and logged, and all members confirm that they have it, and only then is it performed. 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. They can stream high rates of updates by packing multiple updates into a single message.

To give some sense of the relative speed, experiments with 4-node replicated variables undertaken on the Isis and Horus systems in the 1980s suggested that virtual synchrony implementations in typical networks were about 100 times faster than state-machine replication using Paxos, and about 1000 to 10,000 times faster than full-fledged transactional one-copy-serializability. Of course, these sorts of order of magnitude numbers are highly sensitive to the implementation and choice of platform, but they also reflect underlying obligations within the protocols used to implement the models. Modern systems like the Spread Toolkit, Quicksilver, and Corosync can achieve data rates of 10,000 multicasts per second or more, and can scale to large networks with huge numbers of groups or processes.

Most distributed computing platforms support one or more of these models. For example, the widely supported object-oriented CORBA platforms all support transactions and some CORBA products support transactional replication in the one-copy-serializability model. The "CORBA Fault Tolerant Objects standard" is based on the virtual synchrony model. Virtual synchrony was also used in developing the New York Stock Exchange fault-tolerance architecture, the French Air Traffic Control System, the US Navy AEGIS system, IBM's Business Process replication architecture for WebSphere and Microsoft's Windows Clustering architecture for Windows Longhorn enterprise servers.