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Summary: november 2008 | vol. 51 | no. 11 | communications of the acm 86
Doi:10.1145/1400214.1400236
abstract
Alongstandingvisionindistributedsystemsistobuildreliable
systemsfromunreliablecomponents.Anenticingformulation
of this vision is Byzantine fault-tolerant (BFT) state machine
replication, in which a group of servers collectively act as a cor-
rect server even if some of the servers misbehave or malfunc-
tioninarbitrary("Byzantine")ways.Despitethispromise,prac-
titioners hesitate to deploy BFT systems at least partly because
of the perception that BFT must impose high overheads.
In this article, we present Zyzzyva, a protocol that uses
speculation to reduce the cost of BFT replication. In Zyzzyva,
replicas reply to a client's request without first running an
expensivethree-phasecommitprotocoltoagreeontheorder
to process requests. Instead, they optimistically adopt the
order proposed by a primary server, process the request, and
reply immediately to the client. If the primary is faulty, rep-
licas can become temporarily inconsistent with one anoth-
er, but clients detect inconsistencies, help correct replicas
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