Zyzzyva

TLDR

Distributed systems aim to build reliable systems from unreliable components, and Byzantine Fault‑Tolerant state machine replication is a promising approach, but practitioners hesitate to deploy it due to perceived high overheads. The authors present Zyzzyva, a protocol that uses speculation to reduce the cost of BFT replication. Zyzzyva lets replicas reply to client requests by optimistically adopting the primary’s proposed order and processing immediately, while clients detect inconsistencies and help converge on a single total order when the primary is faulty. Zyzzyva achieves near‑theoretical minimum overheads and tens of thousands of requests per second, demonstrating that BFT replication can be practical for demanding services.

Abstract

A longstanding vision in distributed systems is to build reliable systems from unreliable components. An enticing formulation of this vision is Byzantine Fault-Tolerant (BFT) state machine replication, in which a group of servers collectively act as a correct server even if some of the servers misbehave or malfunction in arbitrary (“Byzantine”) ways. Despite this promise, practitioners 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 expensive three-phase commit protocol to agree on the order 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, replicas can become temporarily inconsistent with one another, but clients detect inconsistencies, help correct replicas converge on a single total ordering of requests, and only rely on responses that are consistent with this total order. This approach allows Zyzzyva to reduce replication overheads to near their theoretical minima and to achieve throughputs of tens of thousands of requests per second, making BFT replication practical for a broad range of demanding services.