Read-Write Quorum Systems Made Practical

Abstract: Quorum systems are a powerful mechanism for ensuring the consistency of replicated data. Production systems usually opt for majority quorums due to their simplicity and fault tolerance, but majority quorum systems provide poor throughput and scalability. Alternatively, researchers have invented a number of theoretically "optimal" quorum systems, but the underlying theory ignores many practical complexities such as machine heterogeneity and workload skew. In this paper, we conduct a pragmatic re-examination of quorum systems. We enrich the current theory on quorum systems with a number of practical refinements to find quorum systems that provide higher throughput, lower latency, and lower network load. We also develop a library Quoracle that precisely quantifies the available trade-offs between quorum systems to empower engineers to find optimal quorum systems, given a set of objectives for specific deployments and workloads. Our tool is available online at: https://github.com/mwhittaker/quoracle.