Probabilistically Bounded Staleness for Practical Partial Quorums

TL;DR

This paper introduces Probabilistically Bounded Staleness (PBS), a model that quantifies the staleness and latency trade-offs in partial quorum data stores, explaining their practical acceptability.

Contribution

The paper proposes PBS, a probabilistic model providing bounds on data staleness in partial quorum systems, with analytical solutions and real-world workload evaluations.

Findings

Partial quorums often return data within tens of milliseconds.

PBS provides expected bounds on version and real-time staleness.

Partial quorum systems offer significant latency benefits with acceptable staleness.

Abstract

Data store replication results in a fundamental trade-off between operation latency and data consistency. In this paper, we examine this trade-off in the context of quorum-replicated data stores. Under partial, or non-strict quorum replication, a data store waits for responses from a subset of replicas before answering a query, without guaranteeing that read and write replica sets intersect. As deployed in practice, these configurations provide only basic eventual consistency guarantees, with no limit to the recency of data returned. However, anecdotally, partial quorums are often "good enough" for practitioners given their latency benefits. In this work, we explain why partial quorums are regularly acceptable in practice, analyzing both the staleness of data they return and the latency benefits they offer. We introduce Probabilistically Bounded Staleness (PBS) consistency, which provides expected bounds on staleness with respect to both versions and wall clock time. We derive a closed-form solution for versioned staleness as well as model real-time staleness for representative Dynamo-style systems under internet-scale production workloads. Using PBS, we measure the latency-consistency trade-off for partial quorum systems. We quantitatively demonstrate how eventually consistent systems frequently return consistent data within tens of milliseconds while offering significant latency benefits.