Flexible Paxos: Quorum intersection revisited

TL;DR

Flexible Paxos revisits the quorum intersection requirements of the original Paxos algorithm, allowing non-intersecting quorums across phases, which enhances efficiency and availability in distributed consensus systems.

Contribution

The paper introduces Flexible Paxos, a generalized version of Paxos that relaxes quorum intersection constraints, enabling more flexible and efficient distributed consensus.

Findings

Flexible Paxos is safe and efficient.

It allows non-intersecting quorums across phases.

Reduces quorum sizes to improve availability and speed.

Abstract

Distributed consensus is integral to modern distributed systems. The widely adopted Paxos algorithm uses two phases, each requiring majority agreement, to reliably reach consensus. In this paper, we demonstrate that Paxos, which lies at the foundation of many production systems, is conservative. Specifically, we observe that each of the phases of Paxos may use non-intersecting quorums. Majority quorums are not necessary as intersection is required only across phases. Using this weakening of the requirements made in the original formulation, we propose Flexible Paxos, which generalizes over the Paxos algorithm to provide flexible quorums. We show that Flexible Paxos is safe, efficient and easy to utilize in existing distributed systems. We conclude by discussing the wide reaching implications of this result. Examples include improved availability from reducing the size of second phase quorums by one when the number of acceptors is even and utilizing small disjoint phase-2 quorums to speed up the steady-state.