Flexible Paxos: Quorum intersection revisited (1608.06696v1)

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.

• The paper introduces Flexible Paxos (FPaxos), which relaxes the traditional requirement of quorum intersection in all phases of consensus, necessitating intersection only between quorums from different phases.
• By allowing smaller phase 2 quorums, FPaxos reduces latency and increases throughput compared to traditional Paxos, according to prototype experiments demonstrating improved performance metrics.
• FPaxos enhances fault tolerance and adaptability by supporting alternative quorum systems like grid quorums, enabling systems to tolerate more failures seamlessly during replication.

Summary of "Flexible Paxos: Quorum Intersection Revisited"

The paper "Flexible Paxos: Quorum Intersection Revisited" by Heidi Howard, Dahlia Malkhi, and Alexander Spiegelman introduces an innovative generalization of the Paxos algorithm, proposing Flexible Paxos (FPaxos) that challenges the necessity of intersecting quorums in all phases of the consensus process. The authors argue that the conventional requirement of quorum intersection in both phases of Paxos is overly conservative. Instead, they propose that intersection is only requisite between quorums from different phases, thus offering the potential for significant improvements in efficiency and fault tolerance.

Paxos, a well-established algorithm for achieving distributed consensus, traditionally relies on two phases—preparation and proposal—each necessitating agreement from a quorum of participants. Quorums are typically defined as any majority from a set of participants to ensure intersection. However, FPaxos relaxes this constraint, allowing for non-intersecting quorums within individual phases, thereby reducing quorum sizes without compromising safety.

Key Contributions

1. Flexible Quorums: The notion of flexible quorums elucidates that separate phase quorums need not intersect with each other. FPaxos allows different configurations where quorums intersect only across phases, which is pivotal for maintaining algorithmic safety while enhancing performance by reducing quorum sizes.
2. Improved Performance: By enabling smaller replication quorums (phase 2) compared to majority quorums, FPaxos reduces the latency inherent in waiting for a majority and improves throughput by minimizing the communication overhead during the more common replication phase. Numerical results underscore this with demonstrated decreases in latency and increases in throughput in the paper’s prototype experiments.
3. Enhanced Fault Tolerance: Shifting to flexible quorums can lead to increased system resilience. For instance, FPaxos can handle more failures seamlessly in replication than traditional Paxos, given that certain quorum selections can continue operation even with quorum-participant failures.
4. Grid Quorums and Other Schemes: The introduction of alternative quorum systems, such as grid-based quorums, showcases FPaxos's adaptability. Grid quorums can further optimize quorum sizes, offering different trade-offs between latency, throughput, and fault tolerance, exceeding the limits of majority quorums.

Implications and Future Directions

The theoretical simplification and practical adjustments proposed by FPaxos have substantial implications. For distributed systems leveraging consensus mechanisms, FPaxos presents opportunities to tailor quorum structures to specific use cases, thereby reducing operational overhead and improving fault tolerance without added complexity.

Additionally, the paper discusses the potential for integrating FPaxos into existing systems and how it could foster new consensus algorithms that prioritize scalability, resilience, and efficiency. This insight invites further exploration into dynamic quorum configurations and their practical applications, considering scenarios with low latency and the geometric layout of participants to optimize intersection properties.

In conclusion, "Flexible Paxos" offers a reimagined approach to distributed consensus that, by relaxing conventional quorum intersection requirements, brings to light new possibilities for system architects. Its adoption could mark a shift towards more scalable, fault-tolerant distributed systems, steering future developments in consensus algorithms beyond past limitations. The paper sets a foundation for future research in extending quorum flexibility beyond the domain of static quorum systems, embracing dynamically adaptive models that could revolutionize current consensus protocols.