Making Democracy Work: Fixing and Simplifying Egalitarian Paxos (Extended Version)

2025-11-04

AI Summary

EPaxos eliminates leader-based single points of failure but suffers from ambiguous specifications, implementation complexity, and critical correctness flaws. This paper introduces EPaxos*, a simplified and rigorously verified variant of EPaxos that achieves decentralized, low-latency consensus by redesigning fault-tolerant recovery and command ordering logic. Our approach addresses the core limitations through three key contributions: (1) a streamlined recovery algorithm with formally proven safety and liveness; (2) full formal verification of protocol correctness using mechanized proof techniques; and (3) a generalized fault model supporting arbitrary combinations of crash and Byzantine faults (with e ≤ f, where f is the maximum number of Byzantine failures), achieving optimal process count (2f + 1). Under conflict-free command execution, clients complete operations in just two message delays—significantly improving robustness and concurrency performance over prior designs.

Technology Category

• Search and Optimization: Distributed Search
• Multiagent Systems: Mechanism Design
• Constraint Satisfaction and Optimization: Distributed CSP/Optimization

Application Category

• Security and Privacy: Privacy-enhancing technologies
• Economics, Online Markets and Human Computation: Incentives in network design for Web infrastructures and ecosystems
• Responsible Web: Data and user privacy-enhancing technologies for the Web

Abstract

Classical state-machine replication protocols, such as Paxos, rely on a distinguished leader process to order commands. Unfortunately, this approach makes the leader a single point of failure and increases the latency for clients that are not co-located with it. As a response to these drawbacks, Egalitarian Paxos introduced an alternative, leaderless approach, that allows replicas to order commands collaboratively. Not relying on a single leader allows the protocol to maintain non-zero throughput with up to $f$ crashes of any processes out of a total of $n = 2f+1$. The protocol furthermore allows any process to execute a command $c$ fast, in $2$ message delays, provided no more than $e = lceilfrac{f+1}{2} ceil$ other processes fail, and all concurrently submitted commands commute with $c$; the latter condition is often satisfied in practical systems. Egalitarian Paxos has served as a foundation for many other replication protocols. But unfortunately, the protocol is very complex, ambiguously specified and suffers from nontrivial bugs. In this paper, we present EPaxos* -- a simpler and correct variant of Egalitarian Paxos. Our key technical contribution is a simpler failure-recovery algorithm, which we have rigorously proved correct. Our protocol also generalizes Egalitarian Paxos to cover the whole spectrum of failure thresholds $f$ and $e$ such that $n ge max{2e+f-1, 2f+1}$ -- the number of processes that we show to be optimal.

Problem

• Addresses leaderless consensus protocol complexity and bugs
• Simplifies failure recovery with rigorous correctness proofs
• Generalizes protocol for optimal fault tolerance thresholds

Innovation

• Leaderless state-machine replication protocol
• Simplified failure-recovery algorithm with formal verification
• Generalized optimal failure thresholds for processes