Unbeatable Consensus

TL;DR

This paper introduces the first practical unbeatable consensus protocols in the synchronous message-passing model, surpassing previous protocols in efficiency and fairness through a novel knowledge-based analysis.

Contribution

It develops the first unbeatable consensus protocols with concise descriptions, improving upon existing protocols and providing new insights into consensus logic and fairness.

Findings

Protocols strictly dominate previous solutions

Protocols are efficiently implementable

First fair unbeatable consensus protocol

Abstract

The unbeatability of a consensus protocol, introduced by Halpern, Moses and Waarts in 2001, is a stronger notion of optimality than the accepted notion of early stopping protocols. Using a novel knowledge-based analysis, this paper derives the first practical unbeatable consensus protocols in the literature, for the standard synchronous message-passing model with crash failures. These protocols strictly dominate the best known protocols for uniform and for non-uniform consensus, in some case beating them by a large margin. The analysis provides a new understanding of the logical structure of consensus, and of the distinction between uniform and nonuniform consensus. Finally, the first (early stopping and) unbeatable protocol that treats decision values "fairly" is presented. All of these protocols have very concise descriptions, and are shown to be efficiently implementable.