Unbeatable consensus

Abstract

The unbeatability of a consensus protocol, introduced by Halpern, Moses andWaarts in [15], is a stronger notion of optimality than the accepted notion of early stopping protocols. Using a novel knowledgebased analysis, this paper derives the first practical unbeatable consensus protocols in the literature, for the standard synchronous message-passing modelwith 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.