Early Consensus in Message-Passing Systems Enriched with a Perfect Failure Detector and Its Application in the Theta Model

While lots of consensus algorithms have been proposed for crash-prone asynchronous message-passing systems enriched with a failure detector of the class Ω (the class of eventual leader failure detectors), very few algorithms have been proposed for systems enriched with a failure detector of the class P (the class of perfect failure detectors). Moreover, (to the best of our knowledge) the early decision and stopping notion has not been investigated in such systems. This paper presents an early-deciding/stopping P-based consensus algorithm. A process that does not crash decides (and stops) in at most min(f+2, t+1) rounds, where t is the maximum number of processes that may crash, and f the actual number of crashes (0≤ f≤ t). Differently from what occurs in a synchronous system, a perfect failure detector notifies failures asynchronously. This makes the design of an early deciding (and stopping) algorithm not trivial. Interestingly enough, the proposed algorithm meets the lower bound on the number of rounds for early decision in synchronous systems. In that sense, it is optimal. The paper then presents an original algorithm that implements a perfect failure detector in the Theta model, an interesting model that achieves some form of synchrony without relying on physical clocks. Hence, the stacking of these algorithms provides an algorithm that solves consensus in the Theta model in min(f+2, t+1) communication rounds, i.e., in two rounds when there are no failures, which is clearly optimal.