A Time-Free Assumption to Implement Eventual Leadership

Abstract

Leader-based protocols rest on a primitive able to provide the processes with the same unique leader. Such protocols are very common in distributed computing to solve synchronization or coordination problems. Unfortunately, providing such a primitive is far from being trivial in asynchronous distributed systems prone to process crashes. (It is even impossible in fault-prone purely asynchronous systems.) To circumvent this difficulty, several protocols have been proposed that build a leader facility on top of an asynchronous distributed system enriched with synchrony assumptions. This paper introduces a novel approach to implement an eventual leader protocol, namely a time-free behavioral assumption on the flow of messages that are exchanged. It presents a very simple leader protocol based on this assumption. It then presents a second leader protocol combining this timeless assumption with eventually timely channels. As it considers several assumptions, the resulting hybrid protocol has the noteworthy feature to provide an increased overall assumption coverage. A probabilistic analysis shows that the time-free assumption is practically always satisfied.

Failure Detectors Encapsulate Fairness

“…The next significant fairness-based model sufficient to implement ♦S is proposed in [32] (extended in [33]) for systems consisting of n processes with at most f crash faults in which executions progress in "rounds" (the notion of a round is local to each process, not global), and processes send messages to all other processes in each round. A round terminates at a process when the process has received messages from n − f processes for that round.…”

Time-Free Authenticated Byzantine Consensus

“…Finally, the response from a process to its own queries is assumed to always arrive among the first (n − t) responses that it is waiting for. Henceforth, we reuse the definitions introduced in [21], [22], [24], [25] to define formally the notions of winning link and x-winning process.…”

“…In the crash-failure model, a second approach consists in adding a property that is based on the message pattern. In the latter approach, we have the query-response-based winning messages proposed in [21], [22] and the teta-model proposed in [28]. There exist also hybrid approaches [24], [25].…”

“…The second approach assumes that some pattern of messages cannot happen [21], [22], [24], [25], [26]. This property is abstracted in the notion of winning responses.…”

Eventual Leader Service in Unreliable Asynchronous Systems: Why? How?

“…The following protocol (due to Mostefaoui, Mourgaya, Raynal and Travers [18]) is made up of three tasks executed by each process. Its underlying principles are relatively simple.…”