Synchrony weakened by message adversaries vs asynchrony restricted by failure detectors

Abstract

International audienceA message adversary is a daemon that suppresses messages in round-based message-passing synchronous systems in which no process crashes. A property imposed on a message adversary defines a subset of messages that cannot be eliminated by the adversary. It has recently been shown that when a message adversary is constrained by a property denoted TOUR (for tournament), the corresponding synchronous system and the asynchronous crash-prone read/write system have the same computability power for task solvability. This paper introduces new message adversary properties (denoted SOURCE and QUORUM), and shows that the synchronous round-based systems whose adversaries are constrained by these properties are characterizations of classical asynchronous crash-prone systems (1) in which processes communicate through atomic read/write registers or point-to-point message-passing, and (2) enriched with failure detectors such as Ω and Σ. Hence these properties characterize maximal adversaries, in the sense that they define strongest message adversaries equating classical asynchronous crash-prone systems. They consequently provide strong relations linking round-based synchrony weakened by message adversaries with asynchrony restricted with failure detectors. This not only enriches our understanding of the synchrony/asynchrony duality, but also allows for the establishment of a meaningful hierarchy of property-constrained message adversaries.Cet article étudie les relations entre les modèles synchrones affaiblis par des adversaires supprimant des messages et les modèles asynchrones renforcés par des détecteurs de fautes

Fast consensus under eventually stabilizing message adversaries

“…Agreement problems in dynamic networks with undirected communication graphs have been studied in the work by Kuhn et al [12]; it focuses on the ∆-coordinated consensus problem, which extends consensus by requiring all processes to decide within ∆ rounds of the first decision. Agreement in directed graphs has been considered in [1,3,4,6,17,19,20]. Whereas [6,19] considerably restrict the dynamicity of the communication graphs, e.g., by not allowing stabilizing behavior, which effectively causes them to belong to quite strong classes of network assumptions in the classification of Casteigts et al [5], the algorithms of [3,4,20] allow to solve consensus under very weak network assumptions: [3] only admits single-rooted graphs, whereas [4] provides a consensus algorithm that gracefully degrades to k-set agreement in unfavorable runs under a fairly strong stabilizing message adversary.…”

“…Afek and Gafni [1] introduced (oblivious) message adversaries for specifying network assumptions in this context, and used them for relating problems solvable in wait-free read-write shared memory systems to those solvable in message-passing systems. Raynal and Stainer [17] used message adversaries for exploring the relationship between round-based models and failure detectors.…”

Bremen workshop review

“…Michel discussed a joint work with Stainer [18], following the work presented by Afek. He showed some relations between models of synchrony weakened by message suppressing adversaries and models of asynchrony restricted by failure detectors, and examined the hierarchy and equivalence between these models.…”

Stabilization, Safety, and Security of Distributed Systems

“…It is easier to analyze algorithms in more structured, iterated models of computation [8,10,11,13,18,22,26,28,[43][44][45][46]. It is easier to analyze algorithms in more structured, iterated models of computation [8,10,11,13,18,22,26,28,[43][44][45][46].…”