On the complexity of fault-tolerant consensus

Friday, September 27, 2019 - 2:20pm to 3:10pm
Storey Innovation Center (Room 1400)

Speaker: Dariusz Kowalski
Affiliation: Augusta University
Location: Innovation Center, Room 1400
Time: Friday 9/27/2019 (2:20 - 3:10pm)

The problem of reaching agreement in a distributed message-passing system prone to failures is one of the most important and vibrant problems in distributed computing. This talk concentrates on the impact of processes’ crashes to performance of consensus. Crashes are generated by constrained adversaries – a weakly-adaptive adversary, who has to fix, in advance, the set of f crash-prone processes, and a chain-adaptive adversary, who orders all the processes into k disjoint chains and has to follow this order when crashing them. Apart from these constraints, both of them may crash processes in an adaptive way at any time, thus emulating (almost) worst-case failure scenarios in the system against a given algorithm.

While commonly used strongly-adaptive adversaries model attacks and non-adaptive ones – pre-defined faults, the constrained adversaries model more realistic scenarios when there are fault-prone dependent processes, e.g., in hierarchical or dependable software/hardware systems. In this view, our approach helps to understand better the crash-tolerant consensus in more realistic executions.

We propose time-efficient consensus algorithms against such adversaries. We complement our algorithmic results with (almost) tight lower bounds, and extend the one for weakly-adaptive adversaries to hold also for (syntactically) weaker non-adaptive adversaries. Together with the consensus algorithm against weakly-adaptive adversaries (which automatically translates to the non-adaptive adversaries), these results extend the state-of-the-art of the popular class of non-adaptive adversaries, in particular, the result of Chor, Meritt and Shmoys [JACM 1989], and prove separation gap between the constrained adversaries (including the non-adaptive ones) and the strongly-adaptive adversaries, analyzed by Bar-Joseph and Ben-Or [PODC 1998] and others.

Pooyan Jamshidi