On implementing omega with weak reliability and synchrony assumptions
Marcos K. Aguilera, Carole Delporte-Gallet, Hugues Fauconnier, Sam Toueg · 2003
We study the feasibility and cost of implementing Ω---a fundamental failure detector at the core of many algorithms---in systems with weak reliability and synchrony assumptions. Intuitively, Ω allows processes to eventually elect a common leader. We first give an algorithm that implements Ω in a weak system S where processes are synchronous, but: (a) any number of them may crash, and (b) only the output links of an unknown correct process are eventually timely (all other links can be asynchronous and/or lossy). This is in contrast to previous implementations of Ω which assume that a quadratic number of links are eventually timely, or systems that are strong enough to implement the eventually perfect failure detector P. We next show that implementing Ω in S is expensive: even if we want an implementation that tolerates just one process crash, all correct processes (except possibly one) must send messages forever; moreover, a quadratic number of links must carry messages forever. We then show that with a small additional assumption---the existence of some unknown correct process whose asynchronous links are lossy but fair---we can implement Ω efficiently: we give an algorithm for Ω such that eventually only one process (the elected leader) sends messages.