On implementing omega with weak reliability and synchrony assumptions

  • Authors:

  • Marcos K. Aguilera;Carole Delporte-Gallet;Hugues Fauconnier;Sam Toueg

  • Affiliations:

  • Hp Labs Systems Research Center, Palo Alto, CA;LIAFA, Université D. Diderot, 2 Place Jussieu, 75251, Paris Cedex, France;LIAFA, Université D. Diderot, 2 Place Jussieu, 75251, Paris Cedex, France;University of Toronto, Toronto, Canada

  • Venue:
  • Proceedings of the twenty-second annual symposium on Principles of distributed computing
  • Year:
  • 2003

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Abstract

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.