Simultaneous consensus tasks: a tighter characterization of set-consensus

  • Authors:
  • Yehuda Afek;Eli Gafni;Sergio Rajsbaum;Michel Raynal;Corentin Travers

  • Affiliations:
  • Computer Science Department, Tel-Aviv University, Israel;Department of Computer Science, UCLA, Los Angeles, CA;Instituto de Matemáticas, UNAM, Mexico;IRISA, Rennes Cedex, France;IRISA, Rennes Cedex, France

  • Venue:
  • ICDCN'06 Proceedings of the 8th international conference on Distributed Computing and Networking
  • Year:
  • 2006

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Abstract

We address the problem of solving a task T=(T1,...Tm) (called (m,1)-BG), in which a processor returns in an arbitrary one of m simultaneous consensus subtasks T1,...Tm. Processor pi submits to T an input vector of proposals (propi,1,...,propi,m), one entry per subtask, and outputs, from just one subtask ℓ, a pair (ℓ, propj,l) for some j. All processors that output at ℓ output the same proposal. Let d be a bound on the number of distinct input vectors that may be submitted to T. For example, d=3 if Democrats always vote Democrats across the board, and similarly for Republicans and Libertarians. A wait-free algorithm that immaterial of the number of processors solves T provided m ≥d is presented. In addition, if in each Tj we allow k-set consensus rather than consensus, i.e., for each ℓ, the outputs satisfy |{j | propj , ℓ}| ≤k, then the same algorithm solves T if m ≥⌈d/k ⌉. What is the power of T=(T1,...,Tm) when given as a subroutine, to be used by any number of processors with any number of input vectors? Obviously, T solves m-set consensus since each processor pi can submit the vector (idi,idi,...idi), but can m-set consensus solve T? We show it does, and thus simultaneous consensus is a new characterization of set-consensus. Finally, what if each Tj is just a binary-consensus rather than consensus? Then we get the novel problem that was recently introduced of the Committee-Decision. It was shown that for 3 processors and m=2, the simultaneous binary-consensus is equivalent to (3,2)-set consensus. Here, using a variation of our wait-free algorithms mentioned above, we show that a task, in which a processor is required to return in one of m simultaneous binary-consensus subtasks, when used by n processors, is equivalent to (n,m)-set consensus. Thus, while set-consensus unlike consensus, has no binary version, now that we characterize m-set consensus through simultaneous consensus, the notion of binary-set-consensus is well defined. We have then showed that binary-set-consensus is equivalent to set consensus as it was with consensus.