Abstract
We address the question of the weakest failure detector to circumvent the impossibility of \((2n-2)\)-renaming in a system of up to \(n\) participating processes. We derive that in a restricted class of eventual failure detectors there does not exist a single weakest oracle, but a weakest family of oracles \(\zeta _n\): every two oracles in \(\zeta _n\) are incomparable, and every oracle that allows for solving renaming provides at least as much information about failures as one of the oracles in \(\zeta _n\). As a by product, we obtain one more evidence that renaming is strictly easier to solve than set agreement.
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Notes
In the \(k\)-set consensus task (\(k\)-SC), the processes start with private inputs and produce outputs so that the set of output values is a subset of the set of inputs of size at most \(k\). In \(\mathcal E ^n\), we say simply set consensus (SC) for \((n-1)\)-set consensus.
However its value is bounded between \(\nu ={{m \atopwithdelims ()2n-1}}/{{m-n \atopwithdelims ()(2n-1)-n}}={{m \atopwithdelims ()n}}/ {{2n-1 \atopwithdelims ()n}}\), which is the number of sets of size \(2n-1\) divided by the number of sets of this size covered by each \(\zeta \), and \(\nu \cdot \left[1+ln\left({m-n \atopwithdelims ()m-(2n-1)}\right)\right]\) due to [26].
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Acknowledgments
We are grateful to Eli Gafni, Piotr Zieliński and the anonymous referees of DISC and this journal, for many fertile discussions, helping us to strengthen the results and improve the presentation.
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Afek, Y., Kuznetsov, P. & Nir, I. Renaming and the weakest family of failure detectors. Distrib. Comput. 25, 411–425 (2012). https://doi.org/10.1007/s00446-012-0177-5
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DOI: https://doi.org/10.1007/s00446-012-0177-5