Set Consensus: Captured by a Set of Runs with Ramifications

Abstract: Are (set)-consensus objects necessary? This paper answer is negative. We show that the availability of consensus objects can be replaced by restricting the set of runs we consider. In particular we concentrate of the set of runs of the Immediate-Snapshot-Model (IIS), and given the object we identify this restricted subset of IIS runs. We further show that given an $(m,k)$-set consensus, an object that provides $k$-set consensus among $m$ processors, in a system of $n$, $n>m$ processors, we do not need to use the precise power of the objects but rather their effective cumulative set consensus power. E.g. when $n=3, m=2,$ and $k=1$ and all the 3 processors are active then we only use 2-set consensus among the 3 processors, as if 2-processors consensus is not available. We do this until at least one of the 3 processors obtains an output. We show that this suggests a new direction in the design of algorithms when consensus objects are involved.