How to be a More Efficient Snoop: Refined Probe Complexity of Quorum Sets

Quorums are flexible and well-studied means for implementing fault-tolerant distributed systems. The probe complexity gives the number of probes required to find a quorum of noncrashed processes or to reveal that no such quorum currently exists. In this paper, we refine the original notion of probe complexity by explicitly considering the underlying failure model. A refined probe complexity gives a tight bound on the number of required probes, which is lower than the original probe complexity for most failure models. Additionally, we present a universal probe strategy that is defined for all quorum sets and exhibits the refined probe complexity in the worst case. In contrast, previous probe strategies were limited to special quorum sets, namely to coteries, and meet the original probe complexity only for special (i. e., nondominated) coteries.