Implementing a Flexible Failure Detector That Expresses the Confidence in the System

Traditional unreliable failure detectors are per process oracles that provide a list of nodes suspected of having failed. In [Rossetto et al., 2015], we introduced the Impact failure detector that outputs a trust level value which is the degree of confidence in the system. An impact factor is assigned to each node and an input threshold parameter defines an impact factor limit value, over which the confidence degree on the system is ensured. The impact factor indicates the relative importance of the process in the set S, while the threshold offers a degree of flexibility for failures and false suspicions. We propose in this article two different algorithms, based on query-response message rounds, that implement the Impact FD whose conceptions were tailored to satisfy the Impact FD's flexibility. The first one exploits the time-free message pattern approach while the second one considers a set of bounded timely responses. We also introduced the concept that a process can be PS-accessible (or PS-accessible) which guarantees that the system S will always (or eventually always) be trusted by this process as well as two properties, PR(ITsp*) and PR(ITsp*), that characterize the stability conditions which ensure the confidence (or eventual confidence) of process p on S. In both implementations, if the process that monitors S is PS-accessible or PS-accessible, at every query round, it only waits (or eventually only waits) for a set of responses that satisfy the threshold. A crucial facet of this set of processes is that it is not fixed, i.e., the set of processes can change at each round, which is in accordance with the flexibility feature of the Impact FD.