Deriving Highly Available Quorum Systems from Structural Failure Models

Quorum systems are widely used means to realize agreement and coordination in fault-tolerant distributed systems. Highly available quorum systems are optimal with respect to resilience as they provide quorums with a sufficient number of correct processes despite any assumed fault scenario. Structural failure models describe all assumed fault scenarios by sets of faulty processes, which abstracts from detailed stochastic assumptions and covers both independent and dependent failures. In this paper, we give necessary and sufficient conditions for the existence of highly available quorum systems in terms of structural failure models. These results generalize previous work as we cover both, static and dynamic quorum systems, dependent failures, and consider crash as well as Byzantine failures.