Selected Results from the Latest Decade of Quorum Systems Research

Over the past decade, work on quorum systems in non-traditional scenarios has facilitated a number of advances in the field of distributed systems. This chapter surveys a selection of these results including: Byzantine quorum systems that are suitable for use when parts of the system cannot be trusted; algorithms for the deployment of quorum systems on wide area networks so as to allow for efficient access and to retain load dispersion properties; and probabilistic quorum systems that yield benefits for protocols and applications that can tolerate a small possibility of inconsistency. We also present a framework grounded in Byzantine quorum systems that can be used to explain, compare, and contrast several recent Byzantine fault-tolerant state-machine and storage protocols. The framework provides a path to understanding the number of servers required, the number of faults that can be tolerated, and the number of rounds of communication employed by each protocol.