Quorum Systems in Replicated Databases: Science or Fiction?

A quorum system is a collection of subsets of servers, every two of which intersect. Quorum systems have been suggested as a tool for concurrency control in replicated databases almost twenty years ago. They promised to guarantee strict consistency and to provide high availability and fault-tolerance in the face of server crashes and network partitions. Despite these promises, current commercial replicated databases typically do not use quorum systems. Instead they use mechanisms which guarantee much weaker consistency, if any. Moreover, the interest in quorum systems seems to be waning even in the database research community. This paper attempts to explain why quorum systems have not fulfilled their old promises, and at the same time to argue why the current state of affairs may change. As technological advances bring new capabilities, and new applications bring new requirements, the time may have come to review the validity of some long standing criticisms of quorum systems. Another theme of this paper is to argue that if quorum systems are to play a role in database research, it is not likely to be for their claimed fault-tolerance capabilities. Rather, more attention should be given to a somewhat overlooked feature of quorum systems: they allow load balancing among servers while maintaining strict consistency. Thus quorum systems may offer a way to scale up the throughput of heavily loaded servers.