 
Summary: Optimal Availability Quorum Systems: Theory and
Practice
Yair Amir \Lambda Avishai Wool y
April 3, 1998
Abstract
Quorum systems serve as a basic tool providing a uniform and reliable way to achieve
coordination in a distributed system. They are useful for distributed and replicated
databases, name servers, mutual exclusion, and distributed access control and signatures.
The unavailability of a quorum system is the probability of the event that no live quo
rum exists in the system. When such an event occurs the service is completely halted. The
unavailability is widely accepted as the measure by which quorum systems are evaluated.
In this paper we characterize the optimal availability quorum system in the general
case, when the failure probabilities may take any value in the range 0 ! p i ! 1. Then
we deal with the practical scenario in which the failure probabilities are unknown, but
can be estimated. We give a robust and efficient algorithm that calculates a near optimal
quorum system based on the estimated failure probabilities.
Keywords: Quorum systems, distributed computing, fault tolerance, replication.
1 Introduction
1.1 Motivation
Quorum systems serve as a basic tool providing a uniform and reliable way to achieve coordina
