The Reliability of Voting Mechanisms

In a faulty distributed system, voting is commonly used to achieve mutual exclusion among groups of isolated nodes. Each node is assigned a number of votes, and any group with a majority of votes can perform the critical operations. Vote assignments can have a significant impact on system reliability. In this paper we address the problem of selecting vote assignments in order to maximize the probability that the critical operations can be performed at a given time by some group of nodes. We suggest simple heuristics to assign votes, and show that they give good results in most cases. We also study three particular homogeneous topologies (fully connected, Ethernet, and ring networks), and derive analytical expressions for system reliability. These expressions provide useful insights into the reliability provided by voting mechanisms.