Logical Clocks Are Not Fair: What Is Fair?

Abstract

This paper describes the use of a high-level, precise, and executable language, DistAlgo, for expressing, understanding, running, optimizing, and improving distributed algorithms, through the study of Lamport's algorithm for distributed mutual exclusion. We show how a simplified algorithm, reached by several rounds of better understanding and improvement of the original algorithm, leads to further simplification and improved understanding of fairness. This allows us to use any ordering for fairness, including improved fairness for granting requests in the order in which they are made, over using logical clock values. This leads to the discovery that logical clocks are not fair in general.