Global and logical time in distributed algorithms

Abstract

The use of global time can simplify the design and description of distributed algorithms. It is, however, an expensive mechanism to implement. In this paper we show that in some cases global time can be assumed while designing an algorithm, but need not be implemented—in these cases it can be replaced with Lamport's logical time in a routine manner, which clearly preserves the correctness of the algorithm. The technique is illustrated by three examples. Two existing algorithms are described as if they had been designed while assuming global time; they are Chandy and Lamport's distributed snapshot algorithm, and Banatre and Lapalme's fair agreement algorithm for the distributed auction system Enchere. The third example is a new distributed algorithm for restarting from a saved checkpoint state. It was designed while assuming global time, but can be implemented without it.