Reliable Logical Clocks For Unreliable Process Groups

Abstract

Logical clocks are a powerful tool used as a basic mechanism in a lot of applications run on top of asynchronous distributed systems. A logical clock system is made of a set of logical clocks (one per process) plus a protocol that manages their progress. A logical clock allows a process to timestamp its events. The fundamental property of a logical clock system (called consistency) is the following: if two events are causally related [13] (one being the potential cause, the other being the effect) then the timestamp associated with a cause is smaller than the timestamp associated with its effect. Two main types of logical clocks have been investigated. Scalar clocks are particularly interesting when one is interested in obtaining a system-wide total ordering of events. Vector clocks [6, 14] have been introduced to decide whether two events are causally related or are independent.