The Immediate Dependency Relation: An Optimal Way to Ensure Causal Group Communication

In this paper we present a study on the subject of the Immediate Dependency Relation (IDR), and we show how by extending the IDR relation, one can ensure a global causal delivery in group communication, including in the overlapping group case. The main objective of this paper is to show that the use of the Immediate Dependency Relation (IDR) obliterates the notion that causality is expensive to set up in distributed systems. The IDR minimizes considerably the amount of control information sent per message to ensure causal ordering, without imposing restrictions in interaction (e.g. network topology, redifussion servers, executions models etc.). In order to demonstrate the feasibility of its implementation, we present an optimal broadcast causal protocol inspired by the IDR relation. We show the optimality of the protocol in terms of the amount of control information timestamped per message.