Heterogeneous clock group protocol for causally ordered delivery of messages

Abstract

The vector clock widely used in group protocols cannot be adopted to a scalable group due to communication and computation overheads. In order to reduce the overheads, we discuss a hierarchical group which is composed of local subgroups. Processes in subgroups are synchronised by using physical and linear clocks while processes in a WAN are synchronised by using a vector clock. We discuss how to causally deliver messages by using the local synchronisation mechanisms. We discuss how to reduce the number of messages to be unnecessarily ordered. We evaluate the protocol in terms of number of messages ordered.