Reduction of Unnecessarily Ordered Messages in Scalable Group Communication

In distributed applications, multiple processes are cooperating with one another by exchanging messages in networks. Information systems like cloud computing systems are getting scalable, which are composed of a huge number of processes. In group communications, messages sent by processes have to be causally delivered to every process in a group. The vector clock is widely used in group communication protocols but cannot be adopted to scalable groups due to communication and computation overheads. In the linear clock, the message length is O(1) but some pairs of messages are unnecessarily ordered even if the messages are not required to be causally delivered. In this paper, we discuss a communication protocol for a scalable group where linear clock and physical clock are used to reduce the number of pairs of messages unnecessarily ordered. We consider a group of multiple processes where the maximum offset of every physical clock is the same, i.e. homogeneous clock but maximum delay time between every pair of processes is not the same, i.e. heterogeneous network. We evaluate the protocol and show the number of pairs of unnecessarily ordered messages can be reduced compared with the linear clock protocol.