Communication Protocols in Layered Groups with Heterogeneous Clocks

Abstract

In distributed systems, a group of multiple processes are required to cooperate with each other to achieve some objectives. In this paper, we consider a distributed system where there is o centralized coordinator. Each member process is peer, i.e. each peer has to autonomously make a decision by itself through communicating with other peers in a group. Each peer sends a message to every peer and every other peer receives the message in a group. In group communication, each message sent by a peer has to be causally delivered to every peer. Types of clocks, linear clock, vector clocks, and physica clocks are used to causally order messages. The linear clock can be used in a scalable group since message length is O(1). However, some messages which are not to be causally ordered are ordered. On the other hand, all and only messages to be causally ordered are ordered in the vector clock. However, the vector clock cannot be adopted in a scalable group due to the message length O(n) for the number n of peers of the group. A computer is equipped with a physical clock which is synchronized with a time server. Each peer can read time at a physical clock of a computer where the peer is performed. Messages are totally ordered in terms of physical time when the messages are sent. However, a pair of messages to be causally ordered might be ordered in the reverse order due to difference of times shown by physical clocks. In a scalable group, it is not easy for each peer to send a message to every peer. In this paper, we discuss a hierarchical group to realize a scalable group.