A Sequencing Service for Group Communication

This paper is about the design of a sequencing service for totallyand causally-ordered group communication in a distributed system. A group communication service provides facilities for managing groups of processes and for multicasting messages to all the members of a group. The primary role of the sequencing service is to impose a total order on multicast messages, but we shall show how to build a combined causal order from it, and also a new hybrid order which we call cautal ordering. This design addresses the issue of sequencing many groups, which may overlap, and of using several sequencer processes to do so. In the remainder of this section we briefly explain the requirement for group communication involving overlapping groups, in the context of multi-user applications; we describe existing sequencer-based protocols for totally-ordered multicast to a single group, and we summarise our main contributions in relation to them. Section 2 describes extensions to sequencer-based protocols to handle multiple groups, and to split the sequencing load between several sequencers whenever possible. Section 3 shows how to obtain causal and cautal ordering from total ordering. Section 4 discusses our protocols, and summarises and concludes the paper.