Total ordering group communication protocol based on coordinating sequencers for multiple overlapping groups

This paper presents a total ordering protocol for group communication systems with multiple overlapping groups. Our protocol takes advantages of the simplicity of the sequencer-based approach, but employs multiple sequencers to achieve better load balance. For a given message, the sequencer of the destination group constructs a sequencing array by requesting for relative delivery positions from the sequencers of the overlapping groups. The sequencing array is used by any receiving process to determine the delivery sequence of the message. The notion of logical clock is used for determining the relative delivery sequences of the messages. The coordination between the sequencers is performed in a simple, asynchronous and non-blocking manner. The delivery operation at a receiving process is very simple, and a message can be delivered as soon as it becomes deliverable. These factors amount to a significant saving of the computing and communication overhead for the system. As the protocol demands the minimum effort from the group members it is suitable for mobile computing environment in which mobile devices are typically tight on resources. In the paper, we also present some ways to enhance the performance of the system. Extension of the protocol to encompass the preservation of causality, the dynamic group membership and the failure recovery is included in the paper.