Fault-tolerant Total Order Multicast to asynchronous groups

While Total Order Broadcast (or Atomic Broadcast) primitives have received a lot of attention, the paper concentrates on Total Order Multicast to Multiple Groups in the context of asynchronous distributed systems in which processes may suffer crash failures. "Multicast to Multiple Groups" means that each message is sent to a subset of the process groups composing the system, distinct messages possibly having distinct destination groups. "Total Order" means that all message deliveries must be totally ordered. The paper proposes a protocol for such a multicast primitive. This protocol is based on two underlying building blocks, namely, Uniform Reliable Multicast and Uniform Consensus. Its design characteristics lie in the two following properties. The first one is a minimality property, more precisely, only the sender of a message and processes of its destination groups have to participate in the multicast of the message. The second property is a locality property: no execution of a consensus has to involve processes belonging to distinct groups (i.e., consensus are executed on a "per group" basis). This locality property is particularly useful when one is interested in using the Total Order Multicast primitive in large scale distributed systems. An improvement that reduces the cost of the protocol is also suggested.