Generic Multicast: One Group Communication Primitive to Rule Them All

Group communication primitives have a central role in modern computing infrastructures. They offer a panel of reliability and ordering guarantees for messages, enabling the implementation of complex distributed interactions. In particular, atomic broadcast is a pivotal abstraction for implementing fault-tolerant distributed services. This primitive allows disseminating messages across the system in a total order. There are two group communication primitives closely related to atomic broadcast. Atomic multicast permits targeting a subset of participants, possibly stricter than the whole system. Generic broadcast leverages the semantics of messages to order them only where necessary, that is, when they do not commute at the application level (a conflict). In this paper, we propose to combine all these primitives into a single, more general one, called generic multicast. We formally specify the guarantees offered by generic multicast and present efficient algorithms. Compared to prior works, our solutions offer appealing properties in terms of time and space complexity. In particular, when a run is conflict-free, that is no two messages conflict, a message is delivered after at most three message delays. We explain the logic of of our algorithms, detail their main invariants, and prove them correct. We also present a variation that delivers messages across the system in an order consistent with real-time at the cost of a message delay. This variation is particularly interesting to implement partially-replicated data storage systems.