Highly concurrent logically synchronous multicast

Abstract

We define the logically synchronous multicast problem, which imposes a natural and useful structure on message delivery order in an asynchronous system. In this problem, a computation proceeds by a sequence of multicasts, in which a process sends a message to some arbitrary subset of the processes, including itself. A logically synchronous multicast protocol must make it appear to every process as if each multicast occurs simultaneously at all participants of that multicast (sender plus receivers). Furthermore, if a process continually wishes to send a message, it must eventually be permitted to do so. We present a highly concurrent solution in which each multicast requires at most 4|S| messages, where S is the set of participants in that multicast. The protocol’s correctness is shown using a remarkably simple problem specification stated in the I/O automaton model. We also show that implementing a wait-free solution to the logically synchronous multicast problem is impossible. The author is currently developing a simulation system for algorithms expressed as I/O automata. We conclude the paper by describing how the logically synchronous multicast protocol can be used to distribute this simulation system.