Dynamic Effective Timed Communication Systems

Message passing concurrency is a widely used paradigm in distributed systems research. Communicating finite state machines (CFSM) are probably the simplest model of message passing concurrency—introduced in the eighties, but still the default model in the context of communication protocols, session types, and choreographies. Another well-known family of models are Agha’s actors, which populate the other end of the expressivity and complexity spectrum: actors may behave in ways that even go beyond the computable. We want to avoid the complexities of the actor model and separate out matters that go beyond the computable. Ideally, we want something as simple as cfsms to give semantics to engines of the Anoma specification but with adequate expressive power.

This paper thus introduces a generalization of cfsms, called dynamic effective timed communication systems (DETCs)—somewhat baroque but descriptive: they have arbitrary computable state transitions, can dynamically create new state machines, and come equipped with a clock for each machine. We retain the isolated turn principle of the actor model.

Each machine performs “turns” one after the other: a turn is taking a waiting message, interpreting it, and deciding on a state update for the machine and a collection of actions to take in response, perhaps sending messages or creating new machines. The technical core of the paper is definitions of DETCSs and their labelled transition systems, which can be used to give operational semantics to engine systems of the Anoma specification.