Multiparty Compatibility in Communicating Automata: Characterisation and Synthesis of Global Session Types

Abstract

Multiparty session types are a type system that can ensure the safety and liveness of distributed peers via the global specification of their interactions. To construct a global specification from a set of distributed uncontrolled behaviours, this paper explores the problem of fully characterising multiparty session types in terms of communicating automata. We equip global and local session types with labelled transition systems (LTSs) that faithfully represent asynchronous communications through unbounded buffered channels. Using the equivalence between the two LTSs, we identify a class of communicating automata that exactly correspond to the projected local types. We exhibit an algorithm to synthesise a global type from a collection of communicating automata. The key property of our findings is the notion of multiparty compatibility which non-trivially extends the duality condition for binary session types.

Choreographies in the wild

“…We begin by observing that for all synthesizable T , the culpability states of participants along the reductions of T can be faithfully captured by only considering "1-bounded" configurations (i.e., where each queue contains at most one message). Furthermore, the local and global types in the present work are a subset of those in [4]. Therefore, they enjoy a key property from [4]: for all synthesizable T , each reachable 1-bounded configurations is reachable via a "1-bounded" computation (i.e., an execution such that each step has 1-bounded queues).…”

“…Furthermore, the local and global types in the present work are a subset of those in [4]. Therefore, they enjoy a key property from [4]: for all synthesizable T , each reachable 1-bounded configurations is reachable via a "1-bounded" computation (i.e., an execution such that each step has 1-bounded queues). We can then define "1-bounded" honesty as in Definition 5.8, but using the 1-bounded contract semantics, instead of the unbounded one; the observations above can be used to prove that 1-bounded honesty implies honesty.…”

“…To the best of our knowledge, our approach is the first formal model of distributed systems where contractual agreement is based on the existence of a choreography. Deniélou and Yoshida [4] have given a characterisation of finite-state communicating machines from which it is possible to synthesise a global type, as well as an algorithm to build global types from such machines. Their results could also be used as a foundation for an agreement relation similar the one presented here.…”

“…Their results could also be used as a foundation for an agreement relation similar the one presented here. However, we remark that the global type obtained from the algorithm in [4] is not unique in general, notably due to the lack of special treatment of independent interactions (i.e., G | G , in our notation). Practically, this means that specific fuse relations (such as the ones discussed in Section 4.3) based on this algorithm may become non-deterministic.…”

“…In this case, the top-down approach might not work as expected, because it could be impossible to adapt the end-point type projected from the choreography to the actual interface of the third-party service. In the last few years, approaches mixing top-down and bottom-up composition have been studied to cope with this kind of situation [4,5,6].…”