Abstract De Nicola and Hennessy’s $$\textsc {must}$$ M U S T -preorder is a liveness preserving refinement which states that a server $$q$$ q refines a server $$p$$ p if all clients satisfied by $$p$$ p are also satisfied by $$q$$ q . Owing to the universal quantification over clients, this definition does not yield a practical proof method, and alternative characterisations are necessary to reason over it. Finding these characterisations for asynchronous semantics, i.e. where outputs are non-blocking, has thus far proven to be a challenge, usually tackled via ad-hoc definitions. We show that the standard characterisations of the $$\textsc {must}$$ M U S T -preorder carry over as they stand to asynchronous communication, if servers are enhanced to act as forwarders, i.e. they can input any message as long as they store it back into the shared buffer. Our development is constructive, is completely mechanised in Coq, and is independent of any calculus: our results pertain to Selinger output-buffered agents with feedback. This is a class of Labelled Transition Systems that captures programs that communicate via a shared unordered buffer, as in asynchronous CCS or the asynchronous $$\pi $$ π -calculus. We show that the standard coinductive characterisation lets us prove in Coq that concrete programs are related by the $$\textsc {must}$$ M U S T -preorder. Finally, our proofs show that Brouwer’s bar induction principle is a useful technique to reason on liveness preserving program transformations.

International audience

We propose an interpretation of multiparty sessions with asynchronous communication as Flow Event Structures. We introduce a new notion of asynchronous type for such sessions, ensuring the expected properties for multiparty sessions, including progress. Our asynchronous types, which reflect asynchrony more directly and more precisely than standard global types and are more permissive, are themselves interpreted as Prime Event Structures. The main result is that the Event Structure interpretation of a session is equivalent, when the session is typable, to the Event Structure interpretation of its asynchronous type, namely their domains of configurations are isomorphic.

Choreographic languages describe possible sequences of interactions among a set of agents. Typical models are based on languages or automata over sending and receiving actions. Pomsets provide a more compact alternative by using a partial order to explicitly represent causality and concurrency between these actions. However, pomsets offer no representation of choices, thus a set of pomsets is required to represent branching behaviour. For example, if an agent Alice can send one of two possible messages to Bob three times, one would need a set of 2×2×2 distinct pomsets to represent all possible branches of Alice's behaviour. This paper proposes an extension of pomsets, named branching pomsets, with a branching structure that can represent Alice's behaviour using 2+2+2 ordered actions. We compare the expressiveness of branching pomsets with that of several forms of event structures from the literature. We encode choreographies as branching pomsets and show that the pomset semantics of the encoded choreographies are bisimilar to their operational semantics. Furthermore, we define well-formedness conditions on branching pomsets, inspired by multiparty session types, and we prove that the well-formedness of a branching pomset is a sufficient condition for the realisability of the represented communication protocol. Finally, we present a prototype tool that implements our theory of branching pomsets, focusing on its applications to choreographies.

This short article recaps the purpose of the CONCUR Test-of-Time Award and presents the four papers that received the Award in 2022.

We present a calculus for concurrent reversible multiparty sessions, which improves on recent proposals in several respects: it allows for concurrent and sequential composition within processes and types, it gives a compact representation of the past of processes and types, which facilitates the definition of rollback, and it implements a fine-tuned strategy for backward computation. We propose a refined session type system for our calculus and show that it enforces the expected properties of session fidelity, forward and backward progress, as well as causal consistency. In conclusion, our calculus is a conservative extension of previous proposals, offering enhanced expressive power and refined analysis techniques.

Abstract We present a comprehensive model of structured communications in which self-adaptation and security concerns are jointly addressed. More specifically, we propose a model of multiparty, self-adaptive communications with access control and secure information flow guarantees. In our model, multiparty protocols (choreographies) are described as global types; security violations occur when process implementations of protocol participants attempt to read or write messages of inappropriate security levels within directed exchanges. Such violations trigger adaptation mechanisms that prevent the violations to occur and/or to propagate their effect in the choreography. Our model is equipped with local and global adaptation mechanisms for reacting to security violations of different gravity; type soundness results ensure that the overall multiparty protocol is still correctly executed while the system adapts itself to preserve the participants’ security.

We consider a calculus for multiparty sessions enriched with security levels for messages. We propose a monitored semantics for this calculus, which blocks the execution of processes as soon as they attempt to leak information. We illustrate the use of this semantics with various examples, and show that the induced safety property is compositional and that it is strictly included between a typability property and a security property proposed for an extended calculus in previous work.

Abstract We study a notion of observation for concurrent processes which allows the observer to see the distributed nature of processes, giving explicit names for the location of actions. A general notion of bisimulation related to this observation of distributed systems is introduced. Our main result is that these bisimulation relations, particularized to a process algebra extending CCS, are completely axiomatizable. We discuss in detail two instances of location bisimulations, namely the location equivalence and the location preorder.

A new equivalence between concurrent processes is proposed. It generalizes the well-known bisimulation equivalence to take into account the distributed nature of processes. The result is a noninterleaving semantic theory; concurrent processes are differentiated from processes that are non-deterministic but sequential. The new equivalence, together with its observational version, is investigated for a subset of the language CCS, and various algebraic characterizations are obtained.