Romain Demangeon

Monitoring Networks through Multiparty Session Types

In large-scale distributed infrastructures, applications are realised through communications among distributed components. The need for methods for assuring safe interactions in such environments is recognized, however the existing frameworks, relying on centralised verification or restricted specification methods, have limited applicability. This paper proposes a new theory of monitored π-calculus with dynamic usage of multiparty session types (MPST), offering a rigorous foundation for safety assurance of distributed components which asynchronously communicate through multiparty sessions. Our theory establishes a framework for semantically precise decentralised run-time enforcement and provides reasoning principles over monitored distributed applications, which complement existing static analysis techniques. We introduce asynchrony through the means of explicit routers and global queues, and propose novel equivalences between networks, that capture the notion of interface equivalence, i.e. equating networks offering the same services to a user. We illustrate our static-dynamic analysis system with an ATM protocol as a running example and justify our theory with results: satisfaction equivalence, local/global safety and transparency, and session fidelity.

Full abstraction in a subtyped pi-calculus with linear types

We introduce a concise pi-calculus with directed choices and develop a theory of subtyping. Built on a simple behavioural intuition, the calculus offers exact semantic analysis of the extant notions of subtyping in functional programming languages and session-based programming languages. After illustrating the idea of subtyping through examples, we show type-directed embeddings of two known subtyped calculi, one for functions and another for session-based communications. In both cases, the behavioural content of the original subtyping is precisely captured in the fine-grained subtyping theory in the pi-calculus. We then establish full abstraction of these embeddings with respect to their standard semantics, Morriss contextual congruence in the case of the functional calculus and testing equivalence for the concurrent calculus. For the full abstraction of the embedding of the session-based calculus, we introduce a new proof method centring on non-deterministic computational adequacy and definability. Partially suggested by a technique used by Quaglia and Walker for their full abstraction result, the new proof method extends the framework used in game-based semantics to the May/Must equivalences, giving a uniform proof method for both deterministic and non-deterministic languages.

Nested protocols in session types

We propose an improvement to session-types, introducing nested protocols, the possibility to call a subprotocol from a parent protocol. This feature adds expressiveness and modularity to the existing session-type theory, allowing arguments to be passed and enabling higher-order protocols definition. Our theory is introduced through a new type system for protocols handling subprotocol calls, and its implementation in a session-calculus. We propose validation and satisfaction relations between specification and implementation. Sound behaviour is enforced thanks to the usage of kinds and well-formedness, allowing us to ensure progress and subject reduction. In addition, we describe an extension of our framework allowing subprotocols to send back results.

Practical Interruptible Conversations

The rigorous and comprehensive verification of communication-based software is an important engineering challenge in distributed systems. Drawn from our industrial collaborations [33,28] on Scribble, a choreography description language based on multiparty session types, this paper proposes a dynamic verification framework for structured interruptible conversation programming. We first present our extension of Scribble to support the specification of asynchronously interruptible conversations. We then implement a concise API for conversation programming with interrupts in Python that enables session types properties to be dynamically verified for distributed processes. Our framework ensures the global safety of a system in the presence of asynchronous interrupts through independent runtime monitoring of each endpoint, checking the conformance of the local execution trace to the specified protocol. The usability of our framework for describing and verifying choreographic communications has been tested by integration into the large scientific cyberinfrastructure developed by the Ocean Observatories Initiative. Asynchronous interrupts have proven expressive enough to represent and verify their main classes of communication patterns, including asynchronous streaming and various timeout-based protocols, without requiring additional synchronisation mechanisms. Benchmarks show conversation programming and monitoring can be realised with little overhead.

Structuring Communication with Session Types

Session types are types for distributed communicating processes. They were born from process encodings of data structures and typical interaction scenarios in an asynchronous version of the π-calculus, and are being studied and developed as a potential basis for structuring concurrent and distributed computing, as well as in their own right. In this paper, we introduce basic ideas of sessions and session types, outline their key technical elements, and discuss how they may be usable for programming, drawing from our experience and comparing with existing paradigms, especially concurrent objects such as actors. We discuss how session types can offer a programming framework in which communications are structured both in program text and at run-time.

A Multiparty Multi-session Logic

Recent work on the enhancement of multiparty sessions types with logical annotations enables not only the validation of structural properties of the conversations and on the sorts of the messages, but also the validation of properties on the actual values exchanged. However, the specification and verification of the mutual effects of multiple cross-session interactions is still an open problem. We introduce a multiparty logical proof system with virtual states that enables the tractable specification and validation of fine-grained inter-session correctness properties of processes participating in several interleaved sessions. We present a sound and relatively complete static verification method.

On the Expressiveness of Multiparty Sessions.

This paper explores expressiveness of asynchronous multiparty sessions. We model the behaviours of endpoint implementations in several ways: (i) by the existence of different buffers and queues used to store messages exchanged asynchronously, (ii) by the ability for an endpoint to lightly reconfigure his behaviour at runtime (flexibility), (iii) by the presence of explicit parallelism or interruptions (exceptional actions) in endpoint behaviour. For a given protocol we define several denotations, based on traces of events, corresponding to the different implementations and compare them.

Causal Computational Complexity of Distributed Processes

This paper studies the complexity of π-calculus processes with respect to the quantity of transitions caused by an incoming message. First we propose a typing system for integrating Bellantoni and Cooks characterisation of polynomially-bound recursive functions into Deng and Sangiorgis typing system for termination. We then define computational complexity of distributed messages based on Degano and Priamis causal semantics, which identifies the dependency between interleaved transitions. Next we apply a syntactic flow analysis to typable processes to ensure the computational bound of distributed messages. We prove that our analysis is decidable for a given process; sound in the sense that it guarantees that the total number of messages causally dependent of an input request received from the outside is bounded by a polynomial of the content of this request; and complete which means that each polynomial recursive function can be computed by a typable process.

Embedding Session Types in HML

Recent work on the enhancement of multiparty session types with logical annotations enable the effective verification of properties on (1) the structure of the conversations, (2) the sorts of the messages, and (3) the actual values exchanged. In [3] we extend this work to enable the specification and verification of mutual effects of multiple cross-session interactions. Here we give a sound and complete embedding into the Hennessy-Milner logic to justify the expressiveness of the approach in [3] and to provide it with a logical background that will enable us to compare it with similar approaches.