Core higher-order session processes: tractable equivalences and relative expressiveness

This work proposes tractable bisimulations for the higher-order - calculus with session primitives (HO ) and o ers a complete study of the expressivity of its most significant subcalculi. First we develop three typed bisimulations, which are shown to coincide with contextual equivalence. These characterisations demonstrate that observing as inputs only a specific finite set of higher-order values (which inhabit session types) su ces to reason about HO processes. Next, we identify HO, a minimal, second-order subcalculus of HO in which higher-order applications/abstractions, name-passing, and recursion are absent. We show that HO can encode HO extended with higher-order applications and abstractions and that a first-order session -calculus can encode HO . Both encodings are fully abstract. We also prove that the session -calculus with passing of shared names cannot be encoded into HO without shared names. We show that HO , HO, and are equally expressive; the expressivity of HO enables e ective reasoning about typed equivalences for higher-order processes

On asynchronous session semantics

This paper studies a behavioural theory of the p-calculus with session types under the fundamental principles of the practice of distributed computing — asynchronous communication which is order-preserving inside each connection (session), augmented with asynchronous inspection of events (message arrivals). A new theory of bisimulations is introduced, distinct from either standard asynchronous or synchronous bisimilarity, accurately capturing the semantic nature of session-based asynchronously communicating processes augmented with event primitives. The bisimilarity coincides with the reduction-closed barbed congruence. We examine its properties and compare them with existing semantics. Using the behavioural theory, we verify that the program transformation of multithreaded into event-driven session based processes, using Lauer-Needham duality, is type and semantic preserving. Our benchmark results demonstrate the potential of the sessiontype based translation as semantically transparent optimisation techniques

Reversing Single Sessions

Session-based communication has gained a widespread acceptance in practice as a means for developing safe communicating systems via structured interactions. In this paper, we investigate how these structured interactions are affected by reversibility, which provides a computational model allowing executed interactions to be undone. In particular, we provide a systematic study of the integration of different notions of reversibility in both binary and multiparty single sessions. The considered forms of reversibility are: one for completely reversing a given session with one backward step, and another for also restoring any intermediate state of the session with either one backward step or multiple ones. We analyse the costs of reversing a session in all these different settings. Our results show that extending binary single sessions to multiparty ones does not affect the reversibility machinery and its costs

Explicit connection actions in multiparty session types

This work extends asynchronous multiparty session types (MPST) with explicit connection actions to support protocols with op- tional and dynamic participants. The actions by which endpoints are connected and disconnected are a key element of real-world protocols that is not treated in existing MPST works. In addition, the use cases motivating explicit connections often require a more relaxed form of mul- tiparty choice: these extensions do not satisfy the conservative restric- tions used to ensure safety in standard syntactic MPST. Instead, we de- velop a modelling-based approach to validate MPST safety and progress for these enriched protocols. We present a toolchain implementation, for distributed programming based on our extended MPST in Java, and a core formalism, demonstrating the soundness of our approach. We discuss key implementation issues related to the proposed extensions: a practi- cal treatment of choice subtyping for MPST progress, and multiparty correlation of dynamic binary connections

Globally Governed Session Semantics

This paper proposes a bisimulation theory based on multiparty session types where a choreography specification governs the behaviour of session typed processes and their observer. The bisimulation is defined with the observer cooperating with the observed process in order to form complete global session scenarios and usable for proving correctness of optimisations for globally coordinating threads and processes. The induced bisimulation is strictly more fine-grained than the standard session bisimulation. The difference between the governed and standard bisimulations only appears when more than two interleaved multiparty sessions exist. This distinct feature enables to reason real scenarios in the large-scale distributed system where multiple choreographic sessions need to be interleaved. The compositionality of the governed bisimilarity is proved through the soundness and completeness with respect to the governed reduction-based congruence. Finally, its usage is demonstrated by a thread transformation governed under multiple sessions in a real usecase in the large-scale cyberinfrustracture

Resource Sharing via Capability-Based Multiparty Session Types

Multiparty Session Types (MPST) are a type formalism used to model communication protocols among components in distributed systems, by specifying type and direction of data transmitted. It is standard for multiparty session type systems to use access control based on linear or affine types. While useful in offering strong guarantees of communication safety and session fidelity, linearity and affinity run into the well-known problem of inflexible programming, excluding scenarios that make use of shared channels or need to store channels in shared data structures. In this paper, we develop capability-based resource sharing for multiparty session types. In this setting, channels are split into two entities, the channel itself and the capability of using it. This gives rise to a more flexible session type system, which allows channel references to be shared and stored in persistent data structures. We illustrate our type system through a producer-consumer case study. Finally, we prove that the resulting language satisfies type safety

Declarative Choreographies and Liveness

Part 1: Full PapersInternational audienceWe provide the first formal model for declarative choreographies, which is able to express general omega-regular liveness properties. We use the Dynamic Condition Response (DCR) graphs notation for both choreographies and end-points. We define end-point projection as a restriction of DCR graphs and derive the condition for end-point projectability from the causal relationships of the graph. We illustrate the results with a running example of a Buyer-Seller-Shipper protocol. All the examples are available for simulation in the online DCR workbench at http://dcr.tools/forte19

Typechecking Java Protocols with [St]Mungo

This is a tutorial paper on [St]Mungo, a toolchain based on multiparty session types and their connection to typestates for safe distributed programming in Java language. The StMungo (“Scribble-to-Mungo”) tool is a bridge between multiparty session types and typestates. StMungo translates a communication protocol, namely a sequence of sends and receives of messages, given as a multiparty session type in the Scribble language, into a typestate specification and a Java API skeleton. The generated API skeleton is then further extended with the necessary logic, and finally typechecked by Mungo. The Mungo tool extends Java with (optional) typestate specifications. A typestate is a state machine specifying a Java object protocol, namely the permitted sequence of method calls of that object. Mungo statically typechecks that method calls follow the object’s protocol, as defined by its typestate specification. Finally, if no errors are reported, the code is compiled with javac and run as standard Java code. In this tutorial paper we give an overview of the stages of the [St]Mungo toolchain, starting from Scribble communication protocols, translating to Java classes with typestates, and finally to typechecking method calls with Mungo. We illustrate the [St]Mungo toolchain via a real-world case study, the HTTP client-server request-response protocol over TCP. During the tutorial session, we will apply [St]Mungo to a range of examples having increasing complexity, with HTTP being one of them