Abstract. Event-driven programming is a major paradigm in concurrent and communication-based programming, and a widely adopted approach to building scalable high-concurrency servers. However, traditional event-driven programs are more difficult to read, write and verify than their multi-threaded counterparts due to low-level APIs and fragmentation of control flow across disjoint event handlers. This paper presents a Java language extension and a novel type discipline for type-safe event-driven session programming that counters the problems of traditional event-based programming with abstractions and safety guarantees based on session types, while retaining the expressiveness and performance characteristics of events. The type discipline extends session types and their primitives with asynchronous input, session typecase and session set types, ensuring eventhandling safety and event progress in addition to the standard type soundness and communication safety. The advantages, expressiveness and performance of event-driven session programming are demonstrated through a range of examples and benchmarks, including a session-typed SMTP server.

For higher-order (process) languages, characterising contextual equivalence is a long-standing issue. In the setting of a higher-order π-calculus with session types, we develop characteristic bisimilarity, a typed bisimilarity which fully characterises contextual equivalence. To our knowledge, ours is the first characterisation of its kind. Using simple values inhabiting (session) types, our approach distinguishes from untyped methods for characterising contextual equivalence in higher-order processes: we show that observing as inputs only a precise finite set of higher-order values suffices to reason about higher-order session processes. We demonstrate how characteristic bisimilarity can be used to justify optimisations in session protocols with mobile code communication.

We report on two tools which extend Java with support for static typechecking of communication protocols. Our Mungo tool extends Java with typestate definitions, which allow classes to be associated with state machines defining permitted sequences of method calls. A complementary tool, StMungo, takes a communication protocol specified in the Scribble protocol description language, and generates a typestate specification for each endpoint, capturing the permitted sequences of messages along that channel. Endpoint implementations can be validated by Mungo against their typestate definitions and then compiled as usual with javac. We formalise Mungo's typestate inference system and demonstrate the Scribble, Mungo and StMungo toolchain via a typechecked SMTP client that can communicate with a real-world SMTP server.

Abstract. This paper studies a behavioural theory of the π-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.

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