Parameterized Concurrent Multi-Party Session Types

Session types have been proposed as a means of statically verifying implementations of communication protocols. Although prior work has been successful in verifying some classes of protocols, it does not cope well with parameterized, multi-actor scenarios with inherent asynchrony. For example, the sliding window protocol is inexpressible in previously proposed session type systems. This paper describes System-A, a new typing language which overcomes many of the expressiveness limitations of prior work. System-A explicitly supports asynchrony and parallelism, as well as multiple forms of parameterization. We define System-A and show how it can be used for the static verification of a large class of asynchronous communication protocols.Comment: In Proceedings FOCLASA 2012, arXiv:1208.432

Actor programming with static guarantees

This thesis discusses two methodologies for applying type discipline to concurrent programming with actors: process types, and session types. A system based on each of the two is developed, and used as the basis for a comprehensive overview of process- and session- type merits and limitations. In particular, we analyze the trade-offs of the two approaches with regard to the expressiveness of the resulting calculi, versus the nature of the static guarantees offered. The first system discussed is based on the notion of a \emph{typestate}, that is, a view of an actor's internal state that can be statically tracked. The typestates used here capture what each actor handle \emph{may} be used for, as well as what it \emph{must} be used for. This is done by associating two kinds of tokens with each actor handle: tokens of the first kind are consumed when the actor receives a message, and thus dictate the types of messages that can be sent through the handle; tokens of the second kind dictate messaging obligations, and the type system ensures that related messages have been sent through the handle by the end of its lifetime. The next system developed here adapts session types to suit actor programming. Session types come from the world of process calculi, and are a means to statically check the messaging taking place over communication channels against a pre-defined protocol. Since actors do not use channels, one needs to consider pairs of actors as participants in multiple, concurrently executed---and thus interleaving---protocols. The result is a system with novel, parameterized type constructs to capture communication patterns that prior work cannot handle, such as the sliding window protocol. Although this system can statically verify the implementation of complicated messaging patterns, it requires deviations from industry-standard programming models---a problem that is true for all session type systems in the literature. This work argues that the typestate-based system, while not enforcing protocol fidelity as the session-inspired one does, is nevertheless more suitable for model actor calculi adopted by practical, already established frameworks such as Erlang and Akka

Types for Progress in Actor Programs

Properties in the actor model can be described in terms of the message-passing behavior of actors. In this paper, we address the problem of using a type system to capture liveness properties of actor programs. Specifically, we define a simple actor language in which demands for certain types of messages may be generated during execution, in a manner specified by the programmer. For example, we may want to require that each request to an actor eventually results in a reply. The difficulty lies in that such requests can be generated dynamically, alongside the associated requirements for replies. Such replies might be sent in response to intermediate messages that never arrive, but the property may also not hold for more trivial reasons; for instance, when the code of potential senders of the reply omit the required sending command in some branches of a conditional statement. We show that, for a restricted class of actor programs, a system that tracks typestates can statically guarantee that such dynamically generated requirements will eventually be satisfied.Ope

Automated Inference of Atomic Sets for Safe Concurrent Execution

Atomic sets are a synchronization mechanism in which the programmer specifies the groups of data that must be accessed as a unit. The compiler can check this specification for consistency, detect deadlocks, and automatically add the primitives to prevent interleaved access. Atomic sets relieve the programmer from the burden of recognizing and pruning execution paths which lead to interleaved access, thereby reducing the potential for data races. However, manually converting programs from lock-based synchronization to atomic sets requires reasoning about the program's concurrency structure, which can be a challenge even for small programs. Our analysis eliminates the challenge by automating the reasoning. Our implementation of the analysis allowed us to derive the atomic sets for large code bases such as the Java collections framework in a matter of minutes. The analysis is based on execution traces; assuming all traces reflect intended behavior, our analysis allows safe concurrency by preventing unobserved interleavings which may harbor latent Heisenbugs.unpublishednot peer reviewe