Elena Giachino

Session types revisited

Session types are a formalism to model structured communication-based programming. A session type describes communication by specifying the type and direction of data exchanged between two parties. When session types and session primitives are added to the syntax of standard π-calculus types and terms, they give rise to additional separate syntactic categories. As a consequence, when new type features are added, there is duplication of efforts in the theory: the proofs of properties must be checked both on ordinary types and on session types. We show that session types are encodable in ordinary π types, relying on linear and variant types. Besides being an expressivity result, the encoding (i) removes the above redundancies in the syntax, and (ii) the properties of session types are derived as straightforward corollaries, exploiting the corresponding properties of ordinary π types. The robustness of the encoding is tested on a few extensions of session types, including subtyping, polymorphism and higher-order communications.

A framework for deadlock detection in core ABS

We present a framework for statically detecting deadlocks in a concurrent object-oriented language with asynchronous method calls and cooperative scheduling of method activations. Since this language features recursion and dynamic resource creation, deadlock detection is extremely complex and state-of-the-art solutions either give imprecise answers or do not scale. In order to augment precision and scalability, we propose a modular framework that allows several techniques to be combined. The basic component of the framework is a front-end inference algorithm that extracts abstract behavioral descriptions of methods, called contracts, which retain resource dependency information. This component is integrated with a number of possible different back-ends that analyze contracts and derive deadlock information. As a proof-of-concept, we discuss two such back-ends: (1) an evaluator that computes a fixpoint semantics and (2) an evaluator using abstract model checking.

Behavioral Types in Programming Languages

A recent trend in programming language research is to use behavioral type theory to ensure various correctness properties of large-scale, communication-intensive systems. Behavioral types encompass concepts such as interfaces, communication protocols, contracts, and choreography. The successful application of behavioral types requires a solid understanding of several practical aspects, from their representation in a concrete programming language, to their integration with other programming constructs such as methods and functions, to design and monitoring methodologies that take behaviors into account. Behavioral Types in Programming Languages provides the reader with the first comprehensive overview of the state of the art of these practical aspects, which are summarized as the pragmatics of behavioral types. Each section covers a particular programming paradigm or methodology, providing an ideal reference for programming languages researchers interested the topic, and in identifying the areas as yet unexplored.

Global Escape in Multiparty Sessions

This article proposes a global escape mechanism which can handle unexpected or unwanted conditions changing the default execution of distributed communicational flows, preserving compatibility of the multiparty conversations. Our escape is realised by a collection of asynchronous local exceptions which can be thrown at any stage of the communication and to any subsets of participants in a multiparty session. This flexibility enables to model complex exceptions such as criss-crossing global interactions and error handling for distributed cooperating threads. Guided by multiparty session types, our semantics is proven to provide a termination algorithm for global escapes. Our type system guarantees further safety and liveness properties, such as progress within the session and atomicity of escapes with respect to the subset of involved participants.

Causal-Consistent Reversible Debugging

Reversible debugging provides developers with a way to execute their applications both forward and backward, seeking the cause of an unexpected or undesired event. In a concurrent setting, reversing actions in the exact reverse order in which they have been executed may lead to undo many actions that were not related to the bug under analysis. On the other hand, undoing actions in some order that violates causal dependencies may lead to states that could not be reached in a forward execution. We propose an approach based on causal-consistent reversibility: each action can be reversed if all its consequences have already been reversed. The main feature of the approach is that it allows the programmer to easily individuate and undo exactly the actions that caused a given misbehavior till the corresponding bug is reached. This paper major contribution is the individuation of the appropriate primitives for causal-consistent reversible debugging and their prototype implementation in the CaReDeb tool. We also show how to apply CaReDeb to individuate common real-world concurrent bugs.

Deadlock Analysis of Unbounded Process Networks

Deadlock detection in concurrent programs that create networks with arbitrary numbers of nodes is extremely complex and solutions either give imprecise answers or do not scale. To enable the analysis of such programs, (1) we define an algorithm for detecting deadlocks of a basic model featuring recursion and fresh name generation: the lam programs, and (2) we design a type system for value passing CCS that returns lam programs. As a byproduct of these two techniques, we have an algorithm that is more powerful than previous ones and that can be easily integrated in the current release of TyPiCal, a type-based analyser for pi-calculus.

Deadlock Analysis of Concurrent Objects: Theory and Practice

We present a framework for statically detecting deadlocks in a concurrent object language with asynchronous invocations and operations for getting values and releasing the control. Our approach is based on the integration of two static analysis techniques: (i) an inference algorithm to extract abstract descriptions of methods in the form of behavioral types, called contracts, and (ii) an evaluator that computes a fixpoint semantics returning a finite state model of contracts. A potential deadlock is detected when a circular dependency is found in some state of the model. We discuss the theory and the prototype implementation of our framework. Our tool is validated on an industrial case study based on the Fredhopper Access Server (FAS) developed by SDL Fredhoppper. In particular we verify one of the core concurrent components of FAS to be deadlock-free.

Analysis of deadlocks in object groups

Object groups are collections of objects that perform collective work. We study a calculus with object groups and develop a technique for the deadlock analysis of such systems based on abstract descriptions of methods behaviours.

Causal-Consistent Reversibility in a Tuple-Based Language

Causal-consistent reversibility is a natural way of undoing concurrent computations. We study causal-consistent reversibility in the context of μKlaim, a formal coordination language based on distributed tuple spaces. We consider both uncontrolled reversibility, suitable to study the basic properties of the reversibility mechanism, and controlled reversibility based on a rollback operator, more suitable for programming applications. The causality structure of the language, and thus the definition of its reversible semantics, differs from all the reversible languages in the literature because of its generative communication paradigm. In particular, the reversible behavior of μKlaim read primitive, reading a tuple without consuming it, cannot be matched using channel-based communication. We illustrate the reversible extensions of μKlaim on a simple, but realistic, application scenario.

A Beginner's Guide to the DeadLock Analysis Model

This paper is an introduction to the framework for the deadlock analysis of object-oriented languages we have defined ini¾?[6,5]. We present a basic Java-like language and the deadlock analysis model in an accessible way. We also overview the algorithm for deciding deadlock-freeness by discussing a number of paradigmatic examples. We finally explore the techniques for coping with extensions of the object-oriented language.