Alceste Scalas

Honesty by Typing

We propose a type system for a calculus of contracting processes. Processes may stipulate contracts, and then either behave honestly, by keeping the promises made, or not. Type safety guarantees that a typeable process is honest — that is, the process abides by the contract it has stipulated in all possible contexts, even those containing dishonest adversaries.

A semantic deconstruction of session types

We investigate the semantic foundations of session types, by revisiting them in the abstract setting of labelled transition systems. The crucial insight is a simulation relation which generalises the usual syntax-directed notions of typing and subtyping, and encompasses both synchronous and asynchronous binary session types. This allows us to extend the session types theory to some common programming patterns which are not typically considered in the session types literature.

Lightweight Session Programming in Scala.

Designing, developing and maintaining concurrent applications is an error-prone and time-consuming task; most difficulties arise because compilers are usually unable to check whether the inputs/outputs performed by a program at runtime will adhere to a given protocol specification. To address this problem, we propose lightweight session programming in Scala: we leverage the native features of the Scala type system and standard library, to introduce (1) a representation of session types as Scala types, and (2) a library, called lchannels, with a convenient API for session-based programming, supporting local and distributed communication. We generalise the idea of Continuation-Passing Style Protocols (CPSPs), studying their formal relationship with session types. We illustrate how session programming can be carried over in Scala: how to formalise a communication protocol, and represent it using Scala classes and lchannels, letting the compiler help spotting protocol violations. We attest the practicality of our approach with a complex use case, and evaluate the performance of lchannels with a series of benchmarks.

Choreographies in the wild

We investigate the use of choreographies in distributed scenarios where, as in the real world, mutually distrusting (and possibly dishonest) participants may be unfaithful to their expected behaviour. In our model, each participant advertises its promised behaviour as a contract. Participants may interact through multiparty sessions, created when their contracts allow to synthesise a choreography. We show that systems of honest participants (which always adhere to their contracts) enjoy progress and session fidelity.

Choreography Synthesis as Contract Agreement

We propose a formal model for distributed systems, where each participant advertises its requirements and obligations as behavioural contracts, and where multiparty sessions are started when a set of contracts allows to synthesise a choreography. Our framework is based on the CO2 calculus for contract-oriented computing, and borrows concepts and results from the session type literature. It supports sessions where the number of participants is not determined beforehand, and keeps CO2’s ability to rule out participants that are culpable if contracts are not fulfilled at runtime. We show that we have progress and session fidelity in CO2, as a result of the honesty of participants — i.e., their ability to always adhere to their contracts.

Modelling and Verifying Contract-Oriented Systems in Maude

We address the problem of modelling and verifying contract-oriented systems, wherein distributed agents may advertise and stipulate contracts, but — differently from most other approaches to distributed agents — are not assumed to always behave “honestly”. We describe an executable specification in Maude of the semantics of CO2, a calculus for contract-oriented systems [6]. The honesty property [5] characterises those agents which always respect their contracts, in all possible execution contexts. Since there is an infinite number of such contexts, honesty cannot be directly verified by model-checking the state space of an agent (indeed, honesty is an undecidable property in general [5]). The main contribution of this paper is a sound verification technique for honesty. To do that, we safely over-approximate the honesty property by abstracting from the actual contexts a process may be engaged with. Then, we develop a model-checking technique for this abstraction, we describe an implementation in Maude, and we discuss some experiments with it.

A Linear Decomposition of Multiparty Sessions for Safe Distributed Programming (Artifact)

This artifact contains a version of the Scribble tool that, given a protocol specification with multiple participants, can generate Scala APIs for implementing each participant in a type-safe, protocol abiding way. Crucially, the API generation leverages a decomposition of the multiparty protocol into type-safe peer-to-peer interactions between pairs of participants; and this, in turn, allows to implement the API internals on top of the existing lchannels library for type-safe binary session programming. As a result, several technically challenging aspects in the implementation of multiparty sessions are solved “for free”, at the underlying binary level. This includes distributed multiparty session delegation: this artifact implements it for the first time.

High-performance technical computing with erlang

High-performance Technical Computing (HPTC) is a branch of HPC (High-performance Computing) that deals with scientific applications, such as physics simulations. Due to its numerical nature, it has been traditionally based on low-level or mathematically-oriented languages (C, C++, Fortran), extended with libraries that implement remote execution and inter-process communication (like MPI and PVM). But those libraries just provide what Erlang does out-of-the-box: networking, process distribution, concurrency, interprocess communication and fault tolerance. So, is it possible to use Erlang as a foundation for developing HPTC applications? This paper shows our experiences in using Erlang for distributed number-crunching systems. We introduce two extensions: a simple and efficient foreign function interface (FFI), and an Erlang binding for numerical libraries. We use them as a basis for developing a simple mathematically-oriented programming language (in the style of Matlab™) compiled into Core Erlang. These tools are later used for creating a HPTC framework (based on message-passing) and an IDE for distributed applications. The results of this research and development show that Erlang/OTP can be used as a platform for developing large and scalable numerical applications.

Verifiable abstractions for contract-oriented systems☆

Abstract We address the problem of modelling and verifying contract-oriented systems, wherein distributed agents may advertise and stipulate contracts, but — differently from most other approaches to distributed agents — are not assumed to always respect them. A key issue is that the honesty property, which characterises those agents which respect their contracts in all possible execution contexts, is undecidable in general. The main contribution of this paper is a sound verification technique for honesty, targeted at agents modelled in a value-passing version of the calculus CO2. To do that, we safely over-approximate the honesty property by abstracting from the actual values and from the contexts a process may be engaged with. Then, we develop a model-checking technique for this abstraction, we describe its implementation in Maude, and we discuss some experiments with it.

Multiparty session types, beyond duality

Abstract Multiparty Session Types (MPST) are a well-established typing discipline for message-passing processes interacting on sessions involving two or more participants. Session typing can ensure desirable properties: absence of communication errors and deadlocks, and protocol conformance. We propose a novel MPST theory based on a rely/guarantee typing system , that checks (1) the guaranteed behaviour of the process being typed, and (2) the relied upon behaviour of other processes. Crucially, our theory achieves type safety by enforcing a typing context liveness invariant throughout typing derivations. Unlike “classic” MPST works, our typing system does not depend on global session types, and does not use syntactic duality checks. As a result, our new theory can prove type safety for processes that implement protocols with complex inter-role dependencies, thus sidestepping an intrinsic limitation of “classic” MPST.