From Process Calculi to Klaim and Back

We briefly describe the motivations and the background behind the design of Klaim, a process description language that has proved to be suitable for describing a wide range of distributed applications with agents and code mobility. We argue that a drawback of Klaim is that it is neither a programming language, nor a process calculus. We then outline the two research directions we have pursued more recently. On the one hand we have evolved Klaim to a full-fledged language for distributed mobile programming. On the other hand we have distilled the language into a number of simple calculi that we have used to define new semantic theories and equivalences and to test the impact of new operators for network aware programming

Fair Testing

In this paper we present a solution to the long-standing problem of characterising the coarsest liveness-preserving pre-congruence with respect to a full (TCSP-inspired) process algebra. In fact, we present two distinct characterisations, which give rise to the same relation: an operational one based on a De Nicola-Hennessy-like testing modality which we call should-testing, and a denotational one based on a refined notion of failures. One of the distinguishing characteristics of the should-testing pre-congruence is that it abstracts from divergences in the same way as Milner¿s observation congruence, and as a consequence is strictly coarser than observation congruence. In other words, should-testing has a built-in fairness assumption. This is in itself a property long sought-after; it is in notable contrast to the well-known must-testing of De Nicola and Hennessy (denotationally characterised by a combination of failures and divergences), which treats divergence as catrastrophic and hence is incompatible with observation congruence. Due to these characteristics, should-testing supports modular reasoning and allows to use the proof techniques of observation congruence, but also supports additional laws and techniques. Moreover, we show decidability of should-testing (on the basis of the denotational characterisation). Finally, we demonstrate its advantages by the application to a number of examples, including a scheduling problem, a version of the Alternating Bit-protocol, and fair lossy communication channel

Relating fair testing and accordance for service replaceability

AbstractThe accordance pre-order describes whether a service can safely be replaced by another service. That is, all partners for the original service should be partners for the new service. Partners for a service interact with the service in such a way that always a certain common goal can be reached.We relate the accordance pre-order to the pre-orders known from the linear–branching time spectrum, notably fair testing. The differences between accordance and fair testing include the modeling of termination and success, and the parts of the services that cannot be used reliably by any partner. Apart from the theoretical results, we address the practical relevance of the introduced concepts

A Behavioural Theory for Interactions in Collective-Adaptive Systems

We propose a process calculus, named AbC, to study the behavioural theory of interactions in collective-adaptive systems by relying on attribute-based communication. An AbC system consists of a set of parallel components each of which is equipped with a set of attributes. Communication takes place in an implicit multicast fashion, and interaction among components is dynamically established by taking into account "connections" as determined by predicates over their attributes. The structural operational semantics of AbC is based on Labeled Transition Systems that are also used to define bisimilarity between components. Labeled bisimilarity is in full agreement with a barbed congruence, defined by simple basic observables and context closure. The introduced equivalence is used to study the expressiveness of AbC in terms of encoding broadcast channel-based interactions and to establish formal relationships between system descriptions at different levels of abstraction

A hybrid type system for lock-freedom of mobile processes

We propose a type system for lock-freedom in the π-calculus, which guarantees that certain communications will eventually succeed. Distinguishing features of our type system are: it can verify lock-freedom of concurrent programs that have sophisticated recursive communication structures; it can be fully automated; it is hybrid, in that it combines a type system for lock-freedom with local reasoning about deadlockfreedom, termination, and confluence analyses. Moreover, the type system is parameterized by deadlock-freedom/termination/confluence analyses, so that any methods (e.g. type systems and model checking) can be used for those analyses. A lock-freedom analysis tool has been implemented based on the proposed type system, and tested for non-trivial programs

Linear vs. branching time: A semantical perspective

The discussion of the relative merits of linear versus branching-time goes back to early 1980s. The dominating belief has been that the linear-time framework is not expressive enough semantically, marking linear-time logics as weak. Here we examine this issue from the perspective of process equivalence, one of the most fundamental notions in concurrency theory. We postulate three principles that we view as fundamental to any discussion of process equivalence. First, we take contextual equivalence as the primary notion of equivalence. Second, we require the description of a process to fully specify all relevant behavioral aspects of the process. Finally, we require observable process behavior to be reflected in input/output behavior. Under these postulates the distinctions between the linear and branching semantics tend to evaporate. Applying them to the framework of transducers, we show that our postulates result in a unique notion of process equivalence, which is trace based, rather than tree based

Basic Observables for Processes

We propose a general approach to define behavioural preorders over process terms by considering the pre-congruences induced by three basic observables. These observables provide information about the initial communication capabilities of processes and about their possibility of engaging in an infinite internal chattering. We show that some of the observables-based pre-congruences do correspond to behavioral preorders long studied in the literature. The coincidence proofs shed light on the differences between the must preorder of De Nicola and Hennessy and the fair/should preorder of Cleaveland and Natarajan and of Brinksma, Rensink and Vogler, and on the rôle played in their definition by tests for internal chattering