Giacoma Valentina Monreale

Reactive Systems, Barbed Semantics, and the Mobile Ambients

Reactive systems, proposed by Leifer and Milner, represent a meta-framework aimed at deriving behavioral congruences for those specification formalisms whose operational semantics is provided by rewriting rules. Despite its applicability, reactive systems suffered so far from two main drawbacks. First of all, no technique was found for recovering a set of inference rules, e.g. in the so-called SOS style, for describing the distilled observational semantics. Most importantly, the efforts focused on strong bisimilarity, tackling neither weak nor barbed semantics. Our paper addresses both issues, instantiating them on a calculus whose semantics is still in a flux: Cardelli and Gordons mobile ambients. While the solution to the first issue is tailored over our case study, we provide a general framework for recasting (weak) barbed equivalence in the reactive systems formalism. Moreover, we prove that our proposal captures the behavioural semantics for mobile ambients proposed by Rathke and Sobocinski and by Merro and Zappa Nardelli.

A Life Cycle for the Development of Autonomic Systems: The E-mobility Showcase

Component ensembles are a promising way of building self-aware autonomic adaptive systems. This approach has been promoted by the EU project ASCENS, which develops the core idea of ensembles by providing rigorous semantics as well as models and methods for the whole development life cycle of an ensemble-based system. These methods specifically address adaptation, self-awareness, self-optimization, and continuous system evolution. In this paper, we demonstrate the key concepts and benefits of the ASCENS approach in the context of intelligent navigation of electric vehicles (e-Mobility), which itself is one of the three key case studies of the project.

Labelled Transitions for Mobile Ambients (As Synthesized via a Graphical Encoding)

The paper presents a case study on the synthesis of labelled transition systems (LTSs) for process calculi, choosing as testbed Cardelli and Gordons Mobile Ambients (MAs). The proposal is based on a graphical encoding: each process is mapped into a graph equipped with suitable interfaces, such that the denotation is fully abstract with respect to the usual structural congruence. Graphs with interfaces are amenable to the synthesis mechanism proposed by Ehrig and Konig and based on borrowed contexts (BCs), an instance of relative pushouts, introduced by Leifer and Milner. The BC mechanism allows the effective construction of a LTS that has graphs with interfaces as both states and labels, and such that the associated bisimilarity is automatically a congruence. Our paper focuses on the analysis of a LTS over (processes as) graphs with interfaces, as distilled by exploiting the graphical encoding of MAs. In particular, we use the LTS on graphs to recover a suitable LTS directly defined over the structure of MAs processes.

Soft Constraints for Lexicographic Orders

While classical Constraint Satisfaction Problems (CSPs) concern the search for the boolean assignment of a set of variables that has to satisfy some given requirements, their soft variant considers ordered domains for assignments, thus modeling preferences: the aim is to provide an environment where suitable algorithms (e.g. on constraint propagation) can be stated and proved, and inherited by its instances.

A Decentralized Implementation of Mobile Ambients

We present a graphical implementation for finite processes of the mobile ambients calculus. Our encoding uses unstructured (i.e., non hierarchical) graphs and it is sound and complete with respect to the structural congruence of the calculus (that is, two processes are equivalent iff they are mapped into isomorphic graphs). With respect to alternative proposals for the graphical implementation of mobile ambients, our encoding distinguishes the syntactic structure of a process from the activation order of a process components. Our solution faithfully captures a basic feature of the calculus (ambients can be nested and reductions are propagated across ambient nesting) and it allows to model the reduction semantics via a graph transformation system containing just three rules.

A General Theory of Barbs, Contexts, and Labels

Barbed bisimilarity is a widely used behavioral equivalence for interactive systems: given a set of predicates (denoted “barbs” and representing basic observations on states) and a set of contexts (representing the possible execution environments), two systems are deemed to be equivalent if they verify the same barbs whenever inserted inside any of the chosen contexts. Despite its flexibility and expressiveness, this definition of equivalence is unsatisfactory because often the quantification is over an infinite set of contexts, thus making barbed bisimilarity very hard to be verified. Should a labeled operational semantics be available, more efficient observational equivalences might be adopted. To this end, a series of techniques has been proposed to derive labeled transition systems (LTSs) from unlabeled ones, the main example being Leifer and Milner’s theory of reactive systems. The underlying intuition is that labels should be the “minimal” contexts that allow for a reduction step to be performed. However, minimality is difficult to asses, whereas the set of “intuitively” correct labels is often easily devised by the ingenuity of the researcher. This article introduces a framework that characterizes (weak) barbed bisimilarity via LTSs whose labels are (not necessarily minimal) contexts. Differently from previous proposals, our theory does not depend on the way the labeled transitions are built but instead relies on a simple set-theoretical presentation for identifying those properties such an LTS should verify to (1) capture the barbed bisimilarities of the underlying system and (2) ensure that such bisimilarities are congruences. Furthermore, we adopt suitable proof techniques to make feasible the verification of such properties. To provide a test-bed for our formalism, we instantiate it by addressing the semantics of the Mobile Ambients calculus, recasting its barbed bisimilarities via label-based behavioral equivalences.

From Local to Global Knowledge and Back

Two forms of knowledge are considered: declarative and procedural. The former is easy to extend but it is equipped with expensive deduction mechanisms, while the latter is efficiently executable but it can hardly anticipate all the special cases. In the first part of this chapter (Sections 2 and 3), we first define a syntactic representation of Soft Constraint Satisfaction Problems (SCSPs), which allows us to express dynamic programming (DP) strategies. For the e-mobility case study of ASCENS, we use Soft Constraint Logic Programming (SCLP) to program (in CIAO Prolog) and solve local optimization problems of single electric vehicles. Then we treat the global optimization problem of finding optimal parking spots for all the cars. We provide: (i) a Java orchestrator for the coordination of local SCLP optimizations; and (ii) a DP algorithm, which corresponds to a local to global propagation and back. In the second part of this chapter (Section 4) we assume that different subjects are entitled to decide. The case study concerns a smart grid model where various prosumers (producers-consumers) negotiate (in real time, according to the DEZENT approach) the cost of the exchanged energy. Then each consumer tries to plan an optimal consumption profile (computed via DP) where (s)he uses less energy when it is expensive and more energy when it is cheap, conversely for a producer. Finally, the notion of an aggregator is introduced, whose aim is to sell flexibility to the market.

Encoding Synchronous Interactions Using Labelled Petri Nets

We present an encoding of bound CSP processes with replication into Petri nets with labelled transitions. Through the encoding, the firing semantics of Petri nets models the standard operational semantics of CSP processes, which is both preserved and reflected. This correspondence allows for describing by net semantics the standard CSP observational equivalences. Since the encoding is modular with respect to process syntax, the paper puts on a firm ground the technology transfer between the two formalisms, e.g. recasting into the CSP framework well-established results like decidability of coverability for nets. This work complements previous results concerning the encoding of asynchronous interactions, thus witnessing the expressiveness of open labelled nets in modelling process calculi with alternative communication patterns.

On barbs and labels in reactive systems

Reactive systems (RSs) represent a meta-framework aimed at deriving behavioral congruences for those computational formalisms whose operational semantics is provided by reduction rules. RSs proved a flexible specification device, yet so far most of the efforts dealing with their behavioural semantics focused on idem pushouts (IPOs) and saturated (also known as dynamic) bisimulations. In this paper we introduce a novel, intermediate behavioural equivalence: L-bisimilarity, which is able to recast both its IPO and saturated counterparts. The equivalence is parametric with respect to a set L of RSs labels, and it is shown that under mild conditions on L it is indeed a congruence. Furthermore, L-bisimilarity can also recast the notion of barbed semantics for RSs, proposed by the same authors in a previous paper. In order to provide a suitable test-bed, we instantiate our proposal by addressing the semantics of (asynchronous) CCS and of the calculus of mobile ambients.

Concurrency cannot be observed, asynchronously

The paper is devoted to an analysis of the concurrent features of asynchronous systems. A preliminary step is represented by the introduction of a non-interleaving extension of barbed equivalence. This notion is then exploited in order to prove that concurrency cannot be observed through asynchronous interactions, i.e., that the interleaving and concurrent versions of a suitable asynchronous weak equivalence actually coincide. The theory is validated on some case studies, related to nominal calculi (π-calculus) and visual specification formalisms (Petri nets). Additionally, we prove that a class of systems which is deemed (output-buffered) asynchronous, according to a characterization that was previously proposed in the literature, falls into our theory.