G. Michele Pinna

On the nature of events: another perspective in concurrency

Abstract We introduce yet another event-based formalism, that of event automata, which unifies various concepts to be found in the literature. A characteristic property of these automata is that every state bears information about the events which have happened, and every state change coincides with the happening of an event. With configurations being reachable states, we show that other formalisms, such as (prime or flow) event structures and geometric automata, can be embedded in our model. Various constructions on event automata, such as the partially synchronous products, specialize to those of other formalisms. We discuss various new concepts such as possible events, asymmetric conflict and quiescent states, all of which are of an intensional nature, and which more faithfully model certain phenomena of event-based protocols. Finally, we distinguish between specifications, and automata which satisfy a specification. This distinction seems to open new perspectives for the analysis of refinement and abstraction. We prove compositionality of satisfaction with regard to some synchronization operators.

Process Semantics for Place/Transition Nets with Inhibitor and Read Arcs

In this paper we introduce a truly concurrent semantics for P/T nets with inhibitor and read arcs, called henceforth Contextual P/T nets. The semantics is based on a proper extension of the notion of process to cope with read and inhibitor arcs: we show that most of the properties enjoined by the classical process semantics for P/T nets continue to hold and we substantiate the adequateness of our notion by comparing it with the step semantics.

Non Sequential Semantics for Contextual P/T Nets

The problem of finding a true concurrent semantics for contextual P/T nets has not been deeply studied yet. The interest for such a semantics has been renewed by some recent proposals to equip mobile process algebras with a net semantics based on this model. In [3] we proposed a causal semantics for such nets, in this paper we study the non sequential semantics of contextual P/T nets. The semantics is based on a suitable notion of occurrence net and, equipped with a notion of history preserving bisimulation, is more discriminating of the contextual one in general, whereas both are equivalent if we restrict our attention to positive context only.

Synthesis of Nets with Inhibitor Arcs

The synthesis problem for Petri nets consists in the construction of a net system whose behaviour is specified by a given transition system. In this paper we deal with the synthesis of elementary net systems extended with inhibitor arcs, i.e. arcs that test for absence of tokens in a place. We characterize the class of transitions systems corresponding to the sequential execution of these nets, which is a proper extension of the one obtained by the execution of nets without inhibitor arcs. Finally, we try to minimize the number of inhibitor arcs; we look for conditions guaranteeing that an inhibitor arc is really used, i.e. its presence influences the behaviour of the net.

Functorial Concurrent Semantics for Petri Nets with Read and Inhibitor Arcs

We propose a functorial concurrent semantics for Petri nets extended with read and inhibitor arcs, that we call inhibitor nets. Along the lines of the seminal work of Winskel on safe nets, the truly concurrent semantics is given at a categorical level via a chain of functors leading from the category SW-IN of semi-weighted inhibitor nets to the category Dom of finitary prime algebraic domains. As an intermediate semantic model, we introduce inhibitor event structures, an extension of prime event structures able to faithfully capture the dependencies among events which arise in the presence of read and inhibitor arcs.

On the Nature of Events

We introduce yet another event-based formalism, event automata, which unifies various definitions to be found in the literature. Moreover we discuss various new concepts such as possible events, asymmetric conflict and quiescent states. We also distinguish between specifications and automata which satisfy a specification.

Lending Petri Nets and Contracts

Choreography-based approaches to service composition typically assume that, after a set of services has been found which correctly play the roles prescribed by the choreography, each service respects his role. Honest services are not protected against adversaries. We propose a model for contracts based on a extension of Petri nets, which allows services to protect themselves while still realizing the choreography. We relate this model with Propositional Contract Logic, by showing a translation of formulae into our Petri nets which preserves the logical notion of agreement, and allows for compositional verification.

Contracts as games on event structures

Abstract Event structures are one of the classical models of concurrent systems. The idea is that an enabling X ⊢ e represents the fact that the event e can only occur after all the events in the set X have already occurred. By interpreting events as actions promised by some participants, and by associating each participant with a goal (a function on sequences of events), we use event structures as a formal model for contracts . The states of a contract are sequences of events; a participant has a contractual obligation (in a given state) whenever some of its events is enabled in such a state. To represent the fact that participants are mutually distrusting, we study concurrent games on event structures; there, participants may play by firing events in order to reach their goals, and eventually win, lose or tie. A crucial notion arising in this setting is that of agreement : a participant agrees on a set of contracts if she has a strategy to reach her goals in all the plays conforming to her strategy (or to make another participant sanctionable for not honouring an obligation). Another relevant notion is protection : a participant is protected by her contract when she has a strategy to avoid losing in any contexts, even in those where she has not reached an agreement. We study conditions for obtaining agreement and protection, and we show that these properties mutually exclude each other in a certain class of contracts. We then relate the notion of agreement in contracts with that of compliance in session types. In particular, we show that compliance corresponds to the fact that eager strategies lead to agreement.

Verifying a Time-Triggered Protocol in a Multi-language Environment

The multi-language environment SYNCHRONIE supports the design and formal verification of synchronous reactive systems. Presently, SYNCHRONIE integrates three synchronous languages, ESTEREL, LUSTRE, and ARGOS. In the synchronous approach, not only the system but also its properties can be specified using a synchronous language. In SYNCHRONIE properties can be formalised textually as ESTEREL or LUSTRE programs, or graphically as ARGOS programs. Moreover, properties may also be specified as temporal logic formulas with past or future operators. It is shown how to specify and automatically prove properties of a time-triggered protocol taking advantage of this environment.

Circular Causality in Event Structures

We propose a model of events with circular causality, in the form of a conservative extension of Winskels event structures. We study the relations between this new kind of event structures and Propositional Contract Logic. Provable atoms in the logic correspond to reachable events in our event structures. Furthermore, we show a correspondence between the configurations of this new brand of event structures and the proofs in a fragment of Propositional Contract Logic.