: The behavior of some kinds of systems features a high rate of dynamic evolution. The system running causes the introduction of new components whereas some others disappear, and links between components are dynamically set: a component sometimes interacts with given components and sometimes with others. It is uneasy to capture such evolution inside Petri nets whose structure is fixed; but it is necessary to respect the Petri net semantics and keep the possibility to apply the structural analysis technics. The paper introduces two extensions of Petri nets dealing with this problem, Communicative Nets and Cooperative Nets. They enable to model a system as a collection of nets which encapsulate their behavior, while interacting by means of message sending or a client/ server protocol; a net may instantiate another net, and the links between nets are dynamic. An algorithm is given which captures this dynamicity by building a single fixed net whose behavior is equivalent to a whole system...

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.

We address the problem of alarm correlation in large distributed systems. The key idea is to make use of the concurrence of events in order to separate and simplify the state estimation in a faulty network. Petri nets and their causality semantics are used to model concurrency. Special partially stochastic Petri nets are developed, that establish some kind of equivalence between concurrence and independence. The diagnosis problem is defined as the computation of the most likely history of the net given a sequence of observed alarms. Solutions are provided in four contexts, with a gradual complexity on the structure of observations.

The main result shows the undecidability of (strong) bisimilarity for labelled (place / transition) Petri nets. The technique of the proof applies to the language (or trace) equivalence and the reachability set equality as well, which yields stronger versions with simpler proofs of already known results. The paper also contains two decidability results. One concerns the Petri nets which are deterministic up to bisimilarity, the other concerns semilinear bisimulations and extends the result of [4] for Basic Parallel Processes. 1 Introduction The topic of the paper belongs to an interesting area in the theory of parallelism and concurrency, namely to the area of decidability questions for behavioural equivalences in various classes of (models of) processes. These questions are among the rst ones to ask when developing automated verication methods, for example. There is a large amount of equivalences in the literature (cf. e.g. [9]), nevertheless some of them are felt to be mor...

This paper combines and extends the material of [GG-a/c/d/e], except for the part in [GG-c] on refinement of transitions in Petri nets and the discussion of TCSP-like parallel composition in [GG-e]. An informal presentation of some basic ingredients of this paper appeared as [GG-b]. Among others, the treatment of action refinement in stable and non-stable event structures is new. The research reported here was supported by Esprit project 432 (METEOR), Esprit Basic Research Action 3148 (DEMON), Sonderforschungsbereich 342 of the TU Munchen, ONR grant N00014-92-J-1974 and the Human Capital and Mobility Cooperation Network EXPRESS (Expressiveness of Languages for Concurrency). Contents

S. All local authors can be reached via e-mail at the address last-name@cs.unibo.it. Questions and comments should be addressed to tr-admin@cs.unibo.it. Recent Titles from the UBLCS Technical Report Series 97-4 An Algebra of Actors, M. Gaspari, G. Zavattaro, May 1997. 97-5 On the Turing Equivalence of Linda Coordination Primitives, N. Busi, R. Gorrieri, G. Zavattaro, May 1997 (Revised October 1998). 97-6 A Process Algebraic View of Linda Coordination Primitives, N. Busi, R. Gorrieri, G. Zavattaro, May 1997. 97-7 Validating a Software Architecture with respect to an Architectural Style, P. Ciancarini, W. Penzo, July 1997. 97-8 System Support for Partition-Aware Network Applications, O. Babaoglu, R. Davoli, A. Montresor, R. Segala, October 1997. 97-9 Generalized Semi-Markovian Process Algebra, M. Bravetti, M. Bernardo, R. Gorrieri, October 1997. 98-1 Group Communication in Partitionable Systems: Specification and Algorithms, O. Babao glu, R. Davoli, A. Montresor, April 1998. 98-2...

We consider a discrete event dynamic system (DEDS) obtained by the parallel composition of several subsystems. Each subsystem can be seen as a standard stochastic DEDS. The composed system is provided with true concurrency semantics that emphasize concurrent behaviors of the subsystems. For these semantics, a trajectory appears as a partial order of events. For simplicity, we focus on the case of a global system composed of only two subsystems. We assume that firings in each subsystem are collected by a local sensor, which yields a sequence of transition labels (or events). The objective is to recover the most likely global trajectory of the system from the two (asynchronous) sequences of observations. This is an almost standard hidden state estimation problem, amenable to the clasical Viterbi algorithm. We propose a solution in which this global trajectory is built recursively by two asynchronously cooperating "players, " each one being in charge of one subsystem. These two players run local Viterbi algorithms based on local states of the subsystems, plus some coordination information. This supervising architecture is particularly suited to large modular systems and is currently being applied to the distributed monitoring (and fault diagnosis) of telecommunication networks.

We study a new partial order semantics of Petri nets with read arcs, where read arcs model reading without consuming, which is often more adequate than the destructive-read-and-rewrite modelled in ordinary nets. As basic observations we take ST-traces, which are sequences of transition starts and ends. We define processes of our nets and derive two partial orders modelling causality and start precedence. These partial orders are related to observations and system states just as in the ordinary approach the single partial order of a process is related to firing sequences and reachable markings. Our approach also supports a new view of concurrency as captured by steps. 1 Introduction Describing the runs of a concurrent system by sequences of actions ignores the possible concurrency of these actions, which can be important e.g. for judging the temporal efficiency of the system. Alternatively to this so-called interleaving approach, one can take step sequences, where a step consists of si...

A testing scenario in the sense of De Nicola and Hennessy is developed to measure the worst-case efficiency of asynchronous systems using dense time. For all three variants considered, it is shown that one can equivalently use discrete time; in the discrete versions, one variant coincides with an approach based on discrete time in [Vog95b], and thus we can clarify the assumptions behind this approach. The resulting testing-preorders are characterized with some kind of refusal traces and shown to satisfy some properties that make them attractive as faster-than relations. The three testing-preorders are incomparable in general, but for some interesting classes of systems implications are shown. 1 Introduction In the testing approach of [DNH84], reactive systems are compared by embedding them -- with a parallel composition operator k -- in arbitrary test environments. One variant of testing (must-testing) considers the worst-case behaviour: a system N performs successfully in an environm...

In the domain of concurrent object-oriented programming (COOP) there is a strong need for a formalism to describe concurrency. Such a formalism is offered by Petri nets, which however lack thorough modularisation techniques for coping with models for large systems. An appropriate combination of COOP and Petri nets is required in order to profit from the strength of both methods. This paper gives an overview of ongoing research on PN-TOX, a paradigm and development environment for concurrent object-oriented programming based on Petri nets. First, we describe our object model. Each object has its own Petri net (encapsulated within the object) that models the concurrent behaviour of the object; these objects are called Petri net objects. Cooperation and synchronisation of Petri net objects can be modelled by separate objects describing their cooperation scenarios. Our ideas concerning (single and multiple) inheritance, as part of the integration of Petri nets within the object-oriented pa...