Process versus unfolding semantics for Place/Transition Petri nets

Place/transition (PT) Petri nets are one of the most widely used models of concurrency. However, they still lack, in our view, a satisfactory semantics: on the one hand the ‘token game’ is too intensional, even in its more abstract interpretations in terms of nonsequential processes and monoidal categories; on the other hand, Winskel's basic unfolding construction, which provides a coreflection between nets and finitary prime algebraic domains, works only for safe nets. In this paper we extend Winskel's result to PT nets. We start with a rather general category PTNets of PT nets, we introduce a category DecOcc of decorated (nondeterministic) occurrence nets and we define adjunctions between PTNets and DecOcc and between DecOcc and Occ, the category of occurrence nets. The role of DecOcc is to provide natural unfoldings for PT nets, i.e., acyclic safe nets where a notion of family is used to relate multiple instances of the same place. The unfolding functor from PTNets to Occ reduces to Winskel's when restricted to safe nets. Moreover, the standard coreflection between Occ and Dom, the category of finitary prime algebraic domains, when composed with the unfolding functor above, determines a chain of adjunctions between PTNets and Dom.

We introduce infinitary partial languages for Petri nets and study some of their properties. Earlier results on the interrelation between various Petri net semantics for finite behaviour are shown to be valid for infinite behaviour. As a consequence step sequences and event structures of processes both can be taken as a semantics for Petri nets without any loss of information.Via the introduction of infinite partial words it is possible to express the properties of justice and fairness in a natural way: justice can be characterized as maximality of progress and fairness as ‘fair’ conflict resolution. By this we overcome the ambiguity arising from the lack of discrimination between concurrency and non-determinism in the sequential approach.

In order to model the behaviour of open concurrent systems by means of Petri nets, we introduce open Petri nets, a generalisation of the ordinary model where some places, designated as open, represent an interface between the system and the environment. Besides generalising the token game to reflect this extension, we define a truly concurrent semantics for open nets by extending the Goltz–Reisig process semantics of Petri nets. We introduce a composition operation over open nets, characterised as a pushout in the corresponding category, suitable for modelling both interaction through open places and synchronisation of transitions. The deterministic process semantics is shown to be compositional with respect to such a composition operation. If a net $Z_3$ results as the composition of two nets $Z_1$ and $Z_2$, having a common subnet $Z_0$, then any two deterministic processes of $Z_1$ and $Z_2$ that ‘agree’ on the common part, can be ‘amalgamated’ to produce a deterministic process of $Z_3$. Conversely, any deterministic process of $Z_3$ can be decomposed into processes of the component nets. The amalgamation and decomposition operations are shown to be inverse to each other, leading to a bijective correspondence between the deterministic processes of $Z_3$ and the pair of deterministic processes of $Z_1$ and $Z_2$ that agree on the common subnet $Z_0$. Technically, our result is similar to the amalgamation theorem for data-types in the framework of algebraic specification. A possible application field of the proposed constructions and results is the modelling of interorganisational workflows, recently studied in the literature. This is illustrated by a running example.

Model checking Petri nets with MSVL

Understanding biological pathways is an important activity in the biological domain for drug development. Due to the parallelism and complexity inherent in pathways, computer models that can answer queries about pathways are needed. A researcher may ask `what-if' questions comparing alternate scenarios, that require deeper understanding of the underlying model. In this paper, we present overview of such a system we developed and an English-like high level language to express pathways and queries. Our language is inspired by high level action and query languages and it uses Petri Net execution semantics.

We introduce the notion of <em>strong concatenable process</em> for Petri nets as the least refinement of non-sequential (concatenable) processes which can be expressed abstractly by means of a <em> functor</em> Q[_] from the category of Petri nets to an appropriate category of symmetric strict monoidal categories with free non-commutative monoids of objects, in the precise sense that, for each net N, the strong concatenable processes of N are isomorphic to the arrows of Q[N]. This yields an axiomatization of the causal behaviour of Petri nets in terms of symmetric strict monoidal categories.<br /> <br />In addition, we identify a <em>coreflection</em> right adjoint to Q[_] and we characterize its replete image in the category of symmetric monoidal categories, thus yielding an abstract description of the category of net computations.

Efficient State Space Search for Time Petri Nets

In the paper, a "truly concurrent" and nondeterministic semantics is defined in terms of branching processes of discrete-time Petri nets (DTPNs). These nets may involve infinite numbers of transitions and places, infinite number of tokens in places, and (maximal) steps of concurrent transitions, which allows us to consider this class of DTPNs to be the most powerful class of Petri nets. It is proved that the unfolding (maximal branching process) of the DTPN is the greatest element of a complete lattice constructed on branching processes of DTPNs with step semantics. Moreover, it is shown that this result is true also in the case of maximal transition steps if additional restrictions are imposed on the structure and behavior of the DTPN.

We propose rewriting logic as a unifying framework for a wide range of nets models. We treat in detail place/transition nets and important extensions of the basic model by individual tokens, test arcs, and time. Based on the idea that Petri nets are monoids suggested by Meseguer and Montanari we define a rewriting semantics that maps place/transition nets into rewriting logic specifications. We furthermore generalize this result to a general form of algebraic net specifications subsuming also colored nets as a special case. The soundness and completeness results we state relate the commutative process semantics of nets proposed by Best and Devillers to the model-theoretic semantics of rewriting logic in the sense of natural isomorphisms between suitable functors. In addition we show how place/transition nets with test arcs and timed nets can be equipped with a rewriting semantics and discuss how other extensions can be treated along similar lines. Beyond the conceptual unification of quite different kinds of nets within a single framework, the rewriting semantics can provide a guide for future extensions of nets and help to cope with the growing diversity of models in this field. On the practical side, a major application of the rewriting semantics is its use as a logical and operational representation of net models for formal verification and for the efficient execution and analysis using a rewriting engine such as Maude, which also allows us to specify different execution and analysis strategies in the same rewriting logic language by means of reflection.

We shall describe connections between Petri nets, quantum physics and category theory. The view of Net theory as a kind of discrete physics has been consistently emphasized by Carl-Adam Petri. The connections between Petri nets and monoidal categories were illuminated in pioneering work by Ugo Montanari and Jose Meseguer. Recent work by the author and Bob Coecke has shown how monoidal categories with certain additional structure (dagger compactness) can be used as the setting for an effective axiomatization of quantum mechanics, with striking applications to quantum information. This additional structure matches the extension of the Montanari-Meseguer approach by Marti-Oliet and Meseguer, motivated by linear logic.

Specification and properties of a cache coherence protocol model.- GAMERU: A language for the analysis and design of human communication pragmatics within organizational systems.- Recent results on the complexity of problems related to Petri nets.- Algebraic structure of flows of a regular coloured net.- On equivalent execution semantics of concurrent systems.- Mathematical methods for calculating invariants in Petri nets.- On Petri nets with deterministic and exponentially distributed firing times.- An accurate performance model of CSMA/CD bus LAN.- Petri net models for the ?-calculus.- Fair processes.- Operational Petri net semantics for CCSP.- Infinitary languages of Petri nets and logical sentences.- A strong part of concurrency.- Concurrency and continuity.- Parallel systems specifications with coloured Petri nets and algebraic specifications.- Bibliography of Petri nets.

In the area of specification and modelling of concurrent systems, Petri nets have become a standard tool, and they still work behind the scenes in tools for graph transformation systems (cf. [1]). Moreover there is still potential for crossfertilization between the graph transformation and Petri net community. Even a better understanding of adhesive categories [2] and the related concepts of [3] seems possible in light of the proposed notion of weakly adhesive categories [4], which has emerged during work on the generalization of the co-reflective semantics of Petri nets to the realm of graph transformation and adhesive rewriting systems.KeywordsType ObjectGraph TransformationConcurrent SystemGraph GrammarGraph Transformation SystemThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

In order to model the behaviour of open concurrent systems by means of Petri nets, we introduce open Petri nets, a generalization of the ordinary model where some places, designated as open, represent an interface of the system towards the environment. Besides generalizing the token game to reflect this extension, we define a truly concurrent semantics for open nets by extending the Goltz-Reisig process semantics of Petri nets. We introduce a composition operation over open nets, characterized as a pushout in the corresponding category, suitable to model both interaction through open places and synchronization of transitions. The process semantics is shown to be compositional with respect to such composition operation. Technically, our result is similar to the amalgamation theorem for data-types in the framework of algebraic specifications. A possible application field of the proposed constructions and results is the modeling of interorganizational workflows, recently studied in the literature. This is illustrated by a running example.

Signal Transition Graphs (STGs) are Petri nets, which were introduced to represent a behavior of asynchronous circuits. To derive logic functions from an STG, the reachability graph should be constructed. In the verification of STGs some method based on Occurrence nets (OCN) and its prefix, called unfolding, has been proposed. OCNs can represent both causality and concurrency between two nodes by net structure. In this paper, we propose a method to derive a logic function by generating substate space of a given STG using the structural properties of OCN. The proposed method can be seen as a parallel algorithm for deriving a logic function.

One goal of applied category theory is to understand open systems. We compare two ways of describing open systems as cospans equipped with extra data. First, given a functor L:A→X, a `structured cospan' is a diagram in X of the form L(a)→x←L(b). We give a new proof that if A and X have finite colimits and L preserves them, there is a symmetric monoidal double category whose objects are those of A and whose horizontal 1-cells are structured cospans. Second, given a pseudofunctor F:A→Cat, a `decorated cospan' is a diagram in A of the form a→m←b together with an object of F(m). Generalizing the work of Fong, we show that if A has finite colimits and F:(A,+)→(Cat,×) is symmetric lax monoidal, there is a symmetric monoidal double category whose objects are those of A and whose horizontal 1-cells are decorated cospans. We prove that under certain conditions, these two constructions become isomorphic when we take X=∫F to be the Grothendieck category of F. We illustrate these ideas with applications to electrical circuits, Petri nets, dynamical systems and epidemiological modeling.