G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Computer Science Department, University of Pisa, 1994.  Home/Search   Document Not in Database   Summary   Related Articles   Check

.... to nets with read arcs, like those concerning non sequential processes [22, 28] and event structures [1] Moreover, these nets naturally model read write access to shared memory, where readers are allowed to progress in parallel, with applications to transaction serializability in databases [25, 10], concurrent constraint programming [21, 3] asynchronous systems [27] and process algebras [20] Research partly supported by the EC TMR Network GETGRATS (General Theory of Graph Transformation Systems) through the Technical University of Berlin and the University of Pisa; by the Office of ....

....defined algebra of processes constitutes such a category. Match share categories are then introduced. In Section 4 a lluf and faithful functor is defined from the category of processes of a net N to the match share category CP(N ) generated from N . 2 Contextual Nets Processes Introduced in [25], along the line of the work for C E systems [22] contextual place transition nets are an extension of classical place transitions (P T) nets, in which a new relation, called context relation, is defined. A read arc between a place s and a transition t means that at least one token in s is needed ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, University of Pisa - Department of Computer Science, 1994.

....represent context conditions. The ability of faithfully representing the reading of resources allows contextual nets to model many concrete situations more naturally than classical nets. In recent years they have been used to model concurrent access to shared data (e.g. reading in a database) [34], to provide concurrent semantics to concurrent constraint (CC) programs [27] to model priorities [19] and to specify a net semantics for the calculus [8] Moreover they have been studied for their connections with another powerful formalism for the speci cation of concurrent computations, ....

....context conditions. A transition can re if enough tokens are present in its preconditions and context conditions. In the ring, preconditions are consumed, context conditions remains unchanged and new tokens are generated in the postconditions. This section introduces (marked) contextual P T nets [34] (or c nets for short) that following the lines suggested in [28] for C E systems, add contexts to classical P T nets. To give the de nition of c net we need some notation for multisets. Let A be a set. A multiset of A is a function M : A N. Such a multiset will be denoted sometimes as a formal ....

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G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Universita di Pisa, 1994.

....readings. On the contrary, with contextual nets, besides pre and post sets, transitions also have contexts , that is resources that are necessary for the enabling but are not affected by the firing. Contextual nets have found applications e.g. to transaction serializability in databases [30], concurrent constraint programming [26] and asynchronous systems [34] Independently of CTph and ITph, for contextual nets several different approaches have been proposed that differ for the way in which contexts are read. For example, let us consider the nets N 1 , N 2 and N 3 in Figure 1, ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. Ph.D. thesis, TD-5/94, Dipartimento di Informatica, Universita di Pisa, 1994.

....has a great impact on the resulting notion of transaction. Operational semantics under the ITph. In the ITph, a marking can be thought of as an indexed (over the places) collection of ordered sequences of tokens and each ring must exactly specify which tokens are consumed. In [14] inspired by [42], we presented a stack based approach to the implementation of ITph states. The idea was to choose a canonical interpretation of the tokens that have to be consumed and produced in a ring and to introduce permutation rings with the task of rearranging ordered tokens: A marking is represented as ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. Ph.D. thesis, Computer Science Department, University of Pisa, 1994.

....of concurrent readings. On the contrary, with contextual nets, besides pre and post sets transitions also have contexts , that is resources that are necessary for the enabling, but not a ected by the ring. Contextual nets have found applications e.g. to transaction serializability in databases [16], concurrent constraint programming [12] and asynchronous systems [20] The extensive use of pt nets has given rise to di erent schools of thought concerning their semantic interpretation. In particular, the main distinction is drawn between collective and individual token philosophies (see e.g. ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. Ph.D. thesis, Dipartimento di Informatica, Universita di Pisa, 1994.

....resources (tokens) which are not a ected by the ring of the transition. Read arcs are able to faithfully represent the situations where a resource is read but not consumed (read only accesses) They have been used to model concurrent accesses to shared data (e.g. read operations in a database) [30, 13], to study temporal eciency in asynchronous systems [32] and to give a truly concurrent semantics to concurrent constraint programs [23, 7] Inhibitor arcs have been introduced in [2] to solve a synchronization problem not expressible in classical Petri nets. A study of the expressiveness of ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Department of Computer Science - University of Pisa, 1994.

....unchanged and produces its postconditions. The possibility of faithfully representing a read only access to resources allows contextual nets to model many concrete situations more naturally than classical nets. In recent years they have been used to model concurrent accesses to shared data [16], to provide concurrent semantics to concurrent constraint (CC) programs [13] and to model priorities [8] Several concurrent semantics for contextual nets based on processes and event structures have been de ned in the literature [9, 21, 7, 1] Relying on such concurrent descriptions of ....

....does not suggest an immediate way of checking RHP bisimulation. The decidability of RHP bisimulation on contextual nets is left as an open question. 2 Contextual nets, processes and asymmetric event structures In this section, after introducing the basics of marked contextual P T nets [16], we present the notions of deterministic occurrence contextual net and process. Then we brie y review prime event structures, and asymmetric event structures as de ned in [1] 2.1 Contextual nets We rst recall some notation for multisets. Let A be a set; a multiset of A is a function M : A ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Department of Computer Science - University of Pisa, 1994.

....produced token or an idle token of the original marking. Using multisets instead of ordered sequences would make it impossible to recognize which token was produced by which firing, as it happens for the CTph. The stack based approach. The approach we propose is very similar to the one adopted in [27]: We choose a canonical interpretation of the tokens that have to be consumed and produced in a firing and we introduce permutation firings with the task of rearranging the orderings of the indexed sequences of tokens. A marking is represented as a collection of stacks, one for each place. ....

G. Ristori, Modelling Systems with Shared Resources via Petri Nets, PhD thesis TD-94-05, Department of Computer Science, University of Pisa (1994).

....token or an idle token of the original marking. Using multisets instead of ordered sequences would make it impossible to recognize which token was produced by which firing, as it happens for the CTph. The Stacks Based Approach. The approach we propose is very similar to the one adopted in Ristori (1994): we choose a canonical interpretation of the tokens that are to be consumed and produced in a firing and we introduce permutation firings with the task of rearranging the orderings of the indexed sequences of tokens. Therefore a marking is represented as a collection of stacks, one for each ....

Ristori, G. (1994), Modelling Systems with Shared Resources via Petri Nets. PhD thesis TD 05/94, Department of Computer Science, University of Pisa.

.... connection structure, such as calculus [7] based on P T nets with inhibitor arcs (i.e. testing for absence) 2] Unfortunately, the problem of giving a true concurrent semantics to contextual P T nets has not been deeply studied in the literature: only some contrasting proposals are present [6, 8, 10], where to some extent positive and negative contexts contribute to the causality relation. Our approach to contextual nets has the advantage of keeping separate the causality relation from the testing of the contexts and it works as follows: to deal with inhibitor arcs, we need to extend the ....

....is finer than step bisimulation (as defined in [8] where a multiset of events can happen in the same step iff they can happen in either order. Related Works To the best of our knowledge, the only other works that deal with noninterleaving semantics for contextual nets are [6] 8] and [10]. In [6] Janicki and Koutny propose a step semantics for nets with inhibitor arcs that differs from the one we use in the fact that they permit the execution in the same step of two transitions that cannot happen in any order, namely, if a transition produces a token in a place inhibiting another ....

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G. Ristori, "Modelling Systems with Shared Resources via Petri Nets", Ph.D. Thesis, Universit`a di Pisa, TD-5/94, 1994.

.... interest in contextual nets, i.e. nets that are able to test for presence and absence of tokens, besides the usual flow relation, has grown, mainly due to the use of contextual nets to give a net semantics to the calculus ( 8] studied in [2] or to model the concurrent access to shared data [14, 4], or to their relation with constraint programming [9] or to model priorities [7] Quite clearly contextual Petri nets give a faithful and simpler representation of real systems, for instance timeouts can be modeled just requiring that the various activities (that may be independent to each ....

....as follows: m 0 is a step firing sequence; if m 0 [G 1 im 1 : Gn Gamma1 imn Gamma1 is a step firing sequence and mn Gamma1 [Gn imn then m 0 [G 1 im 1 : Gn Gamma1 imn Gamma1 [Gn imn is a step firing sequence. Our definition of contextual P T net is an extension of the ones in [9, 14]. Definition4. A contextual place transition net is a tuple (S; T; F; I; C; W; m 0 ) where (S; T; F; W;m 0 ) is a place transition net; C S Theta T is the contextual relation; I S Theta T is the inhibiting relation; such that (C [ I) F = and I C = 2 The structural ....

G. Ristori, "Modelling Systems with Shared Resources via Petri Nets", Ph.D. Thesis, Universit`a di Pisa, TD-5/94, 1994.

....activities) In the CTph approach the two firings are always concurrent, since the initial marking enables both t 1 and t 4 , i.e. the execution of t 4 does not modify the enabling condition of t 1 . The Stacks Based Approach. The approach we propose is very similar to the one adopted in [19]: we choose a canonical interpretation of the tokens that are to be consumed and produced in a firing and we introduce permutation firings with the task of rearranging the orderings of the indexed sequences of tokens. A marking becomes a collection of stacks, one for each place, that can be ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets . PhD thesis TD 05/94, Department of Computer Science, University of Pisa, 1994.

....read arcs in [18, 19] The possibility of faithfully representing the reading of resources allows contextual nets to model a lot of concrete situations more naturally than classical nets. In recent years they have been used to model concurrent access to shared data (e.g. reading in a database) [17, 7], to provide a concurrent semantics to concurrent constraint (CC) programs [13] to model priorities [9] to specify a net semantics for the calculus [3] Moreover they have been studied for their connections with another powerful formalism for the representation of concurrent computations, ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Universit`a di Pisa, 1994.

....coincides with their schema semantics, i.e. that two firings yield the same result for all possible interpretations of the net if and only if they correspond to the same process. This same treatment and results have been extended to contextual nets in [FMR94] and for contextual P T nets in [Ris94], where such notion of nets have been defined based on the development of this paper. This line of research based on nets with contexts (either C E or P T) is very important, since it allows to give a suitable theory to most models of computation based on a distributed memory. In particular, it ....

G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, University of Pisa, TD-5/94, 1994.

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G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Computer Science Department, University of Pisa, 1994.

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G. Ristori. Modelling Systems with Shared Resources via Petri Nets. PhD thesis, Computer Science Department, University of Pisa, 1994.

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