Showing papers on "Process architecture published in 2013"

Time and Petri Nets

Abstract: At first glance the concepts of time and of Petri nets are quite contrary: while time determines the occurrences of events in a system, classic Petri nets consider their causal relationships and they represent events as concurrent systems. But if we take a closer look at how time and causality are intertwined we realize that there are many possible ways in which time and Petri nets interact. This book takes a closer look at three time-dependent Petri nets: Time Petri nets, Timed Petri nets, and Petri nets with time windows. The author first explains classic Petri nets and their fundamental properties. Then the pivotal contribution of the book is the introduction of different algorithms that allow us to analyze time-dependent Petri nets. For Time Petri nets, the author presents an algorithm that proves the behavioral equivalence of a net where time is designed once with real and once with natural numbers, so we can reduce the state space and consider the integer states exclusively. For Timed Petri nets, the author introduces two time-dependent state equations, providing a sufficient condition for the non-reachability of states, and she also defines a local transformation for converting these nets into Time Petri nets. Finally, she shows that Petri nets with time-windows have the ability to realize every transition sequence fired in the net omitting time restrictions. These classes of time-dependent Petri nets show that time alone does not change the power of a Petri net, in fact time may or may not be used to force firing. For Time Petri nets and Timed Petri nets we can say that they are Turing-powerful, and thus more powerful than classic Petri nets, because there is a compulsion to fire at some point in time. By contrast, Petri nets with time-windows have no compulsion to fire, their expressiveness power is less than that of Turing-machines. This book derives from advanced lectures, and the text is supported throughout withexamples and exercises. It issuitable for graduate courses in computer science, mathematics, engineering, and related disciplines, and as a reference for researchers.

Dynamic Adaptive Fuzzy Petri Nets for Knowledge Representation and Reasoning

Abstract: Although a promising tool for knowledge representation and reasoning, fuzzy Petri nets (FPNs) still suffer from some deficiencies. First, the parameters in current FPN models, such as weight, threshold, and certainty factor do not accurately represent increasingly complex knowledge-based expert systems and do not capture the dynamic nature of fuzzy knowledge. Second, the fuzzy rules of most existing knowledge inference frameworks are static and cannot be adjusted dynamically according to variations of antecedent propositions. To address these problems, we present a new type of FPN model, dynamic adaptive fuzzy Petri nets, for knowledge representation and reasoning. We also propose a max-algebra based parallel reasoning algorithm so that the reasoning process can be implemented automatically. As illustrated by a numerical example, the proposed model can well represent the experts' diverse experience and can implement the knowledge reasoning dynamically.

Discovering Stochastic Petri Nets with Arbitrary Delay Distributions from Event Logs

Abstract: Capturing the performance of a system or business process as accurately as possible is important, as models enriched with performance information provide valuable input for analysis, operational support, and prediction. Due to their computationally nice properties, memoryless models such as exponentially distributed stochastic Petri nets have earned much attention in research and industry. However, there are cases when the memoryless property is clearly not able to capture process behavior, e.g., when dealing with fixed time-outs.

On Petri net implementation of proactive resilient holistic supply chain networks

Abstract: Holistic supply chain network (H-SCN) systems are a set of supply chains that have inter-dependence relations. Design and management of H-SCN systems are largely ignored in the literature. Modeling of H-SCN systems will provide a computational facility for design and management of H-SCN systems. A model for disruption of the H-SCN system was proposed by us before, which consists of two parts: a model for disruption of a single firm and a model for propagation of disruption. The present paper mainly discusses how these models can be implemented especially with Petri nets. Guidelines with templates of how the Petri net models for H-SCN systems can be constructed in industrial setting are also given to facilitate the use of our model. Since H-SCN systems are complex network systems, this paper may also contribute to the field of the Petri net technique for network systems modeling in the aspect of reduction of complexity arising from vast amounts of construct instances—a common problem that existed in a Petri net model. Indeed, the proposed Petri net model is suitable for any type of supply chains or supply chain networks.

Nonpure Petri Net Supervisors for Optimal Deadlock Control of Flexible Manufacturing Systems

Abstract: This paper illustrates that Petri nets with self-loops are more powerful than pure nets in modeling and control of flexible manufacturing systems. A self-loop in a Petri net cannot be mathematically represented by its incidence matrix. This paper presents a mathematical method to design a maximally permissive Petri net supervisor that is expressed by a set of control places with self-loops. A control place with a self-loop can be represented by a constraint and a self-loop associated with a transition whose firing may lead to an illegal marking. The constraint is designed to ensure that all legal markings are reachable and the self-loop is used to prevent the system from reaching illegal markings by disabling the transition at a specific marking. A marking-reduction approach is developed in order to cut down the considered markings, which can greatly decrease the computational overhead of the proposed method. An integer linear programming model is developed to compress the number of control places, aiming to reduce the structural complexity of the resulting supervisors. Finally, illustrative examples are used to validate the proposed method and to demonstrate that it can obtain an optimal supervisor for some cases that cannot be optimally controlled by pure net supervisors.

Interactive Petri Nets

Abstract: Such concurrent systems as Web services and workflow systems can be viewed as a composition of a set of subsystems. Subsystems interact with each other through a set of message channels in order to perform a task. This work defines a class of Petri nets called interactive Petri nets (IPNs) to model these systems. IPNs can be used to analyze their behavior, find potential problems, and then improve their designs. Compatibility is an important concept for a composed system and reflects the possibility of correct/proper interaction among its subsystems. In order to characterize different cooperative abilities in practice, compatibility and weak compatibility are defined for IPNs. Some relationships among (weak) compatibility, liveness, reversibility, and boundedness are revealed. Based on them, this work proves that the (weak) compatibility problem is co-NP-hard. A taxonomy is also presented for IPNs in order to explore whether some subclasses can be analyzed efficiently. Based on it, we can identify several IPN subclasses, some of which can be analyzed in polynomial time.

An Optimization Approach to Improved Petri Net Controller Design for Automated Manufacturing Systems

Abstract: Sensors and actuators are two indispensable parts in the paradigm of feedback control. Their implementation cost should be properly evaluated and constrained. In the previous work, a Petri net monitor with the least cost is synthesized through integer programming formulation. Despite its technical correctness, the existing method may lead to undesirable results when the net structure contains some shared or unshared resource places of a manufacturing-oriented net model. A necessary and sufficient condition is established to show that certain structures can lead to deadlock-prone supervisors. An efficient algorithm is developed to identify such structures. Furthermore, it is shown that if one can identify such structures at the initial stage, it is possible to achieve desirable controllers for the original systems. The theoretical correctness of the proposed algorithm is discussed. A manufacturing example is provided to illustrate the proposed approach.

A general divide and conquer approach for process mining

Abstract: Operational processes leave trails in the information systems supporting them. Such event data are the starting point for process mining - an emerging scientific discipline relating modeled and observed behavior. The relevance of process mining is increasing as more and more event data become available. The increasing volume of such data (“Big Data”) provides both opportunities and challenges for process mining. In this paper we focus on two particular types of process mining: process discovery (learning a process model from example behavior recorded in an event log) and conformance checking (diagnosing and quantifying discrepancies between observed behavior and modeled behavior). These tasks become challenging when there are hundreds or even thousands of different activities and millions of cases. Typically, process mining algorithms are linear in the number of cases and exponential in the number of different activities. This paper proposes a very general divide-and-conquer approach that decomposes the event log based on a partitioning of activities. Unlike existing approaches, this paper does not assume a particular process representation (e.g., Petri nets or BPMN) and allows for various decomposition strategies (e.g., SESE- or passage-based decomposition). Moreover, the generic divide-and-conquer approach reveals the core requirements for decomposing process discovery and conformance checking problems.

Discovering Petri Nets from Event Logs

Abstract: As information systems are becoming more and more intertwined with the operational processes they support, multitudes of events are recorded by today’s information systems. The goal of process mining is to use such event data to extract process related information, e.g., to automatically discover a process model by observing events recorded by some system or to check the conformance of a given model by comparing it with reality. In this article, we focus on process discovery, i.e., extracting a process model from an event log. We focus on Petri nets as a representation language, because of the concurrent and unstructured nature of real-life processes. The goal is to introduce several approaches to discover Petri nets from event data (notably the α-algorithm, state-based regions, and language-based regions). Moreover, important requirements for process discovery are discussed. For example, process mining is only meaningful if one can deal with incompleteness (only a fraction of all possible behavior is observed) and noise (one would like to abstract from infrequent random behavior). These requirements reveal significant challenges for future research in this domain.

Hybrid Liveness-Enforcing Policy for Generalized Petri Net Models of Flexible Manufacturing Systems

Abstract: This paper proposes a hybrid liveness-enforcing method for a class of Petri nets, which can well model many flexible manufacturing systems. The proposed method combines elementary siphons with a characteristic structure-based method to prevent deadlocks and enforce liveness to the net class under consideration. The characteristic structure-based method is further advanced in this work. It unveils and takes a full advantage of an intrinsically live structure of generalized Petri nets, which hides behind the arc weights, to achieve the liveness enforcement without any external control agent such as monitors. This hybrid method can identify and remove redundant monitors from a liveness-enforcing supervisor designed according to existing policies, improve the permissiveness, reduce the structural complexity of a controlled system, and consequently save the control implementation cost. Several examples are used to illustrate this method.

Design of Optimal Monitor-Based Supervisors for a Class of Petri Nets With Uncontrollable Transitions

Abstract: For a class of Petri nets whose uncontrollable influence subnets are forward synchronization and backward conflict-free nets, this paper proposes an algorithm to transform a given generalized mutual exclusion constraint (GMEC) into an optimal admissible one. Based on the proposed constraint transformation, a method is given to synthesize an optimal monitor-based supervisors. The proposed method utilizes an optimal monitor-based supervisor to enforce a GMEC containing infinite elements, which has not been seen in any prior research. Moreover, it has higher computational efficiency at the expense of application scope than some existing methods. An example is used to illustrate the application of the proposed method.

Mixing paradigms for more comprehensible models

Abstract: Petri nets efficiently model both data- and control-flow. Control-flow is either modeled explicitly as flow of a specific kind of data, or implicit based on the data-flow. Explicit modeling of control-flow is useful for well-known and highly structured processes, but may make modeling of abstract features of models, or processes which are highly dynamic, overly complex. Declarative modeling, such as is supported by Declare and DCR graphs, focus on control-flow, but does not specify it explicitly; instead specifications come in the form of constraints on the order or appearance of tasks. In this paper we propose a combination of the two, using colored Petri nets instead of plain Petri nets to provide full data support. The combined approach makes it possible to add a focus on data to declarative languages, and to remove focus from the explicit control-flow from Petri nets for dynamic or abstract processes. In addition to enriching both procedural processes in the form of Petri nets and declarative processes, we also support a flow from modeling only abstract data- and control-flow of a model towards a more explicit control-flow model if so desired. We define our combined approach, and provide considerations necessary for enactment. Our approach has been implemented in CPN Tools 4.

A New Modified Reachability Tree Approach and Its Applications to Unbounded Petri Nets

Abstract: This paper proposes a new modified reachability tree (NMRT) approach for a class of unbounded generalized Petri nets called ω-independent nets. The NMRT of an ω-independent net consists of all and only reachable markings from its initial marking. Moreover, the applications of the NMRT to deadlock analysis for ω-independent nets are developed. The proposed method has a larger application scope than all the existing methods. Several examples are provided to show its superiority over them.

Concurrency bugs in multithreaded software: modeling and analysis using Petri nets

Abstract: In this paper, we apply discrete-event system techniques to model and analyze the execution of concurrent software. The problem of interest is deadlock avoidance in shared-memory multithreaded programs. We employ Petri nets to systematically model multithreaded programs with lock acquisition and release operations. We define a new class of Petri nets, called Gadara nets, that arises from this modeling process. We investigate a set of important properties of Gadara nets, such as liveness, reversibility, and linear separability. We propose efficient algorithms for the verification of liveness of Gadara nets, and report experimental results on their performance. We also present modeling examples of real-world programs. The results in this paper lay the foundations for the development of effective control synthesis algorithms for Gadara nets.

A New Class of Fuzzy Petri Nets for Knowledge Representation and Reasoning

Abstract: This paper presents a new class of Petri nets called generalised fuzzy Petri nets. The new class extends the existing fuzzy Petri nets by introducing three operators in the form of triangular norms, which are supposed to function as substitute for the min, max and * algebraic product operators. To demonstrate the power and the usefulness of this model, an application of the generalised fuzzy Petri nets in the domain of train traffic control is provided. The new model is more flexible than the classical one as in the former class the user has the chance to define the input/output operators. The proposed approach can be used for knowledge representation and reasoning in decision support systems.

Colouring space - a coloured framework for spatial modelling in systems biology

Abstract: In this paper we introduce a technique to encode spatial attributes of dynamic systems using coloured Petri nets and show how it can be applied to biological systems within the spirit of BioModel Engineering. Our approach can be equally applied to qualitative, stochastic, continuous or hybrid models of the same physical system, and can be used as the basis for multiscale modelling. We illustrate our approach with two case studies, one from the continuous and one from the stochastic paradigm. In this paper we only discuss the case of finite colours, and by unfolding our method can take advantage of all the analytical machinery and simulation techniques that have been developed for the uncoloured family of Petri net classes.

Supervisory Control of Petri Nets with Language Specifications

Abstract: In this chapter we discuss how Petri nets can be used in the framework of supervisory control theory. A discrete event system is defined in such a theory as a language generator: this motivates the need to start the chapter with a short but self-contained introduction to Petri net languages. We consider the monolithic supervisory design that requires to construct the concurrent composition of the plant with the specification, to check this structure for controllability and nonblockingness, and eventually to refine it. We show how Petri nets can be used within this approach and show that while the procedure can always be applied to bounded nets, in the case of unbounded Petri nets it may not be possible to obtain a Petri net supervisor.

Decentralized Diagnosis of Discrete-Event Systems Using Labeled Petri Nets

Abstract: In this paper, we propose an approach to the diagnosis of Petri nets in a decentralized setting that combines the decentralized scheme for automata presented by Debouk with the diagnosis approach for Petri nets based on the notion of basis markings and justifications presented by some of the authors of this paper. The decentralized architecture that we use is composed of a set of sites communicating their diagnosis information with a coordinator that is responsible for detecting the occurrence of failures in the system. In particular, we define three protocols that differ in the amount of information exchanged between the local sites and the coordinator and the rules adopted by the coordinator to compute the global diagnosis states. Finally, we prove that, as in the case of automata, diagnosability is strictly related to the presence of failure ambiguous strings.

An Empirical Investigation on the Design of Process Architectures

Abstract: Large-scale enterprises struggle with an effective alignment of busi- ness processes and IT services with business strategy. While process models play an important role for bridging between strategy and IT, there is a need to systematically organize the huge number of models. Process architecture de- fines an overarching structure for the organization of processes. However, there is a notable research gap on how process architectures are designed in practice. In this paper we address this problem by integrating insights and approaches from practice. We use Grounded Theory to analyze eleven in-depth interviews we conducted. Further, we present findings from studying documents provided by the interviewees. Our contribution is a conceptual framework about process architecture design, along with a classification of process architecture arche- types found in practice. Our results have strong implications since they demon- strate that process architecture design is more complex and context-dependent than assumed.

Optimal Liveness-Enforcing Control for a Class of Petri Nets Arising in Multithreaded Software

Abstract: We investigate the synthesis of optimal liveness-enforcing control policies for Gadara nets, a special class of Petri nets that arises in the modeling of the execution of multithreaded computer programs for the purpose of deadlock avoidance. We consider maximal permissiveness as the notion of optimality. Deadlock-freeness of a multithreaded program corresponds to liveness of its Gadara net model. We present a new control synthesis algorithm for liveness enforcement of Gadara nets that need not be ordinary. The algorithm employs structural analysis of the net and synthesizes monitor places to prevent the formation of a special class of siphons, termed resource-induced deadly-marked siphons. The algorithm also accounts for uncontrollable transitions in the net in a minimally restrictive manner. The algorithm is generally an iterative process and converges in a finite number of iterations. It exploits a covering of the unsafe states that is updated at each iteration. The proposed algorithm is shown to be correct and maximally permissive with respect to the goal of liveness enforcement.

Modeling and Analyzing Self-Adaptive Systems with Context Petri Nets

Abstract: The development of self-adaptive systems requires the definition of the parts of the system that will be adapted, when such adaptations will take place, and how these adaptations will interact with each other. However, foreseeing all possible adaptations and their interactions is a difficult task, opening the possibility to inconsistencies or erroneous system behavior. To avoid inconsistencies, self adaptive systems require a sound programming model that allows to reason about the correctness of the system in spite of its dynamic reconfigurations. This paper presents context Petri nets, a Petri net-based programming model for self-adaptive systems. This model provides a formal definition of adaptations and their interaction, as well as a consistent process for their inclusion in the system. Besides serving as an underlying run-time model to ensure that adaptations and their constraints remain consistent, context Petri nets allow to analyze properties such as reachability, and liveness in the configuration of self-adaptive systems. Context Petri nets thus are a convenient tool to model and analyze the dynamics of self-adaptive systems, both formally and computationally.

Automatic synthesis of VHDL hardware components from IOPT Petri net models

Abstract: The IOPT Petri net class, designed to implement embedded system controllers, combining the well known properties of Petri nets with input and output functions, provides a set of capabilities suitable to specify digital controller systems. This paper presents an automatic code generation tool used to create VHDL controller implementations of IOPT models and describes the underlying code generation strategies. The new tool is integrated under the Web based IOPT-Tools framework (accessible at http://gres.uninova.pt), containing a complete tool-chain ranging from interactive model design and edition, model checking and simulation tools to automatic code generation, allowing the design of embedded system controllers using only graphical tools and without the need to manually write any hardware description or software code.

The optimization problem based on alternatives aggregation Petri nets as models for industrial discrete event systems

Abstract: The construction, set-up and operation of many systems of interest in sectors such as industry, supply chains and communications are complex processes, which may require significant investment of resources. For this reason, the automation of the decision making for achieving the best design and operation of such systems, which may be regarded as discrete event systems (DESs), constitutes an active research field. In this paper, we present a methodology to cope with this process in an efficient way, optimizing not only the behaviour of the DES but also its structure. This kind of problem is usually associated with the so-called combinatorial explosion, since the number of alternative configurations for the DES might be huge. We present an improved algorithm to transform a set of alternative Petri nets, representing alternative structural configurations, into a more compact model called an alternatives aggregation Petri net. In real decision-making problems, where the different alternative structural configurations may share common subnets, this compact model may allow the development of a much more efficient optimization problem than the classic approach of 'divide and conquer'. The achievement of this objective is performed by developing a single and compact model for all of the alternative structural configurations of the DES and the simulation of the most promising of them. In this paper, the mentioned methodology is introduced and its advantages and drawbacks are described in relation with the classic approach.

Survivability Evaluation of Fluid Critical Infrastructures Using Hybrid Petri Nets

Abstract: In this paper we propose a formal, model-checking based procedure to evaluate the survivability of fluid critical infrastructures. To do so, we introduce the Stochastic Time Logic (STL), which allows to precisely express intricate state-based and until-based properties for an important class of hybrid Petri nets. We present an efficient model checking procedure which recursively traverses the underlying state-space of the hybrid Petri net model, and identifies those regions (subsets of the discrete-continuous state space) that satisfy STL formulae. A case study studying the survivability of a water refinery and distribution plant shows the feasibility of our approach.

Synchronized Petri Net: A Formal Specification Model for Multi Agent Systems

Abstract: This paper proposes a formal model for specifying multi agent systems, named SyPN (for Synchronized Petri Net). This model allows the specification of various kinds of agent-based systems’ behaviors, such as individual and collective behaviors. SyPN is an extension of Recursive Petri net allowing synchronization of several nets. In fact, SyPN borrows the specification of dynamic processes from Recursive Petri net and introduces several valuable concepts that enable concise multi agent system specifications, such as: typed places, transitions and tokens, synchronization points, synchronization condition, synchronization relation and binding function. We illustrate our approach by two case studies of remote interactions between agents.

Structure Theory of Petri Nets

Abstract: The aim of this tutorial is to give a concise, but nonetheless not too narrow, overview of definitions and results pertaining centrally to Petri net structure theory. The Petri net model considered in these notes are the classical place/transition nets, as they have been defined in the First Advanced Course held in 1979 and originate back to the late Sixties. Structure theory asks what behavioural properties of a Petri net can be derived from its structural properties. Other aspects of Petri nets are neglected to a large extent in the present notes, such as various extensions and generalisations of central notions and results, as well as almost all algorithmic and complexity-theoretic consequences that accompany the structure-theoretic results. Because full proofs can easily be retrieved from the literature, they are not given, unless they are small and perhaps somewhat characteristic for Petri net oriented reasoning. Proof ideas are often sketched, however, and the sharpness of various results is accentuated by means of examples and counterexamples. A list for further reading is also provided.

Performance optimization for a class of Generalized Stochastic Petri nets

Abstract: This paper considers the problem of optimizing the (long-term) performance of operations that are modeled by Generalized Stochastic Petri nets. The proposed methodology employs the representational power of the GSPN framework in order to articulate an explicit trade-off between the computational tractability of the formulated problem and the operational efficiency of the derived solutions. On the other hand, the solution of the considered formulations is based on recent results regarding the sensitivity analysis of Markov reward processes. A more expansive treatment of the presented results, together with a case study that highlights the relevance of the considered problem and the efficacy of the proposed methodology, can be found in a companion document that is accessible from the website of the second author.

Scheduling and control of real-time systems based on a token player approach

Abstract: Petri nets are a powerful formalism for the specification and verification of concurrent systems, such as sequential systems and manufacturing systems. To deal with real-time systems whose time issues become essential, different extensions of Petri nets with time have been proposed in the literature. In this paper, a new scheduling and control technique for real-time systems modeled by ordinary P-time Petri nets is proposed. Its goal is to provide a scheduling for a particular firing sequence, without any violation of timing constraints ensuring that no deadline is missed. It is based on the firing instant notion and it consists in determining an inequality system generated for a possible evolution (in terms of a feasible firing sequence for the untimed underlying Petri net) of the model. This system can be used to check reachability problems as well as evaluating the performances of the model considered and determining the associated control for a definite functioning mode and it introduces partial order on the execution of particular events.