Stefan Haar

• PhD
• INRIA - Institut National de Recherche en Informatique et en Automatique at National Institute for Research in Computer Science and Control

Unfoldings are a well known partial-order semantics of P/T Petri nets that can be applied to various model checking or verification problems. For high-level Petri nets, the so-called symbolic unfolding generalizes this notion. A complete finite prefix of a P/T Petri net’s unfolding contains all information to verify, e.g., reachability of markings....

A crucial question in analyzing a concurrent system is to determine its long-run behaviour, and in particular, whether there are irreversible choices in its evolution, leading into parts of the reachability space from which there is no return to other parts. Casting this problem in the unifying framework of safe Petri nets, our previous work has pr...

Unfoldings are a well known partial-order semantics of P/T Petri nets that can be applied to various model checking or verification problems. For high-level Petri nets, the so-called symbolic unfolding generalizes this notion. A complete finite prefix of the unfolding of a P/T Petri net contains all information to verify, e.g., reachability of mark...

A crucial question in analyzing a concurrent system is to determine its long-run behaviour, and in particular, whether there are irreversible choices in its evolution, leading into parts of the reachability space from which there is no return to other parts. Casting this problem in the unifying framework of safe Petri nets, our previous work has pr...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Attractors of network dynamics represent the long-term behaviours of the modelled system. Understanding the basin of an attractor, comprising all those states from which the evolution will eventually lead into that attractor, is therefore crucial for understanding the response and differentiation capabilities of a dynamical system. Building on our...

Predicting biological systems’ behaviors requires taking into account many molecular and genetic elements for which limited information is available past a global knowledge of their pairwise interactions. Logical modeling, notably with Boolean Networks (BNs), is a well-established approach that enables reasoning on the qualitative dynamics of netwo...

Predicting the behaviors of complex biological systems, underpinning processes such as cellular differentiation, requires taking into account many molecular and genetic elements for which limited information is available past a global knowledge of their pairwise interactions. Logical modeling, notably with Boolean Networks (BNs), is a well-establis...

Boolean networks (BNs) are widely used to model the qualitative dynamics of biological systems. Besides the logical rules determining the evolution of each component with respect to the state of its regulators, the scheduling of component updates can have a dramatic impact on the predicted behaviours. In this paper, we explore the use of Read (cont...

We address the sequential reprogramming of gene regulatory networks modelled as Boolean networks. We develop an attractor-based sequential reprogramming method to compute all sequential reprogramming paths from a source attractor to a target attractor, where only attractors of the network are used as intermediates. Our method is more practical than...

Boolean networks (BNs) are widely used to model the qualitative dynamics of biological systems. Besides the logical rules determining the evolution of each component with respect to the state of its regulators, the scheduling of component updates can have a dramatic impact on the predicted behaviours. In this paper, we explore the use of Read (cont...

Cellular reprogramming, a technique that opens huge opportunities in modern and regenerative medicine, heavily relies on identifying key genes to perturb. Most of the existing computational methods for controlling which attractor (steady state) the cell will reach focus on finding mutations to apply to the initial state. However, it has been shown,...

Parametric models abstract part of the specification of dynamical models by integral parameters. They are for example used in computational systems biology, notably with parametric regulatory networks, which specify the global architecture (interactions) of the networks, while parameterising the precise rules for drawing the possible temporal evolu...

Parametric models abstract part of the specification of dynamical models by integral parameters. They are for example used in computational systems biology, notably with parametric regulatory networks, which specify the global architecture (interactions) of the networks, while parameterising the precise rules for drawing the possible temporal evolu...

As shown in [3], the usual update modes of Boolean networks (BNs), including synchronous and (generalized) asynchronous, fail to capture behaviours introduced by multivalued refinements. Thus, update modes do not allow a correct abstract reasoning on dynamics of biological systems, as they may lead to reject valid BN models. We introduce a new sema...

In systems biology, models of cellular regulatory processes such as gene regulatory networks or signalling pathways are crucial to understanding the behaviour of living cells. Available biological data are however often insufficient for full model specification. In this paper, we focus on partially specified models where the missing information is...

The modelling of discrete regulatory networks combines a graph specifying the pairwise influences between the variables of the system, and a parametrisation from which can be derived a discrete transition system. Given the influence graph only, the exploration of admissible parametrisations and the behaviours they enable is computationally demandin...

The modelling of discrete regulatory networks combines a graph specifying the pairwise influences between the variables of the system, and a parametrisation from which can be derived a discrete transition system. Given the influence graph only, the exploration of admissible parametrisations and the behaviours they enable is computationally demandin...

We study the applicability of declarative models to encode and describe structured information by means of semantics. Specifically, we introduce D-SPACES, an implementation of constraint systems with space and extrusion operators. Constraint systems are algebraic models that allow for a semantic language-like representation of information in system...

Cellular reprogramming, a technique that opens huge opportunities in modern and regenerative medicine, heavily relies on identifying key genes to perturb. Most of computational methods focus on finding mutations to apply to the initial state in order to control which attractor the cell will reach. However, it has been shown, and is proved in this a...

Diagnosability and opacity are two well-studied problems in discrete-event systems. We revisit these two problems with respect to expressiveness and complexity issues.

In this paper, we address the formal characterization of targets triggering cellular trans-differentiation in the scope of Boolean networks with asynchronous dynamics. Given two fixed points of a Boolean network, we are interested in all the combinations of mutations which allow to switch from one fixed point to the other, either possibly, or inevi...

This work is part of an ongoing effort to understand the dynamics of passenger loads in modern, multimodal transportation networks (TNs) and to mitigate the impact of perturbations. The challenge is that the percentage of passengers at any given point of the TN that have a certain destination, i.e. their distribution over different trip profiles, i...

The notion of constraint system (cs) is central to declarative formalisms from concurrency theory such as process calculi for concurrent constraint programming (ccp). Constraint systems are often represented as lattices: their elements, called constraints, represent partial information and their order corresponds to entailment. Recently a notion of...

This paper continues our work on perturbation analysis of multimodal transportation networks (TNs) by means of a stochastic hybrid automaton (SHA) model. We focus here on the approximate computation, in particular on the major bottleneck consisting in the high dimensionality of systems of stochastic differential balance equations (SDEs) that define...

In this paper, we address the formal characterization of targets triggering cellular trans-differentiation in the scope of Boolean networks with asynchronous dynamics. Given two fixed points of a Boolean network, we are interested in all the combinations of mutations which allow to switch from one fixed point to the other, either possibly, or inevi...

Diagnosis is the task of detecting fault occurrences in a partially observed system. Depending on the posssible observations, a discrete-event system may be diagnosable or not. Active diagnosis aims at controlling the system to render it diagnosable. Past research has proposed solutions for this problem, but their complexity remains to be improved....

In a network with different transportation modes, or multimodal public transportation system (MPTS), modes are linked among one another not by resources or infrastructure elements - which are not shared, e.g., between different metro lines -, but by the flow of passengers between them. Now, the movements of passengers are steered by the destination...

This paper presents a novel technique for process discovery. In contrast to the current trend, which only considers an event log for discovering a process model, we assume two additional inputs: an independence relation on the set of logged activities, and a collection of negative traces. After deriving an intermediate net unfolding from them, we p...

We enrich spatial constraint systems with operators to specify information and processes moving from a space to another. We shall refer to these news structures as spatial constraint systems with extrusion. We shall investigate the properties of this new family of constraint systems and illustrate their applications. From a computational point of v...

The firing rule for Petri nets assumes instantaneous and simultaneous consumption and creation of tokens. In the context of ordinary Petri nets, this poses no particular problem because of the system’s asynchronicity, even if token creation occurs later than token consumption in the firing. With read arcs, the situation changes, and several differe...

The orientation problem for ternary cyclic order relations has been attacked in the literature from combinatorial perspectives, through rotations, and by connection with Petri nets. We propose here a two-fold characterization of orientable cyclic orders in terms of symmetries of partial orders as well as in terms of separating sets (cuts). The resu...

Under maximal semantics, the occurrence of an event a in a concurrent run of an occurrence net may imply the occurrence of other events, not causally related to a, in the same run. In recent works, we have formalized this phenomenon as the reveals relation, and used it to obtain a contraction of sets of events called facets in the context of occurr...

Attractors of network dynamics represent the long-term behaviours of the modelled system. Their characterization is therefore crucial for understanding the response and differentiation capabilities of a dynamical system. In the scope of qualitative models of interaction networks, the computation of attractors reachable from a given state of the net...

Model-based testing has mainly focused on models where concurrency is interpreted as interleaving (like the ioco theory for labeled transition systems), which may be too coarse when one wants concurrency to be preserved in the implementation. In order to test such concurrent systems, we choose to use Petri nets as specifications and define a concur...

We propose a theoretical testing framework and a test generation algorithm for concurrent systems specified with true-concurrency models, such as Petri nets or networks of automata. The semantic model of computation of such formalisms is labelled event structures, which allow to represent concurrency explicitly. We introduce the notions of strong a...

The ioco relation has become a standard in model-based conformance testing. The co-ioco conformance relation is an extension of this relation to concurrent systems specified with true-concurrency models. This relation assumes a global control and observation of the system under test, which is not usually realistic in the case of physically distribu...

In partially observed Petri nets, diagnosis is the task of detecting whether the given sequence of observed labels indicates that some unobservable fault has occurred. Diagnosability is an associated property of the Petri net, stating that in any possible execution, an occurrence of a fault can eventually be diagnosed. In this article, we consider...

The diagnosis problem amounts to deciding whether some specific “fault” event occurred or not in a system, given the observations collected on a run of this system. This system is then diagnosable if the fault can always be detected, and the active diagnosis problem consists in controlling the system in order to ensure its diagnosability. We consid...

The semantics of concurrent processes can be defined in terms of partially ordered sets. Occurrence nets, which belong to the family of Petri nets, model concurrent processes as partially ordered sets of occurrences of local states and local events. On the basis of the associated concurrency relation, a closure operator can be defined, giving rise...

The task of diagnosis consists in detecting, without ambiguity, occurrence of faults in a partially observed system. Depending on the degree of observability, a discrete event system may be diagnosable or not. Active diagnosis aims at controlling the system in order to make it diagnosable. Solutions have already been proposed for the active diagnos...

Model-based testing has mainly focused on models where currency is interpreted as interleaving (like the ioco theory for labeled transition systems), which may be too coarse when one wants concurrency to be preserved in the implementation. In order to test such concurrent systems, we choose to use Petri nets as specifications and define a concurren...

We present a methodology for fault diagnosis in concurrent, partially observable systems with additional fairness constraints. In this weak diagnosis, one asks whether a concurrent chronicle of observed events allows to determine that a non-observable fault will inevitably occur, sooner or later, on any maximal system run compatible with the observ...

Large distributed system inherently display concurrency, i.e. the fact that several independent events may happen in parallel. The more components in a distributed system are independent (or less coupled), the more this feature appears. With its immediate consequence: handling trajectories as sequences of events means representing an explosive numb...

This chapter presents a brief introduction to the theory of automata, formal languages, decidability, and complexity.

Occurrence nets are a well known partial order model for the concurrent behavior of Petri nets. The causality and conflict relations between events, which are explicitly represented in occurrence nets, induce logical dependencies between event occurrences: the occurrence of an event e in a run implies that all its causal predecessors also occur, an...

From a partial observation of the behaviour of a labeled Discrete Event System, \emph{fault diagnosis} strives to determine whether or not a given ''invisible'' fault event has occurred. The \emph{diagnosability problem} can be stated as follows: does the labeling allow for an outside observer to determine the occurrence of the fault, no later than...

The diagnosability problem can be stated as follows: does a given labeled Discrete Event System allow for an outside observer to determine the occurrence of the "invisible" fault, no later than a bounded number of events after that unobservable occurrence, and based on the partial observation of the behaviour? When this problem is investigated in t...

Real-time distributed systems may be modeled in different formalisms such as time Petri nets (TPN) and networks of timed automata (NTA). This paper focuses on translating a 1-bounded TPN into an NTA and considers an equivalence which takes the distribution of actions into account. This translation is extensible to bounded TPNs. We first use S-invar...

We propose a theoretical framework for testing concurrent systems from true concurrency models like Petri nets or networks of automata. The underlying model of computation of such formalisms are labeled event structures, which allow to represent concurrency explicitly. The activity of testing relies on the definition of a conformance relation that...

Occurrence nets are a well known partial order model for the concurrent behavior of Petri nets. The causality and conflict relations between events, which are explicitly represented in occurrence nets, induce logical dependencies between event occurrences: the occurrence of an event e in a run implies that all its causal predecessors also occur, an...

Petri net unfoldings are a useful tool to tackle state-space explosion in verification and related tasks. Moreover, their structure allows to access directly the relations of causal precedence, concurrency, and conflict between events. Here, we explore the data structure further, to determine the following relation: event a is said to reveal event...

One of the key challenges when a fault occurs in a distributed system is to find the original causes for the failure. In some cases, a seemingly unimportant phenomenon leads to a complete network collapse, and finding the root(s) of the dysfunction, as well as the way the fault has spread through the system, is a key issue for correcting the faulty...

We propose a framework for model-based diagnosis of systems with mobility and variable topologies, modelled as graph transformation systems. Generally speaking, model-based diagnosis is aimed at constructing explanations of observed faulty behaviours on the basis of a given model of the system. Since the number of possible explanations may be huge...

We consider asynchronous diagnosis in (safe) Petri net models of distributed systems, using the partial order semantics of occurrence net unfoldings. Unlike the classical case, observability and diagnosability will appear in two different forms each: a strong form associated to interleaving semantics, and a weak form characteristic of nonsequential...

In recent years, classical discrete event fault diagnosis techniques have been extended to Petri Net system models under partial order semantics. In, we showed how to take further advantage of the partial order representation of concurrent processes, by decomposing the unfolding into `facets', formed by subnets whose events either all occur eventua...

In concurrent real-time processes, the speed of individual components has a double impact: on the one hand, the overall latency of a compound process is affected by the latency of its components. But, if the composition has race conditions, the very outcome of the process will also depend on the latency of component processes. Using stochastic Petr...

Web Service orchestrations are compositions of different Web Services to form a new service. The services called during the orchestration guarantee a given Quality of Service (QoS) to the orchestrator, usually in the form of contracts. These contracts can then be used by the orchestrator to deduce the contract it can offer to its own clients, by pe...

Multimedia services and other critical multisite services (e.g., VPN) are becoming mainstream, and they require a guaranteed quality of service (QoS). Services need to be established across several autonomous systems (ASes), often to connect end-users. Thus, provisioning and control of end-to-end QoS requirements arise as one of the main challenges...

Web services orchestrations and choreographies require establishing Quality of Service (QoS) contracts with the user. This is achieved by performing QoS composition, based on contracts established between the orchestration and the called Web services. These contracts are typically stated in the form of hard guarantees (e.g., response time always le...

Web Service orchestrations are compositions of different Web Services to form a new service. The services called during the orchestration guarantee a given performance to the orchestrater, usually in the form of contracts. These contracts can be used by the orchestrater to deduce the contract it can offer to its own clients, by performing contract...

In recent years, classical discrete event fault diagnosis techniques have been extended to Petri net system models under partial order semantics. We propose here to take further advantage of the partial order representation of concurrent processes; we explore the relational structure of occurrence nets to derive a covering relation. It indicates th...

An Input/Output Automaton is an automaton with a finite number of states where each transition is associated with a single inpuf or output interaction. In [1], we introduced a new formalism, in which each transition is associated with a bipartite partially ordered set made of concurrent inputs followed by concurrent outputs. In this paper, we gener...

One challenge in developing wide-area distributed applications is analyzing the system's non-functional properties, including timing constraints and internal dependencies that can affect quality of service. Analysis of non-functional properties requires a precise formal semantics for the language in which the system is written; but labelled transit...

In a live and bounded Free Choice Petri net, pick a non-conflicting transition. Then there exists a unique reachable marking in which no transition is enabled except the selected one. For a routed live and bounded Free Choice net, this property is true for any transition of the net. Consider now a live and bounded stochastic routed Free Choice net,...

We propose an extension of the Finite State Machine framework in distributed systems, using input/output partial order automata (IOPOA). In this model, transitions can be executed non-atomically, reacting to asynchronous inputs on several ports, and producing asynchronous output on those ports. We develop the formal framework for distributed testin...

Multi-media services and other critical multi-site services (e.g. VPN) are becoming mainstream, and require a guaranteed Quality of Service (QoS). Services need to be established across several domains, often to connect multi-domain end-users. Thus, provisioning and control of end-to-end QoS requirements arises as one of the main challenges in X-do...

Web services orchestrations and choreographies require establishing quality of service (QoS) contracts with the user. This is achieved by performing QoS composition, based on contracts established between the orchestration and the called Web services. These contracts are typically stated in the form of hard guarantees (e.g., response time always le...

The way Internet is used changes: demand grows for critical services that cross several provider networks; guaranteeing a required end-to-end quality of service (QoS) across several networks becomes a challenge. Some critical services (e.g. video-conference, VPN etc.) can not be satisfied in a best effort fashion. The use of QoS contracts (service...

The Internet is based on an X-Domain topology: interconnected domains are managed by independent actors (Fig.1). Deploying critical services (e.g. VPN, video-conference etc.) over such a topology requires to be able to guarantee end-to-end QoS. For this, and to guarantee privacy, Service Level Agreements (SLAs) , also called QoS contracts , are com...

An Input/Output Automaton is an automaton with a finite number of state where each transition is associated with a single input or output interaction. In this paper, we consider a generalization of this formalism, the Partial Order Input/Output Automata (POIOA), in which each transition is associated with a partially ordered set of inputs and outpu...

Web services orchestrations require a firm mathematical basis for their development, regarding both their functional and QoS characteristics. We provide such a basis in the form of a model based on colored Petri net systems. Our approach allows evaluating end-to-end QoS of the orchestration with the help of the QoS of the called sites.

Multi-media services and other critical multi-site services (e.g. VPN) are becoming mainstream, and require a guaranteed Quality of Service (QoS). Services need to be established across several Autonomous Systems (ASes), often to connect end-users. Thus, provisioning and control of end-to-end QoS requirements arise as ones of the main challenges in...

Petri net unfoldings have been recognised as an ecien t means to gh t state space explosion when dealing with concurrent and distributed systems. Some recent Petri net-based approaches to fault di- agnosis of distributed systems suggest to factor the problem into local diagnoses based on the unfoldings of local views of the system, which are then c...

Quality of Service (QoS) has been a major concern in the field of network management, even more so for emerging dynamic multimedia applications (Video on Demand, Telefony over IP etc.) that are becoming mainstream. This problem is particularly sensitive in the context of exchanges accross multiple independent and heterogeneous domains (X-domain), w...

Web Services Orchestrations require a firm mathematical basis for their development. We start from the Orc formalism proposed by J. Misra and co-workers, at Austin University. Orc is small and elegant and captures the essence of Orchestrations. We translate Orc into colored Petri net systems, a generalization of Petri nets allowing to handle recurs...

Diagnosis of concurrent and asynchronous systems, such as large telecommunication or information systems, requires powerful mathematical models. The use of Petri net unfoldings allows to formalize diagnosis using partial order semantics, a generalization from the global state model imposed by the use of automata. If, in addition to asynchronicity a...

We consider query optimization techniques for data intensive P2P applications. We show how to adapt an old technique from deductive databases, namely Query-Sub-Query (QSQ), to a setting where autonomous and distributed peers share large volumes of interelated data.We illustrate the technique with an important telecommunication problem, the diagnosi...

In this paper, we study supervisory control of a class of discrete event systems with simultaneous event occurrences, which we call concurrent discrete event systems, under partial observation. The behavior of the system is described by a language over ...

Diagnosis of concurrent and asynchronous systems, such as large telecommuni- cation or information systems, requires powerful mathematical models. The use of Petri net unfoldings allows to formalize diagnosis using partial order semantics, a general- ization from the global state model imposed by the use of automata. If, in addition to asynchronici...

Developing applications over a distributed and asynchronous architecture without the need for synchronization services is going to become a central track for distributed computing. This research track will be central for the domain of autonomic computing and self-management. Distributed constraint solving, distributed observation, and distributed o...

Distributed architectures for network management have been the subject of a large research eort, but distributed algorithms that im- plement the corresponding functions have been much less investigated. In this paper we describe novel algorithms for model-based distributed fault diagnosis.

This paper deals with distributed and asynchronous discrete event systems diagnosis. This paper has proposed an unfolding approach to the distributed diagnosis of concurrent and asynchronous discrete event dynamical systems. This presentation was informal, based on a toy illustrative examples. Fault diagnosis for distributed discrete event systems...

In truly asynchronous, distributed systems, neither global state nor global time are available. The diagnosis approach with Petri net unfoldings, motivated by the problem of event correlation in telecommunications network management and proposed, uses only local states in combination with a partial order model of time. Here, we give a definition of...

In this paper we study the diagnosis of distributed asynchronous systems with concurrency. Diagnosis is performed by a peer-to-peer distributed architecture of supervisors. Our approach relies on PN unfoldings and event structures, as means to manipulate trajectories of systems with concurrency. This report is an extended version of the paper with...

In truly asynchronous, distributed systems, neither global state nor global time are available. Automata-based diagnosis therefore reaches its limitations there; a different approach, based on Petri net unfoldings, was proposed in [9]. It is motivated by the problem of event correlation in telecommunications network management, and uses only local...

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