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43
Probabilistic QoS and Soft Contracts for TransactionBased Web Services Orchestrations
 IEEE Trans. on Services Computing
, 2008
"... Abstract—Service level agreements (SLAs), or contracts, have an important role in Web services. These contracts define the obligations and rights between the provider of a Web service and its client, with respect to the function and the Quality of Service (QoS). For composite services like orchestra ..."
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Cited by 46 (17 self)
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Abstract—Service level agreements (SLAs), or contracts, have an important role in Web services. These contracts define the obligations and rights between the provider of a Web service and its client, with respect to the function and the Quality of Service (QoS). For composite services like orchestrations, such contracts are deduced by a process called QoS contract 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 less than 5 msec). Using hard bounds is not realistic, however, and more statistical approaches are needed. In this paper, we propose using soft probabilistic contracts instead, which consist of a probability distribution for the considered QoS parameter—in this paper, we focus on timing. We show how to compose such contracts to yield a global probabilistic contract for the orchestration. Our approach is implemented by the TOrQuE tool. Experiments on TOrQuE show that overly pessimistic contracts can be avoided and significant room for safe overbooking exists. An essential component of SLA management is then the continuous monitoring of the performance of called Web services to check for violations of the agreed SLA. We propose a statistical technique for runtime monitoring of soft contracts.
McMillan’s complete prefix for contextual nets
, 2007
"... In a seminal paper, McMillan proposed a technique for constructing a finite complete prefix of the unfolding of bounded (i.e., finitestate) Petri nets, which can be used for verification purposes. Contextual nets are a generalisation of Petri nets suited to model systems with readonly access to re ..."
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Cited by 17 (8 self)
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In a seminal paper, McMillan proposed a technique for constructing a finite complete prefix of the unfolding of bounded (i.e., finitestate) Petri nets, which can be used for verification purposes. Contextual nets are a generalisation of Petri nets suited to model systems with readonly access to resources. When working with contextual nets, a finite complete prefix can be obtained by applying McMillan’s construction to a suitable encoding of the contextual net into an ordinary net. However, it has been observed that if the unfolding is itself a contextual net, then the complete prefix can be significantly smaller than the one obtained with the above technique. A construction for generating such a contextual complete prefix has been proposed for a special class of nets, called readpersistent. In this paper we propose an algorithm that works for arbitrary semiweighted, bounded contextual nets. The construction explicitly takes into account the fact that, unlike in ordinary or readpersistent nets, an event can have several different histories in general contextual net computations.
Complete finite prefixes of symbolic unfoldings of safe time Petri nets
 In ICATPN, volume 4024 of LNCS
, 2006
"... Abstract. Monitoring realtime concurrent systems is a challenging task. In this paper we formulate (modelbased) supervision by means of hidden state history reconstruction, from event (e.g. alarm) observations. We follow a socalled true concurrency approach using time Petri nets: the model define ..."
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Cited by 16 (7 self)
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Abstract. Monitoring realtime concurrent systems is a challenging task. In this paper we formulate (modelbased) supervision by means of hidden state history reconstruction, from event (e.g. alarm) observations. We follow a socalled true concurrency approach using time Petri nets: the model defines explicitly the causal and concurrency relations between the observable events, produced by the system under supervision on different points of observation, and constrained by time aspects. The problem is to compute onthefly the different partial order histories, which are the possible explanations of the observable events. We do not impose that time is observable: the aim of supervision is to infer the partial ordering of the events and their possible firing dates. This is achieved by considering a model of the system under supervision, given as a time Petri net, and the onthefly construction of an unfolding, guided by the observations. Using a symbolic representation, this paper presents a new definition of the unfolding of time Petri nets with dense time.
Unfoldingbased diagnosis of systems with an evolving topology
, 2008
"... Abstract. We propose a framework for modelbased diagnosis of systems with mobility and variable topologies, modelled as graph transformation systems. Generally speaking, modelbased diagnosis is aimed at constructing explanations of observed faulty behaviours on the basis of a given model of the sy ..."
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Cited by 15 (7 self)
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Abstract. We propose a framework for modelbased diagnosis of systems with mobility and variable topologies, modelled as graph transformation systems. Generally speaking, modelbased 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 exploit the unfolding as a compact data structure to store them, along the lines of previous work dealing with Petri net models. Given a model of a system and an observation, the explanations can be constructed by unfolding the model constrained by the observation, and then removing incomplete explanations in a pruning phase. The theory is formalised in a general categorical setting: constraining the system by the observation corresponds to taking a product in the chosen category of graph grammars, so that the correctness of the procedure can be proved by using the fact that the unfolding is a right adjoint and thus it preserves products. The theory thus should be easily applicable to a wide class of system models, including graph grammars and Petri nets. 1
Time Supervision of Concurrent Systems using Symbolic Unfoldings of Time Petri Nets
, 2005
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Configuration Structures, Event Structures and Petri Nets
"... In this paper the correspondence between safe Petri nets and event structures, due to Nielsen, Plotkin and Winskel, is extended to arbitrary nets without selfloops, under the collective token interpretation. To this end we propose a more general form of event structure, matching the expressive powe ..."
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Cited by 12 (1 self)
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In this paper the correspondence between safe Petri nets and event structures, due to Nielsen, Plotkin and Winskel, is extended to arbitrary nets without selfloops, under the collective token interpretation. To this end we propose a more general form of event structure, matching the expressive power of such nets. These new event structures and nets are connected by relating both notions with configuration structures, which can be regarded as representations of either event structures or nets that capture their behaviour in terms of action occurrences and the causal relationships between them, but abstract from any auxiliary structure. A configuration structure can also be considered logically, as a class of propositional models, or—equivalently— as a propositional theory in disjunctive normal from. Converting this theory to conjunctive normal form is the key
Event Structure Semantics for Nominal Calculi
 Proceedings of CONCUR 2006, 17th International Conference on Concurrency Theory, Lecture Notes in Computer Science
, 2006
"... Abstract. Event structures have been used for giving true concurrent semantics to languages and models of concurrency such as CCS, Petri nets and graph grammars. Although certain nominal calculi have been modeled with graph grammars, and hence their event structure semantics could be obtained as i ..."
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Cited by 10 (0 self)
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Abstract. Event structures have been used for giving true concurrent semantics to languages and models of concurrency such as CCS, Petri nets and graph grammars. Although certain nominal calculi have been modeled with graph grammars, and hence their event structure semantics could be obtained as instances of the general case, the main limitation is that in the case of graph grammars the construction is more complex than strictly necessary for dealing with usual nominal calculi and, speaking in categorical terms, it is not as elegant as in the case of Petri nets. The main contribution of this work is the definition of a particular class of graph grammars, called persistent, that are expressive enough to model name passing calculi while simplifying the denotational domain construction, which can be expressed as an adjunction. Finally, we apply our technique to derive event structure semantics for picalculus and joincalculus processes. 1
Unfolding Semantics of Graph Transformation
, 2007
"... Several attempts have been made of extending to graph grammars the unfolding semantics originally developed by Winskel for (safe) Petri nets, but only partial results were obtained. In this paper we fully extend Winskel’s approach to singlepushout grammars providing them with a categorical concurre ..."
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Cited by 10 (8 self)
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Several attempts have been made of extending to graph grammars the unfolding semantics originally developed by Winskel for (safe) Petri nets, but only partial results were obtained. In this paper we fully extend Winskel’s approach to singlepushout grammars providing them with a categorical concurrent semantics expressed as a