A Forward On-The-Fly Approach in Controller Synthesis of Time Petri Nets

P. Heidira, H. Boucheneb ?A Forward On-The-Fly Approach in Controller Synthesis of Time Petri Nets?, Petri Nets - Manufacturing and Computer Science, Pawel Pawlewski (Ed.), ISBN: 978-953-51-0700-2, InTech, August 2012

On Consistency of Operational Transformation Approach

The Operational Transformation (OT) approach, used in many collaborative editors, allows a group of users to concurrently update replicas of a shared object and exchange their updates in any order. The basic idea of this approach is to transform any received update operation before its execution on a replica of the object. This transformation aims to ensure the convergence of the different replicas of the object, even though the operations are executed in different orders. However, designing transformation functions for achieving convergence is a critical and challenging issue. Indeed, the transformation functions proposed in the literature are all revealed incorrect. In this paper, we investigate the existence of transformation functions for a shared string altered by insert and delete operations. From the theoretical point of view, two properties - named TP1 and TP2 - are necessary and sufficient to ensure convergence. Using controller synthesis technique, we show that there are some transformation functions which satisfy only TP1 for the basic signatures of insert and delete operations. As a matter of fact, it is impossible to meet both properties TP1 and TP2 with these simple signatures.Comment: In Proceedings Infinity 2012, arXiv:1302.310

Symbolic Model-Checking of Optimistic Replication Algorithms

The original publication is available at www.springerlink.comInternational audienceThe Operational Transformation (OT) approach, used in many collaborative editors, allows a group of users to concurrently update replicas of a shared object and exchange their updates in any order. The basic idea of this approach is to transform any received update operation before its execution on a replica of the object. This transformation aims to ensure the convergence of the different replicas of the object. However, designing transformation algorithms for achieving convergence is a critical and challenging issue. In this paper, we address the verification of OT algorithms with a symbolic model-checking technique. We show how to use the difference bound matrices to explore symbolically infinite state-spaces of such systems and provide symbolic counterexamples for the convergence property

TCTL model checking of Time Petri Nets

International audienceIn this paper, we consider \emph{subscript} TCTL for Time Petri Nets (TPN-TCTL) for which temporal operators are extended with a time interval, specifying a time constraint on the firing sequences. We prove that the model-checking of a TPN-TCTL formula on a bounded TPN is decidable and is a PSPACE-complete problem. We propose a zone based state space abstraction that preserves marking reachability and traces of the TPN. As for Timed Automata (TA), the abstraction may use an over-approximation operator on zones to enforce the termination. A coarser (and efficient) abstraction is then provided and proved exact w.r.t. marking reachability and traces (LTL properties). Finally, we consider a subset of TPN-TCTL properties for which it is possible to propose efficient on-the-fly model-checking algorithms. Our approach consists in computing and exploring the zone based state space abstractio

Contraction of the ITCPN state space

We show here how to contract the ITCPN state space. We distinguish three levels of contraction that translate the ITCPN state space into one well timed and coherent timed automation. We consider here only equivalence based on delays. To achieve more contractions, the equivalence based on delays can be completed with equivalence based on colours as shown in [4

Experiments in Model-Checking Optimistic Replication Algorithms

This paper describes a series of model-checking experiments to verify optimistic replication algorithms based on Operational Transformation (OT) approach used for supporting collaborative edition. We formally define, using tool UPPAAL, the behavior and the main consistency requirement (i.e. convergence property) of the collaborative editing systems, as well as the abstract behavior of the environment where these systems are supposed to operate. Due to data replication and the unpredictable nature of user interactions, such systems have infinitely many states. So, we show how to exploit some features of the UPPAAL specification language to attenuate the severe state explosion problem. Two models are proposed. The first one, called concrete model, is very close to the system implementation but runs up against a severe explosion of states. The second model, called symbolic model, aims to overcome the limitation of the concrete model by delaying the effective selection and execution of editing operations until the construction of symbolic execution traces of all sites is completed. Experimental results have shown that the symbolic model allows a significant gain in both space and time. Using the symbolic model, we have been able to show that if the number of sites exceeds 2 then the convergence property is not satisfied for all OT algorithms considered here. A counterexample is provided for every algorithm

A secure protocol based on a sedentary agent for mobile agent environments

The main challenge when deploying mobile agent environments pertains to security issues concerning mobile agents and their executive platform. This paper proposes a secure protocol which protects mobile agents against attacks from malicious hosts in these environments. Protection is based on the perfect cooperation of a sedentary agent running inside a trusted third host. Results show that the protocol detects several attacks, such as denial of service, incorrect execution and re-execution of the mobile agent code. Results also indicate that the traffic generated and run time are barely affected

On formalizing UML2 activities using TPNets: case studies

ABSTRACT: Transactional Petri Nets (TPNets) are a new class of high-level Zero-Safe Nets (ZSNs), defined as a more suitable semantic framework for UML2 activity diagrams. Indeed, they ensure reactivity and synchronization of concurrent flows triggering with their junction. Reactivity is guaranteed due to the real time massive cancellation semantics based on the definition of new dynamic enabling rules and the imposed priority among executions. Global synchronization in turn is assured thanks to non-locality principle, an outcome of exploiting atomic stable transactions. Rewriting logic is defined as the operational semantics framework of TPNets

Maximal good step graph methods for reducing the generation of the state space

This paper proposes an effective method based on the two main partial order techniques which are persistent sets and covering step graph techniques, to deal with the state explosion problem. First, we introduce a new definition of sound steps, the firing of which enables to extremely reduce the state space. Then, we propose a weaker sufficient condition about how to find the set of sound steps at each current marking. Next, we illustrate the relation between maximal sound steps and persistent sets, and propose a concept of good steps. Based on the maximal sound steps and good steps, a construction algorithm for generating a maximal good step graph (MGSG) of a Petri net (PN) is established. This algorithm first computes the maximal good step at each marking if there exists one, otherwise maximal sound steps are fired at the marking. Furthermore, we have proven that an MGSG can effectively preserve deadlocks of a Petri net. Finally, the change performance evaluation is made to demonstrate the superiority of our proposed method, compared with other related partial order techniques