Souheib Baarir

The GreatSPN tool: recent enhancements

GreatSPN is a tool that supports the design and the qualitative and quantitative analysis of Generalized Stochastic Petri Nets (GSPN) and of Stochastic Well-Formed Nets (SWN). The very first version of GreatSPN saw the light in the late eighties of last century: since then two main releases where developed and widely distributed to the research community: GreatSPN1.7 [13], and GreatSPN2.0 [8]. This paper reviews the main functionalities of GreatSPN2.0 and presents some recently added features that significantly enhance the efficacy of the tool.

On the Formal Verification of Middleware Behavioral Properties

Distribution middleware is often integrated as a COTS, providing distribution facilities for critical, embedded or large-scale applications. So far, typical middleware does not come with a complete analysis of their behavioral properties. In this paper, we present our work on middleware modeling and the verification of its behavioral properties; the study is applied to our middleware architecture: PolyORB. Then we present the tools and techniques deployed to actually verify the behavioral properties of our model: Petri nets, temporal logic and advanced algorithms to reduce the size of the state space. Finally, we detail some properties we verify and assess our methodology.

Exploiting Partial Symmetries in Well-formed nets for the Reachability and the Linear Time Model Checking Problems

Abstract Taking advantage of the symmetries of a system is an efficient way to cope with the combinatory explosion involved by the verification process. Whereas numerous algorithms and tools efficiently deal with the verification of a symmetrical formula on a symmetrical model, the management of partial symmetries is still an open research topic. In this work, We present the design and the evaluation of two methods applicable on coloured Petri nets. These two methods are extensions of the symbolic reachability graph construction for the well-formed Petri nets. The first algorithm, called the extended symbolic reachability graph construction, tackles the reachability problem. The second one called the symbolic synchronized product, checks a partially symmetric linear time formula on a net. The evaluations show that these two methods outperform the previous approaches dealing with partial symmetries. Furthermore they are complementary ones since the former while being less general gives better results than the latter when applied to the reachability problem.

On the use of exact lumpability in partially symmetrical well-formed nets

Well-formed nets (WNs) have proved an efficient model for building quotient reachability graphs that can be used either for qualitative or performance analysis. However, local asymmetries often break any possibility of grouping states into classes, thus drastically reducing the interest of the approach. An efficient solution has been proposed for qualitative analysis, which relies on a separate representation of the asymmetries in a so called control automaton. The quotient graph is then obtained by synchronizing the transitions of the WN model with the transitions of the control automaton. In this paper, we improve this approach to quantitative analysis. We show that it can be used to build an aggregated graph that is isomorphic to a Markov chain which verifies exact lumpability. Theoretical considerations and practical experiments show that our method outperforms previous approaches.

Shiftability and filter bank design using Morlet wavelet

A multi-resolution representation through wavelet transform has proved to be beneficial for many signal processing applications. For example, Morlet wavelet has shown good performance in tasks like audio coding and image enhancement. Unfortunately, wavelet transforms are unstable when the input signal is shifted in position. Prior works formalize this problem by defining a type of translation invariance called shiftability. Shiftability constraint is equivalent to the constraint that the response power of the transform in the sub-band is preserved with respect to translations of the input signal. In this paper, we propose constraints for filter bank design that guarantee shiftability. Also, we use the proposed constraints to design a filter bank that implements the Morlet wavelet transform. We use the Morlet wavelet based on the fact that it has good properties in joint time-frequency localization and it has shown approximate shiftability. The filter bank proposed presents good results with respect to shiftability.A multi-resolution representation through wavelet transform has proved to be beneficial for many signal processing applications. For example, Morlet wavelet has shown good performance in tasks like audio coding and image enhancement. Unfortunately, wavelet transforms are unstable when the input signal is shifted in position. Prior works formalize this problem by defining a type of translation invariance called shiftability. Shiftability constraint is equivalent to the constraint that the response power of the transform in the sub-band is preserved with respect to translations of the input signal. In this paper, we propose constraints for filter bank design that guarantee shiftability. Also, we use the proposed constraints to design a filter bank that implements the Morlet wavelet transform. We use the Morlet wavelet based on the fact that it has good properties in joint time-frequency localization and it has shown approximate shiftability. The filter bank proposed presents good results with respect to shiftability.

Crocodile: a symbolic/symbolic tool for the analysis of symmetric nets with bag

The use of high-level nets, such as colored Petri nets, is very convenient for modeling complex systems in order to have a compact, readable and structured specification. Symmetric Nets with Bags (SNB) were introduced to cope with this goal without introducing a burden due to the underlying complexity of the state space. The structure of bags allows through exploitation of symmetries to provide a compact quotient state space representation (similarly to the construction proposed in GreatSPN). In this paper, we present Crocodile, the first implementation of a modeling environment and model checker dedicated to SNB. Its goal is first to be a proof of concept for experimenting the quotient graph techniques together with hierarchical set decision diagrams. A second objective is to enable experimentation of modeling techniques with this new class of Petri nets.

Complementary Formal Approaches for Dependability Analysis

Evaluating the robustness of digital circuits with respect to soft errors has become an important part of the design flow for many applications. The identification of the most or less critical registers is often necessary, in order to reach the lowest overheads while achieving a given application-level robustness. The goal here is to identify those soft errors actually harmful for the system, not to compute the Soft Error Rate. In this context, we investigate new approaches based on formal techniques to improve design-time robustness evaluations at least for the most critical blocks in a circuit. Preliminary results are shown, focusing on the evaluation of self-healing (or self-repairing) capabilities.

Towards Distributed Software Model-Checking Using Decision Diagrams

Symbolic data structures such as Decision Diagrams have proved successful for model-checking. For high-level specifications such as those used in programming languages, especially when manipulating pointers or arrays, building and evaluating the transition is a challenging problem that limits wider applicability of symbolic methods. We propose a new symbolic algorithm, EquivSplit, allowing an efficient and fully symbolic manipulation of transition relations on Data Decision Diagrams. It allows to work with equivalence classes of states rather than individual states. Experimental evidence on the concurrent software oriented benchmark BEEM shows that this approach is competitive.

Formalization of fUML: An Application to Process Verification

Much research work has been done on formalizing UML Activity Diagrams for process modeling to verify different kinds of soundness properties (deadlock, unreachable activities and so on) on process models. However, these works focus mainly on the control-flow aspects of the process and have done some assumptions on the precise execution semantics defined in natural language in the UML specification. In this paper, we define a first-order logic formalization of fUML (Foundational Subset of Executable UML), the official and precise operational semantics of UML, in order to apply model checking techniques and therefore verify the correctness of fUML-based process models. Our formalization covers the control-flow, data-flow, resources, and timing dimensions of processes in a unified way. A working implementation based on the Alloy language has been developed. The implementation showed us that many kinds of behavioral properties not commonly supported by other approaches and implying multiple dimensions of the process can be efficiently checked.

Efficient lumpability check in partially symmetric systems

State space based performance analysis of stochastic models may be impaired by the state space explosion but such problem can be mitigated in symmetrical behaving systems by aggregating equivalent states and transitions. An effective way of exploiting symmetries when the system is modeled using the stochastic well-formed net (SWN) formalism, is to generate the symbolic reachability graph (SRG) and automatically derive a lumped continuous time Markov chain (CTMC) of the same size as the SRG from it. For partially symmetric systems, the extended SRG (ESRG) can be used instead, but the derivation of a lumped CTMC in this case is not as direct as in the SRG case: in fact the ESRG structure might need a refinement to satisfy the lumpability conditions. In this paper a new efficient algorithm to derive a lumped CTMC from the ESRG is presented, and the results obtained by experimenting its implementation within the GreatSPN environment are discussed. The algorithm combines the Paige and Tarjans partition refinement algorithm (extended to work with weighted arcs) and a previously proposed lumpability check algorithm (built specifically for the use with the ESRG) and outperforms both of them. The implementation of the algorithm within the GreatSPN environment would allow the several users that have chosen this package to apply the proposed technique