Imed Abbassi

Event-B Based Approach for Verifying Dynamic Composite Service Transactional Behavior

Verifying Web service composition in a dynamic environment remains one of the most difficult tasks despite the efforts and the previous proposed research works because new services can be composed during the execution step and others can automatically appear, disappear, or be updated. To achieve the Web service composition specification and verification, we introduce a new concept, called dynamic pattern. A dynamic pattern is an extension of a static one. Then, we propose to formalize dynamic Web Service composition in Event-B using dynamic patterns. The resulting model is progressively verified using proofs. We use animator of model (ProB) to detect a variety of problems, such as deadlocks or other unexpected behavior of a model.

A Formal Approach for Enforcing Transactional Requirements in Web Service Compositions

Service based applications leveraging different features are today the underpinnings of enterprise computing. However, current Web services composition systems make only use of functional requirements in the selection process of component Web services while transactional consistency is a crucial parameter of most business applications. In this work, we propose a formal approach for enforcing transactional requirements in Web service compositions using Event-B formalism. Our solution enables specifying composite Web services where partner Web services are selected at runtime according to the transactional requirements that are defined based on the Acceptable Termination States concept. The resulting composite Web service is compliant with the consistency requirements expressed by business application designers. Moreover, we incrementally combine proof based models with model checking to trace possible errors and then come back to make proofs of the proof obligations.

Genetic-Based Approach for ATS and SLA-aware Web Services Composition

Composite applications leveraging several functionalities offered by Web services are today the underpinnings of enterprise computing. There are two kinds of requirements of web service composition: QoS requirements and transactional requirements. The QoS requirements are recorded in service-level agreements SLAs contract. The transactional requirements are expressed by designers as a set of accepted termination states ATS. In this paper, we propose a novel selection and composition approach called ATS and SLA-aware Web services composition WSC. Our approach aims at finding composite service compliant with ATS and better satisfying the SLA contract. As the ATS and SLA-aware WSC is NP-hard problem, an efficient algorithm is required. This work adopts genetic algorithm to deal with combinatorial problem.

A Formal Approach for Verifying QoS Variability in Web Services Composition Using EVENT-B

The main issues for the fulfillment service level agreements (SLA) are concerned with problem of variability of QoS properties (vQoS). Indeed, the QoS properties may evolve frequently either because of internal changes or because of workload fluctuations. To solve the vQoS problem, we first introduced three variability operators: replicate, delete and replace. These operators will be used to reconfigure CWS when the SLA contract is violated. The first two operators are used to add and remove Web service instances, while the last one is used to substitute some faulty Web services. Then, we proposed an incremental approach for modeling and verifying the composites services (CWSs) reconfiguration using Event-B. We start by abstractly specifying the main requirements and then we refine them through several steps to model CWSs. The consistency of each model and the relationship between an abstract model and its refinements are obtained by formal proofs. Finally, we used ProB model-checker to trace possible design errors. We have exploited the LTL for dynamic reconfigurations to characterize the correct behavior of CWSs reconfiguration.

Combining Dynamic Workflow and Transactional Semantics Using a Pattern-Based Approach

In this paper, we propose a new paradigm, called dynamic transactional pattern, for specifying flexible and reliable composite Web services in a pervasive environment. This new paradigm is a convergence concept of dynamic Workflow patterns and advanced transactional model. It can be seen both as a dynamic coordination and a structural transaction. Indeed, it combines dynamic Control-Flow flexibility and transactional processing reliability.

Modeling and Verifying the Transactional and QoS-aware Services Composition Using Event-B

Composite applications leveraging the functionalities offered by Web services are today the underpinnings of enterprise computing. However, current Web services composition systems make only use of functional requirements in the selection process of component Web services, while the transactional consistency and quality of service (QoS) are crucial parameters of most business applications. In this paper, we propose a novel approach for Web services selection and composition based on transactional requirements and QoS constraints. The transactional requirements are defined by the engineers using the acceptable termination states concept. We give a formalization of our approach in Event-B. We incrementally combine the modeling and verification activities related to formal development process supported by Event-B method. The verification activity is based on the proof and model-checking.

Formal Behavioral Modeling for Verifying SCA Composition with Event-B

With the emergence of Service Component Architecture (SCA), all interests were focused on representing this architecture in a formal way in order to be able to prevent the specifications failures. In this context, our recent works were interested in formalizing structural properties of the SCA specifications, particularly in defining structural compatibility between connected services. In fact, verifying structural compatibility is necessary but not sufficient. In this paper we intend to represent, in a first step, the SCA behavioral properties by means of Event-B invariants and events. In a second step, we established behavioral compatibility between services interacting together which is considered as a delicate task and has a great importance in guaranteeing reliable communication between services. The consistency and the validity of the obtained model have been proved by the Event-B dedicated tools.

Towards correct cloud resource allocation in FOSS applications

Abstract Cloud computing is a new computing paradigm used for building on demand free and open source software (FOSS) applications. However, due to the lack of an explicit and formal description of the resource perspective in the existing FOSS applications, the correctness of Cloud resources management cannot be verified. The main objective of this paper is to propose a formal definition of the resource perspective in FOSS applications as a step towards ensuring a correct and consistent Cloud resource allocation in FOSS application modeling. Hence, we developed a Cloud Resources Allocation Model (CRAM4FOSS) for FOSS applications using the Event-B method. This model is used to formally validate the consistency of Cloud resource allocation for FOSS applications at design time, and to analyze and check its correctness according to the user’s requirements and the resource’s capabilities. The correctness and the consistency of our CRAM4FOSS model have been established into two phases : first, the ProB model-checker is used to detect the most obvious errors and validate the Event-B model by playing some scenarios, then a proof activity is performed to discharge the generated proof obligations that ensure the correctness of the model.

A Genetic-Based Adaptive Approach for Reliable and Efficient Service Composition

Composite services often run in dynamically changing environments where new composition requirements can be added and Web services may disappear or their quality of service may decline. The service composition requirements are classified into three groups: functional, nonfunctional, and transactional requirements. The functional computing requirements concern functional capabilities of Web services. The nonfunctional computing requirements are defined as service-level agreements (SLA) constraints. The transactional computing requirements define behavioral constraints and are expressed by designers using the acceptable termination states (ATS) concept. In this work, we proposed an adaptive service composition (ASC) approach. Such an approach is adaptive as it can be used to either define or reconfigure composite services according to SLA and ATS constraints. It introduces a new recovery capability (RC) concept for capturing the recoverability level supported by Web services. We adopted a genetic-based algorithm to model the ASC approach. The choice of such an algorithm is justified by the nature of the problem of our ASC process which belongs to the NP-hard class. We experimentally showed the effectiveness of our ASC approach and empirically studied the service composition RC in particular.

A Formal Approach for Correct Elastic Package-Based Free and Open Source Software Composition in Cloud

Cloud environments have been increasingly used to deploy and run software while providing a high level performance with a low operating cost. Most of the existing software applications are nowadays distributed as Package-based Free and Open Source (PFOS) applications. Different requirements must be considered while configuring PFOS software. These requirements can be classified into two classes: dependency and capacity requirements.