The original publication is available at www.springerlink.comInternational audienceThe aim of this paper is to gradually introduce formalism in the requirement engineering phase in order to facilitate its validation. We analyze and elicit our requirements with KAOS, specify them into Event-B language, and then use the animation technique to rigourously validate the derived formal specification and consequently its semi-formal counterpart goal model against original customers' requirements

Mashkoor, Atif

Matoussi, Abderrahman

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Towards validation of requirements models

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HAL Id: hal-00431272https://hal.archives-ouvertes.fr/hal-00431272Submitted on 15 Jan 2010HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.Towards validation of requirements modelsAtif Mashkoor, Abderrahman MatoussiTo cite this version:Atif Mashkoor, Abderrahman Matoussi. Towards validation of requirements models. The SecondInternational Conference on ASM, Alloy, B and Z (ABZ 2010), Feb 2010, Orford (Québec), Canada.pp.404, ￿10.1007/978-3-642-11811-1_38￿. ￿hal-00431272￿Towards validation of requirements modelsAtif Mashkoor1 and Abderrahman Matoussi21 LORIA – Nancy UniversitéVandœuvre lès Nancy, France{firstname.lastname}@loria.fr2 LACL – Université Paris-EstCréteil Cedex, France{firstname.lastname}@univ-paris12.frAbstract. The aim of this paper is to gradually introduce formalism in the re-quirement engineering phase in order to facilitate its validation. We analyze andelicit our requirements with KAOS, specify them into Event-B language, and thenuse the animation technique to rigourously validate the derived formal specifica-tion and consequently its semi-formal counterpart goal model against originalcustomers’ requirements.1 IntroductionThe use of formal methods for software development is escalating over the period oftime. Most of the formal methods refine the initial mathematical model up to an ex-tent where final refinement contains enough details for an implementation. The input tothis formal specification phase is often the documents obtained during the requirementsanalysis activity which are either textual or semi-formal. Now there is a traceability gapbetween analysis and specification phases as verification of the semi-formal analysismodel is difficult because of poor understandability of lower level of formalism of ver-ification tools and validation of the formal specification is difficult for customers due totheir inability to understand formal models.Our objective is to bridge this gap by a gradual introduction of formalism into therequirement model in order to facilitate its validation. We analyse our requirementswith KAOS (Knowledge Acquisition in autOmated Specification) [1] which is a goal-oriented methodology for requirements modeling, then we translate the KAOS goalmodel, following our derived precise semantics, into an Event-B [2] formal specifica-tion with the help of the platform RODIN3, and finally we rigourously animate our spec-ification with the help of the animator Brama [3], incorporated into platform RODIN,in order to validate the conformance of the specification to original requirements. Withthis approach we aim to reap benefits at two levels: customers can be involved intothe development right form the start and consequently the requirement errors can bedetected right on the spot.3 http://rodin-b-sharp.sourceforge.net2 Atif Mashkoor and Abderrahman Matoussi2 KAOS and Event-BTo analyze our requirements, we use KAOS which builds a data model in UML-likenotation. The main KAOS goal defines an objective the system should meet, usuallythrough the cooperation of multiple agents such as devices or humans, followed byseveral sub goals. Contrary to other requirements methods, such as i* [4], KAOS iswell suited for our purpose because it can be extended with an extra step of formalitywhich can fill in the gap between requirements and the later phases of development.The choice of Event-B as a formal specification language is due to its similarity andcomplementarity with KAOS. Firstly, Event-B is based on set theory with the abilityto use standard first-order predicate logic facilitating the integration with the KAOSrequirements model that is based on first-order temporal logic. Secondly, both Event-Band KAOS have the notion of refinement (constructive approach). Finally, KAOS andEvent-B have the ability to model both the system and its environment.3 DiscussionFig. 1. The rigorous require-ments validation processThere are two main steps of our approach. First we trans-late our goal model into an Event-B specification withthe help of our Event-B semantics, and later we animatethe specification in order to validate that captured re-quirements are in accordance with original customer re-quirements. The whole process of validation is summedup by figure 1. Following is the brief elaboration of ourapproach:3.1 The semantics stepThe first step of our approach aims to express the KAOSgoal model with Event-B by staying at the same level ofabstraction which allows us to give this expression pre-cise semantics. To achieve this objective, we use Event-B to formalize the KAOS refinement patterns that ana-lysts use to generate a KAOS goal hierarchy. We primar-ily focus on most frequently used "Goal Patterns": theAchieve goals. The assertions in Achieve goals are ex-pressed as following: G-Guard ⇒ ⋄G-PostCond, whereG-Guard and G-PostCond are predicates. Symbol ⇒denotes the classical logical implication. Symbol ⋄ (theopen diamond) represents the temporal operator "even-tually" which ensures that a predicate must occur "atsome time in future". Hence, such assertions demonstrate that from a state in whichG-Guard holds, we can reach sooner or later to another state in which G-PostCondholds.Towards validation of requirements models 3If we refer to the concepts of guard and postcondition that exist in Event-B, a KAOSgoal can be considered as a postcondition of the system, since it means that a propertymust be established. The crux of our formalization is to express each KAOS goal asa B event, where the action represents the achievement of the goal. Then, we will usethe Event-B refinement relation and additional custom-built proof obligations to deriveall the subgoals of the system by means of B events. One may wonder whether theformalization of KAOS target predicates (i.e. the predicate after the diamond symbol)as B post conditions is adequate, since the execution of B events is not mandatory. Atthis very high level of abstraction, there is only one event for representing the parentgoal. In accordance with the Event-B semantics, if the guard of the event is true, then theevent necessarily occurs. For the new events built by refinement and associated to thesubgoals, we guarantee by construction that no event prevent the post conditions to beestablished. For that, we have proposed Event-B semantics for each KAOS refinementpattern by constructing set-theoretic mathematical models. This process continues untilthe complete specification of KAOS goal model into Event-B. A detailed discussion onthis step can be found in [5]. Formalization of the goal patterns other than Achieve goalsis a work in progress.3.2 The animation stepFollowing the precise semantics discussed in previous section, we derive an initialEvent-B specification of the KAOS goal model. The aim of this animation step is tovalidate this derived specification. Our approach to address this issue is based on fol-lowing hypothesis: we presume if the animation of the specification reveals the samebehavior that we intended while writing our goal model, then the Event-B specificationwould be considered as a valid formal representation of the customers’ requirements.In order to achieve this, we execute our specification to check its behavior with theapproach defined in [6]. We rigourously animate the specification at each refinementstep. It not only indicates any deviation from original requirements right on the spot butalso helps fixing the specification errors. If any deviation from the intended behavior isdiscovered, we go back to the source and rectify the error. The process continues untilthe specification fully adheres to the requirements.4 Conclusion and future workWe present an approach to validate a semi-formal requirement model by the animationof its formal counterpart. We express a KAOS goal model capturing users’ requirementsinto an Event-B specification language for a stepwise requirement validation process.At theoretical level our approach seems promising as we have obtained some initialresults at its both steps independently. However, we hope that our proposed combinedapproach of analysis, specification and validation is also feasible collectively. We aimto target at transportation domain [7] to test our hypothesis.4 Atif Mashkoor and Abderrahman Matoussi5 AcknowledgementsThis work has been partially supported by the ANR project TACOS (Ref: ANR-06-SETI-017). We would also like to thank Jean-Pierre Jacquot and Régine Laleau fortheir valuable comments.References1. Van Lamsweerde, A.: Requirements Engineering: From System Goals to UML Models toSoftware Specifications, Wiley (2009)2. Abrial, J.R.: Modeling in Event-B: System and Software Engineering, Cambridge UniversityPress (2009)3. Servat, T.: BRAMA: A New Graphic Animation Tool for B Models, In: 7th InternationalConference of B Users (B’07), Springer-Verlag, Besançon, France (2007)4. Yu, E.: Towards Modeling and Reasoning Support for Early-Phase Requirements Engineer-ing, In: 3rd IEEE International Symposium on Requirements Engineering (RE’97), IEEE,Annapolis, USA (1997)5. Matoussi, A.: Expressing KAOS Goal Models with Event-B, In Doctoral Symposium of16th International Symposium on Formal Methods (FM’09-DS), Eindhoven, The Netherlands(2009)6. Mashkoor, A., Jacquot, J.P., Souquières, J.: Transformation Heuristics for Formal Require-ments Validation by Animation, In: 2nd International Workshop on the Certification of Safety-Critical Software Controlled Systems (SafeCert’09), York, UK (2009)7. Mashkoor, A., Jacquot, J.P, Souquières, J.: B Événementiel pour la Modélisation du Domaine:Application au Transport, In: Approches Formelles dans l’Assistance au Développement deLogiciels (AFADL’09), Toulouse, France (2009)

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