Abstract. The task of automatically composing Web services involves two main composition processes, vertical and horizontal composition. Vertical composition consists of defining an appropriate combination of simple processes to perform a composition task. Horizontal composition process consists of determining the most appropriate Web service, from among a set of functionally equivalent ones for each component process. Several recent research efforts have dealt with the Web service composition problem. Nevertheless, most of them tackled only the vertical composition of Web services despite the growing trend towards functionally equivalent Web services. In an attempt to facilitate and streamline the process of horizontal composition of Web services while taking the above limitation into consideration, this work includes two main contributions. The first is a generic formalization of any Web service composition problem based on a constraint optimization problem (COP); this formalization is compatible to any Web service description language. The second contribution is an incremental userintervention-based protocol to find the optimal composite Web service according to some predefined criteria at run-time. Our goal is i) to deal with many crucial natural features of Web services such as dynamic and distributed environment, uncertain and incomplete Web service information, etc; and ii) to allow human user intervention to enhance the solving process. Three approaches are described in this work, a centralized approach, a distributed approach and a multi-agent approach to deal with realistic domains.

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Meeting scheduling (MS) represents an important real-world group decision application that denotes one of the actual combinatorial problems. Solving this problem consists of scheduling all the meetings while satisfying all the constraints related to both the users and the meetings. However, given human nature, the solution is usually delineated by the encountering of conflicting preferences. Most of existing research efforts allow the relaxation of the users' preferences in order to reach an agreement between all the participants, which is not always possible. In addition, they do not deal with the achievement of any level of local consistency to enhance the efficiency of the solving process, and finally, they do not address the real difficulty of distributed systems, which is the complexity of message passing operations.Here we propose a new approach to facilitate and streamline the scheduling meetings process in any organization. This approach is based on the distributed reinforcement of arc consistency model, which takes into account the difficulties mentioned above. The present work focuses mainly on satisfying meetings hosts' preferences as much as possible, while taking into consideration all users' availability. The underlying selfish protocol is able to efficiently reach the best solution for the host of the meeting (according to the predefined criteria) whenever possible. This process is achieved with the minimal number of exchanged messages and while

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