Lucas Bordeaux

Describing and reasoning on Web services using process algebra

We argue that essential facets of Web services, and especially those useful to understand their interaction, can be described using process-algebraic notations. Web service description and execution languages such as BPEL are essentially process description languages; they are based on primitives for behaviour description and message exchange which can also be found in more abstract process algebras. One legitimate question is therefore whether the formal approach and the sophisticated tools introduced for process algebra can be used to improve the effectiveness and the reliability of Web service development. Our investigations suggest a positive answer, and we claim that process algebras provide a very complete and satisfactory assistance to the whole process of Web service development. We show on a case study that readily available tools based on process algebra are effective at verifying that Web services conform to their requirements and respect properties. We advocate their use both at the design stage and for reverse engineering issues. More prospectively, we discuss how they can be helpful to tackle choreography issues.

When are two web services compatible

Whether two web services are compatible depends not only on static properties like the correct typing of their message parameters, but also on their dynamic behaviour. Providing a simple description of the service behaviour based on process-algebraic or automata-based formalisms can help detecting many subtle incompatibilities in their interaction. Moreover, this compatibility checking can to a large extent be automated if we define the notion of compatibility in a sufficiently formal way. Based on a simple behavioural representation, we survey, propose and compare a number of formal definitions of the compatibility notion, and we illustrate them on simple examples.

Using process algebra for web services: early results and perspectives

Web services are computational entities distributed on the web whose goal is to cooperate in order to work out simple or complex tasks. In this paper, we advocate the use of process algebra as an abstract and formal description formalism to tackle several issues raised in the context of web services. Abstract processes are helpful to describe services at different levels of expressiveness depending on the goal at hand and to compose them in order to build more complicated services. A great interest of using process algebra is that formal reasoning is made possible at any time and for many purposes (e.g. composition correctness) thanks to the existence of state-of-the-art tools. Abstract descriptions may also be used as a first step to develop certified web services following a well-defined method. We discuss all these ideas in this paper, reinforcing them with simple examples.

Exploiting Fixable, Removable, and Implied Values in Constraint Satisfaction Problems

Complete algorithms for constraint solving typically exploit properties like (in)consistency or interchangeability, which they detect by means of incomplete yet effective algorithms and use to reduce the search space. In this paper, we study a wide range of properties which includes most of the ones used by existing CSP algorithms as well as some which have not yet been considered in the literature, and we investigate their use in CSP solving. We clarify the relationships between these notions and characterise the complexity of the problem of checking them. Following the CSP approach, we then determine a number of relaxations (for instance local versions) which provide sufficient conditions whose detection is tractable. This work is a first step towards a comprehensive framework for CSP properties, and it also shows that new notions still remain to be exploited.

Computing equilibria using interval constraints

Finding Nash equilibria is a hard computational problem which is central to game theory and whose applications range from decision-making to the analysis of multi-agent systems. Despite considerable recent interest and significant recent improvements, the problem remains essentially open in the case of n-person games. We investigate the use of interval-based constraint solving techniques to compute equilibria. We report on experiments made using several encodings of randomly-generated games into continuous CSP, and draw conclusions regarding both the scalability of interval methods for game-theoretic applications and the impact of the symbolic representation of polynomials and of the choice of the propagation technique on the speed of resolution.

Towards automated reasoning on the properties of numerical constraints

A general approach to improving constraint solving is to take advantage of information on the structure and particularities of the considered constraints. Specific properties can determine the use of customized solvers, or they can be used to improve solver cooperation and propagation strategies. We propose a framework in which properties are seen as abstractions of the underlying constraints, and relate them to the literature on abstract reasoning. We mainly exemplify this framework on numerical constraints, where such properties as monotonicity and convexity are important. In particular, we show how deductions can be made on such constraints to dynamically infer properties. We overview connections with recent works, and we give guidelines and examples on how this kind of tool can be integrated into existing or customized constraint-solvers.