Knowledge-based systems must be able to "intelligently" manage a large amount of information coming from different sources and at different moments in time. Intelligent systems must be able to cope with a changing world by adopting a "principled" strategy. Many formalisms have been put forward in the artificial intelligence (Al) and database (DB) literature to address this problem. Among them, belief revision is one of the most successful frameworks to deal with dynamically changing worlds. Formal properties of belief revision have been investigated by Alchourron, Gardenfors, and Makinson, who put forward a set of postulates stating the properties that a belief revision operator should satisfy. Among these properties, a basic assumption of revision is that the new piece of information is totally reliable and, therefore, must be in the revised knowledge base. Different principles must be applied when there are two different sources of information and each one has a different view of the situation-the two views contradicting each other. If we do not have any reason to consider any of the sources completely unreliable, the best we can do is to "merge" the two views in a new and consistent one, trying to preserve as much information as possible. We call this merging process arbitration. In this paper, we investigate the properties that any arbitration operator should satisfy. In the style of Alchourron, Gardenfors, and Makinson we propose a set of postulates, analyze their properties, and propose actual operators for arbitration

We argue that essential facets of Web Services (WSs), and especially, those useful to understand their interaction, are best described using process-algebraic notations. WS 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 languages such as Process Algebras (PAs). One legitimate question is therefore whether the WSs community can benefit from the sophisticated languages and tools developed in the PA area. Our investigations suggest a positive answer and we claim that PAs provide solutions to a number of challenges raised by the WSs paradigm, among which are central issues of orchestration and choreography. We show on a case study that readily available tools based on PA are effective at verifying that compositions of services obtained by choreography and orchestration conform their requirements and respect properties. We suggest a general framework based on a mapping between PA and WSs written in BPEL, and illustrate both the modelling of services by PA and the use of reasoning tools.