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In this paper, we analyze the logical definitions of model-based diagnosis recently presented in the literature, and we propose a unified framework (based on the integration of abductive and consistency-based reasoning) in which most of such definitions can be captured. This allows us to single out the existence of a spectrum of alternatives in the logical definition of diagnosis. A lot of attention in the paper is devoted to analyzing the differences among the definitions in the spectrum. In particular, we show that the definitions can be compared on the basis of their restrictiveness and we relate such a restrictiveness with the completeness of the model of the system to be diagnosed. Key words: model-based reasoning, theory of diagnosis, abductive reasoning, consistency-based reasoning.Dans cet article, les auteurs analysent les dkfinitions logiques de diagnostics bases sur un modtle dont il a kte question rkcemment dans certains ouvrages. 11s proposent un cadre unifie (bask sur I'intkgration du raisonnement abductif et du raisonnement base sur la consistance) B I'interieur duquel la plupart de ces definitions peuvent Otre regroupees. %ette particularit6 permet de mettre en lumitre I'existence d'un spectre d'alternatives dans la definition logique du diagnostic.Cet article accorde une attention toute particulikre B I'analyse des diffkrences entre les dkfinitions du spectre. En outre, les auteurs dkmontrent que les definitions peuvent 2tre comparkes en fonction de leur caracttre restrictif, qui est ensuite mis en relation avec la complktude du modkle du systtme faisant I'objet d'un diagnostic.Mots clPs : raisonnement bask sur un modtle, thtorie du diagnostic, raisonnement abductif, raisonnement base sur la consistance.

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The paper introduces and discusses the notion of decomposition of a configuration problem within the framework of a structured logical approach. The paper describes under which conditions a given configuration problem can be decomposed into a set of noninteracting subproblems and how to exploit such a decomposition, both for improving the performance of the configurator and for supporting interactive configuration. Different kinds of decomposition are considered, but all of them exploit, as much as possible, the explicit representation of the partonomic relations in the FPC language, a KL-One like representation formalism augmented with constraints for expressing complex interrole relations. The paper introduces a notion of boundness among constraints, which is used for formally specifying different types of decomposition. One decomposition strategy aims at singling out the components and subcomponents that are directly related to the constraints put by the user's requirements; the configurator exploits such decomposition by first configuring that portion of the product and then configuring the parts that are not related to the user's requirements. Another decomposition strategy verifies whether the set of constraints for the product to be configured can be split into a set of noninteracting problems. In such a case the configurator solves the configuration problem by splitting the whole search space into a set of smaller search spaces. Different combinations of these two decomposition techniques are considered, and the impact of the decomposition strategies on the performance of the configurator is evaluated via a set of experiments using the configuration of computer systems as a test bed. The results of the experiments show a significant reduction of the computational effort~both in terms of number of backtrackings and in CPU time! when decomposition strategies are used.