To achieve scalability, modern Internet services often rely on distributed databases with consistency models for transactions weaker than serializability. At present, application programmers often lack techniques to ensure that the weakness of these consistency models does not violate application correctness. In this talk I will present criteria to check whether applications that rely on a database providing only weak consistency are robust, i.e., behave as if they used a database providing serializability, and I will focus on a consistency model called Parallel Snapshot Isolation. The results I will outline handle systematically and uniformly several recently proposed weak consistency models, as well as a mechanism for strengthening consistency in parts of an application.

Dans la littérature mathématique, le mot diffusion fait référence à plusieurs définitions différentes. Je m'intéresse à la diffusion dite “macroscopique” caractérisée par l'existence d'une densité de particules évoluant selon la loi de Fick. Je porterai mon attention sur le modèle des miroirs en dimension 2 et sur sa généralisation en dimension quelconque. Ce modèle de gaz de Lorentz sur réseau permet un énoncé clair et rigoureux de la loi de Fick. Une approche analytique nous a permis de simplifier le problème et de réaliser une étude numérique qui permet de conjecturer la validité de la loi de Fick en dimension 3. Je présenterai l'étude complète de façon très visuelle et abordable.

We study the theory of contextual equivalence in the untyped lambda-calculus, generated by taking the normal forms as observables. Introduced by Morris in 1968, this is the original extensional lambda theory H+ of observational equivalence. On the syntactic side, we show that this lambda-theory validates the omega-rule, thus settling a long-standing open problem. On the semantic side, we provide sufficient and necessary conditions for relational graph models to be fully abstract for H+. We show that a relational graph model captures Morris's observational pre-order exactly when it is extensional and lambda-König. Intuitively, a model is lambda-König when every lambda-definable tree has an infinite path which is witnessed by some element of the model.