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Snapshot isolation (SI) is a widely used consistency model for transaction processing, implemented by most major databases and some of transactional memory systems. Unfortunately, its classical definition is given in a low-level operational way, by an idealised concurrency-control algorithm, and this complicates reasoning about the behaviour of applications running under SI. We give an alternative specification to SI that characterises it in terms of transactional dependency graphs of Adya et al., generalising serialisation graphs. Unlike previous work, our characterisation does not require adding additional information to dependency graphs about start and commit points of transactions. We then exploit our specification to obtain two kinds of static analyses. The first one checks when a set of transactions running under SI can be chopped into smaller pieces without introducing new behaviours, to improve performance. The other analysis checks whether a set of transactions running under a weakening of SI behaves the same as when running under SI.

Abstract. We propose a process calculus to model high level wireless systems, where the topology of a network is described by a digraph. The calculus enjoys features which are proper of wireless networks, namely broadcast communication and probabilistic behaviour.We first focus on the problem of composing wireless networks, then we present a compositional theory based on a probabilistic generalisation of the well known may-testing and must-testing preorders. Also, we define an extensional semantics for our calculus, which will be used to define both simulation and deadlock simulation preorders for wireless networks. We prove that our simulation preorder is sound with respect to the may-testing preorder; similarly, the deadlock simulation preorder is sound with respect to the musttesting preorder, for a large class of networks. We also provide a counterexample showing that completeness of the simulation preorder, with respect to the may testing one, does not hold. We conclude the paper with an application of our theory to probabilistic routing protocols.

Abstract. Modern Internet services often achieve scalability and availability by relying on large-scale distributed databases that provide consistency models for transactions weaker than serialisability. We investigate the classical problem of transaction chopping for a promising consistency model in this class-parallel snapshot isolation (PSI), which weakens the classical snapshot isolation to allow more efficient large-scale implementations. Namely, we propose a criterion for checking when a set of transactions executing on PSI can be chopped into smaller pieces without introducing new behaviours, thus improving efficiency. We find that our criterion is more permissive than the existing one for chopping serialisable transactions. To establish our criterion, we propose a novel declarative specification of PSI that does not refer to implementation-level concepts and, thus, allows reasoning about the behaviour of PSI databases more easily. Our results contribute to building a theory of consistency models for modern large-scale databases.

Abstract. We present a timed broadcast process calculus for wireless networks at the MAC-sublayer where time-dependent communications are exposed to collisions. We define a reduction semantics for our calculus which leads to a contextual equivalence for comparing the external behaviour of wireless networks. Further, we construct an extensional LTS (labelled transition system) which models the activities of stations that can be directly observed by the external environment. Standard bisimulations in this novel LTS provide a sound proof method for proving that two systems are contextually equivalent. In addition, the main contribution of the paper is that our proof technique is also complete for a large class of systems.

Abstract. We present a timed process calculus for modelling wireless networks in which individual stations broadcast and receive messages; moreover the broadcasts are subject to collisions. Based on a reduction semantics for the calculus we define a contextual equivalence to compare the external behaviour of such wireless networks. Further, we construct an extensional LTS (labelled transition system) which models the activities of stations that can be directly observed by the external environment. Standard bisimulations in this LTS provide a sound proof method for proving systems contextually equivalence. We illustrate the usefulness of the proof methodology by a series of examples. Finally we show that this proof method is also complete, for a large class of systems.