8 research outputs found
A lower bound on web services composition
A web service is modeled here as a finite state machine. A composition
problem for web services is to decide if a given web service can be constructed
from a given set of web services; where the construction is understood as a
simulation of the specification by a fully asynchronous product of the given
services. We show an EXPTIME-lower bound for this problem, thus matching the
known upper bound. Our result also applies to richer models of web services,
such as the Roman model
Reducing Clocks in Timed Automata while Preserving Bisimulation
Model checking timed automata becomes increasingly complex with the increase
in the number of clocks. Hence it is desirable that one constructs an automaton
with the minimum number of clocks possible. The problem of checking whether
there exists a timed automaton with a smaller number of clocks such that the
timed language accepted by the original automaton is preserved is known to be
undecidable. In this paper, we give a construction, which for any given timed
automaton produces a timed bisimilar automaton with the least number of clocks.
Further, we show that such an automaton with the minimum possible number of
clocks can be constructed in time that is doubly exponential in the number of
clocks of the original automaton.Comment: 28 pages including reference, 8 figures, full version of paper
accepted in CONCUR 201
A parametric analysis of the state-explosion problem in model checking
AbstractIn model checking, the state-explosion problem occurs when one checks a nonflat system, i.e., a system implicitly described as a synchronized product of elementary subsystems. In this paper, we investigate the complexity of a wide variety of model-checking problems for nonflat systems under the light of parameterized complexity, taking the number of synchronized components as a parameter. We provide precise complexity measures (in the parameterized sense) for most of the problems we investigate, and evidence that the results are robust
Behavioural Preorders on Stochastic Systems - Logical, Topological, and Computational Aspects
Computer systems can be found everywhere: in space, in our homes, in our
cars, in our pockets, and sometimes even in our own bodies. For concerns of
safety, economy, and convenience, it is important that such systems work
correctly. However, it is a notoriously difficult task to ensure that the
software running on computers behaves correctly.
One approach to ease this task is that of model checking, where a model of
the system is made using some mathematical formalism. Requirements expressed in
a formal language can then be verified against the model in order to give
guarantees that the model satisfies the requirements.
For many computer systems, time is an important factor. As such, we need our
formalisms and requirement languages to be able to incorporate real time.
We therefore develop formalisms and algorithms that allow us to compare and
express properties about real-time systems. We first introduce a logical
formalism for reasoning about upper and lower bounds on time, and study the
properties of this formalism, including axiomatisation and algorithms for
checking when a formula is satisfied.
We then consider the question of when a system is faster than another system.
We show that this is a difficult question which can not be answered in general,
but we identify special cases where this question can be answered. We also show
that under this notion of faster-than, a local increase in speed may lead to a
global decrease in speed, and we take step towards avoiding this.
Finally, we consider how to compare the real-time behaviour of systems not
just qualitatively, but also quantitatively. Thus, we are interested in knowing
how much one system is faster or slower than another system. This is done by
introducing a distance between systems. We show how to compute this distance
and that it behaves well with respect to certain properties.Comment: PhD dissertation from Aalborg Universit
Construction incrémentale de spécifications de systèmes critiques intégrant des procédures de vérification
Cette thèse porte sur l'aide à la construction de machines d'états UML de systèmes réactifs. Elle vise à définir un cadre théorique et pragmatique pour mettre en œuvre une approche incrémentale caractérisée par une succession de phases de construction, évaluation et correction de modèles. Ce cadre offre des moyens de vérifier si un nouveau modèle est conforme à ceux définis durant les étapes précédentes sans avoir à demander une description explicite des propriétés à vérifier. Afin de pouvoir analyser les machines d'états, nous leur associons une sémantique LTS ce qui nous a conduit à définir une procédure de transformation automatique de machines d'états en LTS. Dans un premier temps, nous avons défini et implanté des techniques de vérification de relations de conformité de LTS (red, ext, conf, et confrestr). Dans un second temps, nous nous sommes intéressés à la définition d'un cadre de construction incrémentale dans lequel plusieurs stratégies de développement peuvent être mises en œuvre en s'assurant que le modèle final élaboré sera une implantation conforme à la spécification initiale. Ces stratégies reposent sur des combinaisons de raffinements qui peuvent être de deux types : le raffinement vertical pour éliminer l'indéterminisme et ajouter des détails ; le raffinement horizontal pour ajouter de nouvelles fonctionnalités sans ajouter d'indéterminisme. Enfin, nous transposons la problématique de construction incrémentale d'une machine d'états à la construction d'architectures dont les composants sont des machines d'états. Des conditions sont définies pour assurer la conformité entre des architectures dans le cas de la substitution de composants.This thesis focuses on supporting construction of UML state machines of reactive systems. It aims at developing a theoretic and pragmatic framework to implement an incremental approach characterized by a succession of construction, evaluation and correction of models. This framework provides the means to verify whether a new model is consistent with those defined in the previous steps without requiring an explicit description of the properties to be verified. To analyze the state machines, we associated with them a LTS semantics which led us to define a procedure for automatic transformation of state machines in LTS. Initially, we have defined and implemented verification technique of conformance relations on LTS (red, ext, conf and confrestr). In a second step, we have defined a framework for incremental construction in which several development strategies can be implemented ensuring that the final developed model will be an implementation consistent with the initial specification. These strategies are based on combination of refinements that may be of two types: vertical refinement to eliminate nondeterminism and add details, and the horizontal refinement to add new features without adding nondeterminism. Finally, we transpose the problem of incremental construction of state machines to the construction of architectures whose components are state machines. Conditions are defined to ensure conformance between architectures in the case of substitution of components