88 research outputs found
On the Executability of Interactive Computation
The model of interactive Turing machines (ITMs) has been proposed to
characterise which stream translations are interactively computable; the model
of reactive Turing machines (RTMs) has been proposed to characterise which
behaviours are reactively executable. In this article we provide a comparison
of the two models. We show, on the one hand, that the behaviour exhibited by
ITMs is reactively executable, and, on the other hand, that the stream
translations naturally associated with RTMs are interactively computable. We
conclude from these results that the theory of reactive executability subsumes
the theory of interactive computability. Inspired by the existing model of ITMs
with advice, which provides a model of evolving computation, we also consider
RTMs with advice and we establish that a facility of advice considerably
upgrades the behavioural expressiveness of RTMs: every countable transition
system can be simulated by some RTM with advice up to a fine notion of
behavioural equivalence.Comment: 15 pages, 0 figure
Lending Petri nets and contracts
Choreography-based approaches to service composition typically assume that,
after a set of services has been found which correctly play the roles
prescribed by the choreography, each service respects his role. Honest services
are not protected against adversaries. We propose a model for contracts based
on a extension of Petri nets, which allows services to protect themselves while
still realizing the choreography. We relate this model with Propositional
Contract Logic, by showing a translation of formulae into our Petri nets which
preserves the logical notion of agreement, and allows for compositional
verification
A Branching Time Model of CSP
I present a branching time model of CSP that is finer than all other models
of CSP proposed thus far. It is obtained by taking a semantic equivalence from
the linear time - branching time spectrum, namely divergence-preserving coupled
similarity, and showing that it is a congruence for the operators of CSP. This
equivalence belongs to the bisimulation family of semantic equivalences, in the
sense that on transition systems without internal actions it coincides with
strong bisimilarity. Nevertheless, enough of the equational laws of CSP remain
to obtain a complete axiomatisation for closed, recursion-free terms.Comment: Dedicated to Bill Roscoe, on the occasion of his 60th birthda
Report on the Standardization Project ``Formal Methods in Conformance Testing''
This paper presents the latest developments in the “Formal Methods in Conformance
Testing” (FMCT) project of ISO and ITU–T. The project has been initiated to study
the role of formal description techniques in the conformance testing process. The goal
is to develop a standard that defines the meaning of conformance in the context of formal
description techniques. We give an account of the current status of FMCT in the
standardization process as well as an overview of the technical status of the proposed
standard. Moreover, we indicate some of its strong and weak points, and we give some
directions for future work on FMCT
On coalgebras with internal moves
In the first part of the paper we recall the coalgebraic approach to handling
the so-called invisible transitions that appear in different state-based
systems semantics. We claim that these transitions are always part of the unit
of a certain monad. Hence, coalgebras with internal moves are exactly
coalgebras over a monadic type. The rest of the paper is devoted to supporting
our claim by studying two important behavioural equivalences for state-based
systems with internal moves, namely: weak bisimulation and trace semantics.
We continue our research on weak bisimulations for coalgebras over order
enriched monads. The key notions used in this paper and proposed by us in our
previous work are the notions of an order saturation monad and a saturator. A
saturator operator can be intuitively understood as a reflexive, transitive
closure operator. There are two approaches towards defining saturators for
coalgebras with internal moves. Here, we give necessary conditions for them to
yield the same notion of weak bisimulation.
Finally, we propose a definition of trace semantics for coalgebras with
silent moves via a uniform fixed point operator. We compare strong and weak
bisimilation together with trace semantics for coalgebras with internal steps.Comment: Article: 23 pages, Appendix: 3 page
Justness: A Completeness Criterion for Capturing Liveness Properties (Extended Abstract)
This paper poses that transition systems constitute a good model of
distributed systems only in combination with a criterion telling which paths
model complete runs of the represented systems. Among such criteria, progress
is too weak to capture relevant liveness properties, and fairness is often too
strong; for typical applications we advocate the intermediate criterion of
justness. Previously, we proposed a definition of justness in terms of an
asymmetric concurrency relation between transitions. Here we define such a
concurrency relation for the transition systems associated to the process
algebra CCS as well as its extensions with broadcast communication and signals,
thereby making these process algebras suitable for capturing liveness
properties requiring justness.Comment: An extended abstract of this paper appears in Proc. FoSSaCS'1
Applications of Fair Testing
In this paper we present the application of the fair testing pre-order, introduced in a previous paper, to the specification and analysis of distributed systems. This pre-order combines some features of the standard testing pre-orders, viz. the possibility to refine a specification by the resolution of nondeterminism, with a powerful feature of standard observation congruence, viz. the fair abstraction from divergences. Moreover, it is a pre-congruence with respect to all standard process-algebraic combinators, thus allowing for the standard algebraic proof techniques by substitution and rewriting. In this paper we will demonstrate advantages of the fair testing pre-order by the application to a number of examples, including a scheduling problem, a version of the Alternating Bit-protocol, and fair communication channels
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