Chris Tofts

Processes with Probabilities, Priority and Time

A probabilistic calculus, derived from Milners SCCS, WSCCS is presented. We define a notion of bisimulation for probabilistic processes and show that it is a congruence. A simple equational characterisation is shown to be both sound and complete for finite processes. We present many examples including some extended ones. The larger examples show both the expressive power of WSCCS and the availability of simple proof methods for some complex systems.

A Calculus and logic of resources and processes

Recent advances in logics for reasoning about resources provide a new approach to compositional reasoning in interacting systems. We present a calculus of resources and processes, based on a development of Milner’s synchronous calculus of communication systems, SCCS, that uses an explicit model of resource. Our calculus models the co-evolution of resources and processes with synchronization constrained by the availability of resources. We provide a logical characterization, analogous to Hennessy–Milner logic’s characterization of bisimulation in CCS, of bisimulation between resource processes which is compositional in the concurrent and local structure of systems.

Systems Modelling via Resources and Processes: Philosophy, Calculus, Semantics, and Logic

We describe a programme of research in resource semantics, concurrency theory, bunched logic, and stochastic processes, as applied to mathematical systems modelling. Motivated by a desire for structurally and semantically rigorous discrete event modelling tools, applicable to enterprise-scale as well as component-scale systems, we introduce a new approach to compositional reasoning based on a development of SCCS with an explicit model of resource. Our calculus models the co-evolution of resources and processes with synchronization constrained by the availability of resources. We provide a simple denotational semantics as a parametrization of Abramskys synchronization trees semantics for SCCS. We also provide a logical characterization, analogous to Hennessy-Milner logics characterization of bisimulation in CCS, of bisimulation between resource processes which is compositional in the concurrent and local structure of systems. We discuss applications to ideas such as location and access control.

Errata for Formal Aspects of Computing (2006) 18: 495---517 and their consequences

We present a correction for an error that occurs in the following paper Formal Aspects of Computing (2006) 18:495–517. Atfirst sight, the error appears tobe simply amisplacedquantifier in thedefinitionof bisimulation.Weexplain, however, that the error and its correction reveal a subtle interaction between the substructural connectives of MBI and the resource–process calculus SCRP. We begin with a specific example which illustrates the error. We include also the known typographical errors. We include also a statement of the consequences of these errata for the paperElectronic Notes in Theoretical Computer Science 172, 545–587, 2007, which builds directly upon Formal Aspects of Computing (2006) 18:495–517 and which illustrates the significance of these errata.

Symbolic approaches to probability distributions in process algebra

Abstract. One of the major problems in modelling with process algebras is that of state explosion. With the addition of information describing duration, models become more exposed to growth in the size of the description of their behaviour. We describe a symbolic approach to the description of duration information within a probabilistic calculus that has two benefits: firstly we can describe behaviour without instantiating distributions; secondly we can control the growth in our system representation as a trade against model accuracy. We can compute upper bounds on performance without the inclusion of additional state information in our systems. A large example, from the discrete event simulation literature, is presented to demonstrate the effectiveness of the approach.

Process Algebra as Modelling

Abstract After 25 years of research (19 personally) into process algebras, I ask what areas of mathematics other than the analysis of concurrent computation could or indeed should its basic approach be applied? In particular, I identify the two areas of reductionism and model comprehension, upon which I believe that process algebra has the potential to have a major impact.

Taking a RaSP to Enterprise Stakeholder Dissonance

We present a model based approach to the resolution of conflict between the many stakeholders involved in large scale information system deployment and use. The work is based on the authors experience of more than

Efficiently Modelling Resource in a Process Algebra

20B of outsourcing contracts. The approach, rapid scenario planning (RaSP) yields concrete evidence of the underlying assumptions that drive design and deployment, as well as critical insights into systems and business dynamics. Social and long term management and governance benefits are also obvious. Our implementation of the methodology is supported by specific tools, however equivalent general purpose tools can be used to support this methodology.