On CSP and the Algebraic Theory of Effects

We consider CSP from the point of view of the algebraic theory of effects, which classifies operations as effect constructors or effect deconstructors; it also provides a link with functional programming, being a refinement of Moggi's seminal monadic point of view. There is a natural algebraic theory of the constructors whose free algebra functor is Moggi's monad; we illustrate this by characterising free and initial algebras in terms of two versions of the stable failures model of CSP, one more general than the other. Deconstructors are dealt with as homomorphisms to (possibly non-free) algebras. One can view CSP's action and choice operators as constructors and the rest, such as concealment and concurrency, as deconstructors. Carrying this programme out results in taking deterministic external choice as constructor rather than general external choice. However, binary deconstructors, such as the CSP concurrency operator, provide unresolved difficulties. We conclude by presenting a combination of CSP with Moggi's computational {\lambda}-calculus, in which the operators, including concurrency, are polymorphic. While the paper mainly concerns CSP, it ought to be possible to carry over similar ideas to other process calculi

Compositional schedulability analysis of real-time actor-based systems

We present an extension of the actor model with real-time, including deadlines associated with messages, and explicit application-level scheduling policies, e.g.,"earliest deadline first" which can be associated with individual actors. Schedulability analysis in this setting amounts to checking whether, given a scheduling policy for each actor, every task is processed within its designated deadline. To check schedulability, we introduce a compositional automata-theoretic approach, based on maximal use of model checking combined with testing. Behavioral interfaces define what an actor expects from the environment, and the deadlines for messages given these assumptions. We use model checking to verify that actors match their behavioral interfaces. We extend timed automata refinement with the notion of deadlines and use it to define compatibility of actor environments with the behavioral interfaces. Model checking of compatibility is computationally hard, so we propose a special testing process. We show that the analyses are decidable and automate the process using the Uppaal model checke

Successful Termination in Timed CSP

In previous work the authors investigated the inconsistencies of how successful termination was modelled in Hoare, Brookes and Roscoe's original CSP. This led to the definition of a variant of CSP, called CSPt. CSPt presents a solution to these problems by means of adding a termination axiom to the original process axioms. In this paper we investigate how successful process termination is modelled in Reed and Roscoe's Timed CSP, which is the temporal version of Hoare's original untimed CSP. We discuss the issues that need to be considered when selecting termination axioms for Timed CSP, based on our experiences in defining CSPt. The outcome of this investigation and discussion is a collection of candidate successful termination axioms that could be added to the existing Timed CSP models, leading to an improved treatment of successful termination within the Timed CSP framework. We outline how these termination axioms would be added to the family of semantic models for Timed CSP. Finally, we outline what further work needs to be done once these new models for Timed CSP have been defined. For example, it would then be possible to define timed versions of the new more flexible parallel operators introduced in CSPt

Failure Trace Semantics for a Process Algebra with Time-outs

This paper extends a standard process algebra with a time-out operator, thereby increasing its absolute expressiveness, while remaining within the realm of untimed process algebra, in the sense that the progress of time is not quantified. Trace and failures equivalence fail to be congruences for this operator; their congruence closure is characterised as failure trace equivalence

Failure Trace Semantics for a Process Algebra with Time-outs

This paper extends a standard process algebra with a time-out operator, thereby increasing its absolute expressiveness, while remaining within the realm of untimed process algebra, in the sense that the progress of time is not quantified. Trace and failures equivalence fail to be congruences for this operator; their congruence closure is characterised as failure trace equivalence

Modeling and verification of web service composition based interorganizational workflows

Interorganisationale Workflows sind Arbeitsabläufe, welche die Grenzen einer Organisation verlassen und einen Rahmen für Kooperationen der verschiedenen autonomen Organisationen zur Verfügung stellen. Ein wichtiger Punkt für den Entwurf solcher Workflows ist die Balance zwischen Offenheit und Abgrenzung, wobei erstere für Kooperationen und letztere die für den Schutz von Know-how benötigt wird. Workflow Sichten stellen ein effizientes Werkzeug für diesen Zweck zur Verfügung. Durch Offenlegung von bestimmten Teilen eines Prozesses, können Organisationen sowohl kooperieren als auch das Know-how schützen. Diese Dissertation präsentiert nun eine Methode für die korrekte Konstruktion von Workflow Sichten. Es wird angenommen, dass Organisationen Web Service orientierte Technologien zur Modellierung und Implementierung von interorganisationalen Workflows verwenden. Die Anwendung von Web Services bietet Organisationen viele Vorteile. Den eigentlichen Mehrwert von Web Services stellt aber die Kompositionsfähigkeit dar. Verfügbare Web Services können dadurch von anderen Choreographien und Orchestrationen (wieder-)verwendet werden. Die Notwendigkeit der Implementierung von Systemen von Null weg kann minimiert werden. Die zentralen Anforderungen sind einerseits eine Architektur mit adäquatem Potential, andererseits die Verifikation der Korrektheit. Diese Dissertation präsentiert nun eine Architektur zur Modellierung von Web Service Composition basierten interorganisationalen Workflows, genannt föderierte Choreographien, die verglichen mit anderen Architekturen verschiedene Vorteile anbieten. Darüber hinaus werden Algorithmen und Techniken zur Verifikation der strukturellen und temporalen Korrektheit vorgestellt. Strukturelle Korrektheit prüft, ob die Strukturen der beteiligten Prozesse zusammenpassen. Temporale Korrektheit überprüft, ob ein interorganisationaler Workflow, der aus mehreren Choreographien und Orchestrationen besteht hinsichtlich der lokalen und globalen Bedingungen fehlerfrei ist. Mit Hilfe dieser Techniken kann die strukturelle und temporale Konformität des Modells zur Designzeit überprüft werden. Falls das Modell nicht strukturell oder temporal konform ist, können nötige Änderungen durchgeführt werden, sodass die korrekte Ausführung zur Laufzeit garantiert werden kann. Die Überprüfung der Konformität zur Designzeit reduziert die Prozesskosten vor allem wegen den folgenden zwei Gründen: Erstens, die entdeckten Fehler zur Designzeit sind normalerweise billiger als jene, die zur Laufzeit entdeckt werden und zweitens, Fehlerbehandlungsmechanismen können verhindert werden, die wiederum Zusatzkosten verursachen. Zusätzlich zu der vorgestellten Architektur wird eine allgemeinere Architektur zusammen mit den passenden Konformitätsprüfungsalgorithmen präsentiert. Der Ansatz ist Platform- und sprachunabhängig und die Algorithmen sind verteilt.Interorganizational workflows are workflows that cross the boundaries of a single organization and provide a framework for cooperation of different autonomous organizations. An important issue when designing such workflows is the balance between the openness needed for cooperation and the privacy needed for protection of business know-how. Workflow views provide an efficient tool for this aim. By exposure of only selected parts of a process, organizations can both cooperate and protect their business logic. This dissertation presents a technique for a correct construction of workflow views. It is assumed that organizations and partners use web services and web service related technology to model and implement interorganizational workflows. Application of web services offers several advantages for organizations. The real surplus of web services is their capability of being composed to more complex systems. Available web services can be reused by other choreographies and orchestrations and the need for development of new systems from scratch can be minimized. The essential requirements are on the one hand an architecture with adequate capabilities and on the other hand, verification of correctness. This dissertation proposes an architecture for modeling web service composition based interorganizational workflows, called \emph{federated choreographies}, that provides several advantages compared to existing proposals. Moreover, algorithms and techniques for verification of structural and temporal correctness of interorganizational workflows are proposed. Structural conformance checks if the structures of the involved processes match. Temporal conformance checks if an interorganizational workflow composed of choreographies and orchestrations is temporally error-free with respect to local and global temporal constraints. The proposed algorithms can be applied for checking the structural and temporal conformance of the federated choreographies at design-time. If the model is not structurally or temporally conformant, necessary modifications can be done such that the correct execution of the flow at run-time can be guaranteed. The conformance checking at design time reduces the cost of process because of two reasons: first, errors detected at design time are normally cheaper than those detected at run time and second, exception handling mechanisms can be avoided which are, in turn, coupled with additional costs. In addition to the proposed architecture, a more general architecture together with the conformance checking algorithms and techniques for interorganizational workflows are presented. The presented approach is language and platform independent and algorithms work in a distributed manner

The timed failures - stability model for CSP

We present a mathematical model which is the most abstract allowing (i) a fully compositional semantics for timed CSP and (ii) a natural abstraction map into the standard failures/divergences model of untimed CSP. We discuss in detail the construction and properties of this model, and explore the variety of nondeterministic behaviour it encompasses. We argue that, at least in some sense, this model is definitive for timed CSP