Søren Debois

Bigraphical models of context-aware systems

As part of ongoing work on evaluating Milners bigraphical reactive systems, we investigate bigraphical models of context-aware systems, a facet of ubiquitous computing. We find that naively encoding such systems in bigraphs is somewhat awkward; and we propose a more sophisticated modeling technique, introducing Plato-graphical models, alleviating this awkwardness. We argue that such models are useful for simulation and point out that for reasoning about such bigraphical models, the bisimilarity inherent to bigraphical reactive systems is not enough in itself; an equivalence between the bigraphical reactive systems themselves is also needed.

A model checker for Bigraphs

We present a model checking tool for Bigraphical Reactive Systems that may be instantiated as a model checker for any formalism or domain-specific modelling language encoded as a Bigraphical Reactive System. We describe the implementation of the tool, and how it can be used to verify correctness properties of some infinite-state models by applying a static analysis to reaction rules that permits the exclusion of some infinite branches of execution shown to always be free of violations. We give a proof of correctness for this method, and illustrate the usage of the tool with two examples --- a textbook implementation of the Dining Philosophers problem, and an example motivated by a ubiquitous computing application.

Blockchains for Business Process Management - Challenges and Opportunities

Blockchain technology offers a sizable promise to rethink the way interorganizational business processes are managed because of its potential to realize execution without a central party serving as a single point of trust (and failure). To stimulate research on this promise and the limits thereof, in this article, we outline the challenges and opportunities of blockchain for business process management (BPM). We first reflect how blockchains could be used in the context of the established BPM lifecycle and second how they might become relevant beyond. We conclude our discourse with a summary of seven research directions for investigating the application of blockchain technology in the context of BPM.

A Case for Declarative Process Modelling: Agile Development of a Grant Application System

We report on a recent industrial project carried out by Exformatics A/S in which the company used the declarative DCR Graphs notation to model and implement the grant application process of a Danish foundation. We present the process and discuss the advantages of the approach and challenges faced both while modelling and implementing the process. Finally, we discuss current work on extensions to the DCR Graphs notation aiming to address the challenges raised by the case study and to support the declarative, agile approach.

A Graphical Approach to Progress for Structured Communication in Web Services

We investigate a graphical representation of session invocation interdependency in order to prove progress for the pi-calculus with sessions under the usual session typing discipline. We show that those processes whose associated dependency graph is acyclic can be brought to reduce. We call such processes transparent processes. Additionally, we prove that for well-typed processes where services contain no free names, such acyclicity is preserved by the reduction semantics. Our results encompass programs (processes containing neither free nor restricted session channels) and higher-order sessions (delegation). Furthermore, we give examples suggesting that transparent processes constitute a large enough class of processes with progress to have applications in modern session-based programming languages for web services.

Sortings for reactive systems

We investigate sorting or typing for Leifer and Milners reactive systems. We focus on transferring congruence properties for bisimulations from unsorted to sorted systems. Technically, we give a general definition of sorting; we adapt Jensens work on the transfer of congruence properties to this general definition; we construct a predicate sorting, which for any decomposible predicate P filters out agents not satisfying P; we prove that the predicate sorting preserves congruence properties and that it suitably retains dynamics; and finally, we show how the predicate sortings can be used to achieve context-aware reaction.

Safety, Liveness and Run-Time Refinement for Modular Process-Aware Information Systems with Dynamic Sub Processes

We study modularity, run-time adaptation and refinement under safety and liveness constraints in event-based process models with dynamic sub-process instantiation. The study is part of a larger programme to provide semantically well-founded technologies for modelling, implementation and verification of flexible, run-time adaptable processaware information systems, moved into practice via the Dynamic Condition Response (DCR) Graphs notation co-developed with our industrial partner. Our key contributions are: (1) A formal theory of dynamic subprocess instantiation for declarative, event-based processes under safety and liveness constraints, given as the DCR* process language, equipped with a compositional operational semantics and conservatively extending the DCR Graphs notation; (2) an expressiveness analysis revealing that the DCR* process language is Turing-complete, while the fragment corresponding to DCR Graphs (without dynamic sub-process instantiation) characterises exactly the languages that are the union of a regular and an omega-regular language; (3) a formalisation of run-time refinement and adaptation by composition for DCR* processes and a proof that such refinement is undecidable in general; and finally (4) a decidable and practically useful sub-class of run-time refinements. Our results are illustrated by a running example inspired by a recent Electronic Case Management solution based on DCR Graphs and delivered by our industrial partner. An online prototype implementation of the DCR* language (including examples from the paper) and its visualisation as DCR Graphs can be found at http://tiger.itu.dk:8020/.

Hierarchical Declarative Modelling with Refinement and Sub-processes

We present a new declarative model with composition and hierarchical definition of processes, featuring (a) incremental refinement, (b) adaptation of processes, and (c) dynamic creation of sub-processes. The approach is motivated and exemplified by a recent case management solution delivered by our industry partner Exformatics A/S. The approach is achieved by extending the Dynamic Condition Response (DCR) graph model with interfaces and composition along those interfaces. Both refinement and sub-processes are then constructed in terms of that composition. Sub-processes take the form of hierarchical (complex) events, which dynamically instantiate sub-processes. The extensions are realised and supported by a prototype simulation tool.

On the Construction of Sorted Reactive Systems

We develop a theory of sorted bigraphical reactive systems. Every application of bigraphs in the literature has required an extension, a sorting, of pure bigraphs. In turn, every such application has required a redevelopment of the theory of pure bigraphical reactive systems for the sorting at hand. Here we present a general construction of sortings. The constructed sortings always sustain the behavioural theory of pure bigraphs (in a precise sense), thus obviating the need to redevelop that theory for each new application. As an example, we recover Milners local bigraphs as a sorting on pure bigraphs. Technically, we give our construction for ordinary reactive systems, then lift it to bigraphical reactive systems. As such, we give also a construction of sortings for ordinary reactive systems. This construction is an improvement over previous attempts in that it produces smaller and much more natural sortings, as witnessed by our recovery of local bigraphs as a sorting.

A verification environment for bigraphs

We present the BigMC tool for bigraphical reactive systems that may be instantiated as a verification tool for any formalism or domain-specific modelling language encoded as a bigraphical reactive system. We introduce the syntax and use of BigMC, and exemplify its use with two small examples: a textbook “philosophers” example, and an example motivated by a ubiquitous computing application. We give a tractable heuristic with which to approximate interference between reaction rules, and prove this analysis to be safe. We provide a mechanism for state reachability checking of bigraphical reactive systems, based upon properties expressed in terms of matching, and describe a checking algorithm that makes use of the causation heuristic.