Thomas T. Hildebrandt

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.

Declarative Event-Based Workflow as Distributed Dynamic Condition Response Graphs

We present Dynamic Condition Response Graphs (DCR Graphs) as a declarative, event-based process model inspired by the workflow language employed by our industrial partner and conservatively generalizing prime event structures. A dynamic condition response graph is a directed graph with nodes representing the events that can happen and arrows representing four relations between events: condition, response, include, and exclude. Distributed DCR Graphs is then obtained by assigning roles to events and principals. We give a graphical notation inspired by related work by van der Aalst et al. We exemplify the use of distributed DCR Graphs on a simple workflow taken from a field study at a Danish hospital, pointing out their flexibility compared to imperative workflow models. Finally we provide a mapping from DCR Graphs to Buchi-automata.

Nested dynamic condition response graphs

We present an extension of the recently introduced declarative process model Dynamic Condition Response Graphs (DCR Graphs) to allow nested sub-graphs and a new milestone relation between events. The extension was developed during a case study carried out jointly with our industrial partner Exformatics, a danish provider of case and workflow management systems. We formalize the semantics by giving first a map from Nested to (flat) DCR Graphs with milestones, and then extending the previously given mapping from DCR Graphs to Buchi-automata to include the milestone relation.

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.

From Paper Based Clinical Practice Guidelines to Declarative Workflow Management

We present a field study of oncology workflow, involving doctors, nurses and pharmacists at Danish hospitals and discuss the obstacles, enablers and challenges for the use of computer based clinical practice guidelines. Related to the CIGDec approach of Pesic and van der Aalst we then describe how a sub workflow can be described in a declarative workflow management system: the Resultmaker Online Consultant (ROC). The example demonstrates that declarative primitives allow to naturally extend the paper based flowchart to an executable model without introducing a complex cyclic control flow graph.

Extending howe's method to early bisimulations for typed mobile embedded resources with local names

We extend Howes method to prove that input-early strong and -delay contextual bisimulations are congruences for the Higher-order mobile embedded resources (Homer) calculus, a typed higher order process calculus with active mobile processes, nested locations and local names which conservatively extends the syntax and semantics of higher-order calculi such as Plain CHOCS and HOpi. We prove that the input-early strong and -delay contextual bisimulation congruences are sound co-inductive characterisations of barbed bisimulation congruence and in fact complete in the strong case. The extension of Howes method provides considerably simpler congruence proofs than established previously for similar calculi for mobile processes in nested locations.

A CPS encoding of name-passing in higher-order mobile embedded resources

We present an encoding of the synchronous π-calculus in the calculus of Higher-order mobile embedded resources (Homer), a pure higher-order calculus with mobile processes in nested locations, defined as a simple, conservative extension of the core process-passing subset of Thomsens Plain CHOCS. We prove that our encoding is fully abstract with respect to barbed bisimulation and sound with respect to barbed congruence. Our encoding demonstrates that higher-order process-passing together with mobile resources in, possibly local, named locations are sufficient to express π-calculus name-passing. The encoding uses a novel continuation passing style to facilitate the encoding of synchronous communication.

Bigraphical Semantics of Higher-Order Mobile Embedded Resources with Local Names

Bigraphs have been introduced with the aim to provide a topographical meta-model for mobile, distributed agents that can manipulate their own linkages and nested locations, generalising both characteristics of the @p-calculus and the Mobile Ambients calculus. We give the first bigraphical presentation of a non-linear, higher-order process calculus with nested locations, non-linear active process mobility, and local names, the calculus of Higher-Order Mobile Embedded Resources (Homer). The presentation is based on Milners recent presentation of the @l-calculus in local bigraphs. The combination of non-linear active process mobility and local names requires a new definition of parametric reaction rules and a representation of the location of names. We suggest localised bigraphs as a generalisation of local bigraphs in which links can be further localised.

Formalizing higher-order mobile embedded business processes with binding bigraphs

We propose and formalize HomeBPEL, a higher-order WSBPEL-like business process execution language where processes are firstclass values that can be stored in variables, passed as messages, and activated as embedded sub-instances. A sub-instance is similar to a WSBPEL scope, except that it can be dynamically frozen and stored as a process in a variable, and then subsequently be thawed when reactivated as a sub-instance. We motivate HomeBPEL by an example of pervasive health care where treatment guidelines are dynamically deployed as sub processes that may be delegated dynamically to other workflow engines and in particular stay available for disconnected operation on mobile devices. We provide a formal semantics based on binding bigraphical reactive systems implemented in the BPL Tool as part of the Bigraphical Programming Languages project at ITU. The semantics is an extension of a semantics given previously for a simplified subset of WS-BPEL and exploits the close correspondence between bigraphs and XML to provide a formalized run-time format very close to standard WS-BPEL syntax, which also constitutes the representation of frozen sub-instances.

Contracts for Cross-organizational Workflows as Timed Dynamic Condition Response Graphs

We conservatively extend the declarative Dynamic Condition Response (DCR) Graph process model, introduced in the PhD thesis of the second author, to allow for discrete time deadlines. We prove that safety and liveness properties can be verified by mapping finite timed DCR Graphs to finite state transition systems. We exemplify how deadlines can introduce time-locks and deadlocks and violate liveness. We then prove that the general technique for safe distribution of DCR Graphs provided in previous work can be extended to timed DCR Graphs. We exemplify the use of timed DCR Graphs and the distribution technique in praxis on a timed extension of a cross-organizational case management process arising from a previous case study. The example shows how a timed DCR Graph can be used to describe the global contract for a timed workflow process involving several organizations, which can then be distributed as a network of communicating timed DCR Graphs describing the local contract for each organization.