Extending the ERS approach for workflow modelling in Event-B

Abstract

The Event Refinement Structures (ERS) approach augments the Event-B formal method with hierarchical diagrams, providing explicit support for control fow and refinement relationships. ERS was originally designed to decompose the atomicity of the events in Event-B and later enriched with control flow combinators.
Combining graphical workflow approaches with formal methods has been a subject of interest in both industry and academia, resulting in a diversity of approaches. In this thesis, we present an approach for workflow modelling that addresses both control flow and data handling. ERS is used for control flow, while Event-B mathematical notation supports the data handling. This separation simplifies the modelling by avoiding an extensive number of patterns, though separation does not mean the independence of control flow from data handling. The dependency is achieved by the ERS semantics, which are acquired by transforming the diagrams to Event-B. This combination not only benefits from the verification capabilities of Event-B and the graphical nature of ERS, but also supports incremental modelling through refinement and hierarchy.
Our studies resulted in extending the ERS approach to support more flexible behaviour like unbounded replication and exception handling. Unbounded replication is needed when the number of instances of a flow to be executed is unknown and additional instances can be initiated during execution. We also enhance some of the existing ERS combinators such as the loop. We validate our approach and extensions by applying them to two complex work flows, the fire dispatch system and the travel agency booking system. Finally, we extend the ERS formal language with new translation rules to support our new ERS extensions. We formally define the new translation rules of ERS to Event-B, using the Augmented Backus-Naur Form (ABNF), to be easily integrated in the ERS plug-in. The ERS plug-in is a tool providing automatic generation of part of the Event-B model representing types and sequencing. We also evaluate the ERS combinators in control flow modelling against already published criteria.

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Identifiers

ORCID for Dana Dghaym: orcid.org/0000-0002-2196-2749

ORCID for Michael Butler: orcid.org/0000-0003-4642-5373

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