Semantics, Verification, and Implementation of Workflows ... - YAWL

Abstract
Workflow systems aim to provide automated support for the conduct of certain
business processes. Workflow systems are driven by workflow specifications
which among others, capture the execution interdependencies between various
activities. These interdependencies are modelled by means of different control
flow constructors, e.g., sequence, choice, parallelism and synchronisation. It has
been shown in the research on workflow patterns that the support for and the
interpretation of various control flow constructs varies substantially across workflow
systems. Two of the most problematic patterns relate to the OR-join and
to cancellation.
An OR-join is used in situations when we need to model “wait and see” behaviour
for synchronisation. Different approaches assign a different (often only
intuitive) semantics to this type of join, though they do share the common theme
that synchronisation is only to be performed for active paths. Depending on
context assumptions this behaviour may be relatively easy to deal with, though
in general its semantics is complicated, both from a definition point of view (in
terms of formally capturing a desired intuitive semantics) and from a computational
point of view (how does one determine whether an OR-join is enabled).
Many systems and languages struggle with the semantics and implementation of
the OR-join because its non-local semantics require a synchronisation depending
on an analysis of future execution paths. This may require some non-trivial reasoning.
The presence of cancellation features and other OR-joins in a workflow
further complicates the formal semantics of the OR-join. The cancellation feature
is commonly used to model external events that can change the behaviour
of a running workflow. It can be used to either disable activities in certain parts
of a workflow or to stop currently running activities. Even though it is possible
to cancel activities in workflow systems using some sort of abort function, many
workflow systems do not provide direct support for this feature in the workflow
language. Sometimes, cancellation affects only a selected part of a workflow and
other activities can continue after performing a cancellation action. As cancellation
occurs naturally in business scenarios, comprehensive support in a workflow
language is desirable. We take on the challenge of providing formal semantics,
verification techniques as well as an implementation for workflows with those
features.
This thesis addresses three interrelated issues for workflows with cancellation
regions and OR-joins. The concept of the OR-join is examined in detail in the
context of the workflow language YAWL, a powerful workflow language designed
to support a collection of workflow patterns and inspired by Petri nets. The ORiii