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

Chapter 1. Introduction
OR-joins poses new challenges for deciding the correctness of workflows. Their introduction
may lead to new properties that need to be checked, the design of new
algorithmic approaches and the management of their computational complexity.
1.8 Approach
This thesis addresses three interrelated issues for workflows with cancellation and
OR-joins. As YAWL has a formal foundation in Petri nets, provides comprehensive
support for the workflow patterns including direct support for cancellation
regions and OR-joins, and has an open-source implementation, YAWL represents
an ideal candidate through which the results in this thesis can be expressed 8 .
The ground work for this thesis starts with a reconsideration of the OR-join semantics
in YAWL. Next, possible verification techniques are explored for YAWL
workflows with cancellation and OR-joins. To try and manage the complexity of
verification, reduction rules are proposed.
1. OR-join semantics: An approach based on the translation of workflows
with cancellation regions and OR-joins into reset nets is proposed to define
a decidable and general OR-join concept with non-local semantics [FS98].
Reset nets are Petri-nets with reset arcs. A reset arc removes all tokens
from a place when its transition fires. Through the behaviour of reset arcs,
the behaviour of cancellation regions can be captured in a natural manner.
The OR-join approach uses the backwards coverability algorithm of Wellstructured
Transition Systems [FS98, FS01, FRSB02] to cater for workflows
with an infinite state space.
2. Verification techniques: Four desirable properties for workflows with
cancellation regions and OR-joins are proposed. These are soundness, weak
soundness, irreducible cancellation regions, and immutable OR-joins. Using
the notions of coverability and reachability, these properties can be formulated
and algorithmic approaches can be derived.
3. Reduction rules: When dealing with large and complex workflows, workflow
verification can easily become intractable. Hence, it becomes necessary
to consider potential optimisation techniques. A number of soundness preserving
reduction rules are proposed both at the reset net level and an at
the YAWL net level. Applying these reduction rules to YAWL workflows
may decrease the size of the workflow and hence, the time taken for verification.
The proposed reduction rules for reset nets are based on the reduction
rules for Petri nets [Mur89, DE95]. These rules then form the inspiration
for reduction rules for YAWL nets.
8 However, the results and insights presented in this thesis are definitely not limited to YAWL;
they are applicable to a large set of languages used by various systems and standards
PhD Thesis – c○ 2006 M.T.K Wynn – Page 7