Verification of WF-nets Citation for published version (APA): Verbeek, H. M. W. (2004). Verification of WF-nets. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR577287 DOI: 10.6100/IR577287 Document status and date: Published: 01/01/2004 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. 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Verbeek. - Eindhoven : Technische Univer- siteit Eindhoven, 2004. - Proefschrift. - ISBN 90-386-1918-9 NUR 982 Keywords: Workflow management systems / Verification / Petri nets / WF-nets The work in this thesis has been carried out under the auspices of the research school Beta (Business Engineering and Technology Application). Beta Dissertation Series D65 Printed by University Press Facilities, Eindhoven Cover based on the jester’s image on Marillion’s Real to Reel album by Mark Wilkinson Verification of WF-nets PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR AAN DE TECHNISCHE UNIVERSITEIT EINDHOVEN, OP GEZAG VAN DE RECTOR MAGNIFICUS, PROF.DR. R.A. VAN SANTEN, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN PROMOTIES IN HET OPENBAAR TE VERDEDIGEN OP DONDERDAG 10 JUNI 2004 OM 16.00 UUR DOOR HENRICUS MARINUS WILHELMUS VERBEEK GEBOREN TE EINDHOVEN Dit proefschrift is goedgekeurd door de promotoren: prof.dr.ir. W.M.P. van der Aalst en prof.dr. K.M. van Hee Copromotor: dr.ir. T. Basten “Quod scripsi, scripsi” Pontius Pilate PREFACE 13 years ago, I started as a scientific engineer at the department of Mathematics and Computing Science, Eindhoven. When I left the mil- itary service, I had to find a job. So, I went to see Kees van Hee and Lou Somers to ask them whether I could mention them as references when applying for a job. Instead, Marc Voorhoeve told me that there just might be a vacant position that would suit me fine. For my mas- ter’s thesis, I had been working on ExSpect, and the scientific engineer who happened to be working on ExSpect had just left for the United States. To make a long story short, by coincedence, I became a scien- tific engineer. For several years, I worked on the ExSpect toolset. In the mean time, Kees van Hee moved to Bakkenist Management Consultants (nowadays a part of Deloitte and Touche), which left the group more or less headless for some time. At that time, Wil van der Aalst was still only an assistant professor. However, as his star was ris- ing with his Petri-net related work on workflow management, he was promoted to associate professor and he took over the control of the group. As a result, at the end of 1996, we started working on Woflan: a tool that exploits Petri-net analysis techniques to verify existing work- flow process definitions. So far, I had only been involved in developing software (ExSpect, Woflan), and had never given much thought on doing research. How- ever, this was about to change. Wil encouraged me to take up this part of my job that had long since been forgotten (so it seemed). In 2001, I had cooperated in several publications and the time seemed perfect for another step: writing this PhD thesis. Wil probed me whether I would like to do such a thing, and we both concluded it would be a good idea. At the moment, you’re reading the end result of this decision. vii Preface From the above, it will be clear that I wouldn’t have written this thesis if it were not for the help of many people. First of all, of course, my parents. The first two years at secondary school were not easy for me, and they helped me through this phase with their support. I also want to thank the people from my secondary school for allowing me to con- tinue at a level which seemed at that time too difficult for me. In con- trast with secondary school, university went fine. Still, I want to thank some of my brother students: Eduard Diepstraten, Rik Koenders, Mark van Helvoirt, Han Toan Lim, and Math van Diepen. Thanks to you, I had a good time at university. Next, I would like to thank the members of the Information Systems group at the department of Mathematics and Computing Science group, and especially Kees van Hee, Lou Somers, Marc Voorhoeve, Wil van der Aalst, Twan Basten, Marco Langenhuizen, and Jeroen Schuijt. In 2000, I followed Wil when he moved to the Information and Technology group of the Technology Management department. I want to thank all members of this group for their support and trust in me. I am indebted to Wil van der Aalst for his encouragements and support over the years, and to Kees van Hee and Twan Basten for supervising me while writing this thesis. Last, but not least, I want to thank three persons that are very dear to me. Marielle, Marijn, Vera, this one’s for you: the best way is with an old cliché; it’s simply the best way is an old cliché, always the best is with an old cliché; I’ll leave it to the best way, it’s and old cliché Cliché, Fish A concluding note on the jester figure that appears on the cover. Some of the readers might recognize this figure from Marillion’s Real to Reel album. These readers might also know that this figure is an abstraction from the jester figure as it appears on their Script for a Jester’s Tear album. As such, this jester figure on the cover is an abstraction from an abstraction. A workflow net is also an abstraction from an abstraction, as the workflow process model the workflow net is based on is already an abstraction of the real-world process. Therefore, I linked the work- flow net on the cover to the jester. viii CONTENTS 1 Introduction 1 1.1 Example. 2 1.2 Woflan. 7 1.3 Road map . 9 2 Workflow management 13 2.1 Workflow. 13 2.2 Abstractions. 17 2.2.1 Perspectives . 17 2.2.2 Abstraction from resources. 20 2.2.3 Abstraction from data and triggers . 21 2.2.4 Abstraction from task content and operations . 22 2.3 Verification issues . 22 2.3.1 Soundness . 23 2.3.2 Inheritance of behavior. 24 2.4 Approach. 25 2.5 Related work . 28 2.5.1 Workflow management systems . 28 2.5.2 Workflow process definitions . 29 2.5.3 Verification of workflow process definitions . 30 2.5.4 Petri nets . 32 2.5.5 WF-nets . 32 2.5.6 Base of this thesis . 33 2.6 Conclusions. 34 3 Nets and properties 37 3.1 Nets . 37 3.1.1 Structure . 38 3.1.2 Net state . 42 ix Contents 3.1.3 Firing rule . 44 3.2 Running example . .45 3.3 WF-nets. 46 3.4 Net properties . 49 3.4.1 Structural properties . 49 3.4.2 Behavioral properties . 56 3.5 System equivalence. .67 3.6 Conclusions. 72 4 Deciding soundness 73 4.1 Structural properties related to soundness. 73 4.2 Liveness and boundedness vs. soundness . 79 4.2.1 Proper completion. 79 4.2.2 Option to complete . 81 4.2.3 No dead tasks . 82 4.3 Behavioral error sequences . 83 4.3.1 Non-live sequences. 84 4.3.2 Unbounded sequences . 88 4.4 Reduction techniques . 92 4.4.1 Liveness and boundedness preserving reduction rules. 93 4.4.2 Reduction rules for free-choice nets. ..