Workflow Patterns Put Into Context

Softw Syst Model (2012) 11:319–323 DOI 10.1007/s10270-012-0233-4

EXPERT’S VOICE

Workﬂow patterns put into context

W. M. P. van der Aalst · A. H. M. ter Hofstede

Received: 20 January 2012 / Accepted: 23 January 2012 / Published online: 14 February 2012 © The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract In his paper “Approaches to Modeling Business of sentences are quoted from ten of our publications, which Processes. A Critical Analysis of BPMN, Workflow Pat- appeared between 2000 and 2011. Many of these quotations terns and YAWL”, Egon Börger criticizes the work of the are out of context, e.g., quotations related to specification, Workflow Patterns Initiative in a rather provocative manner. analysis, and implementation are muddled up. Therefore, Although the workflow patterns and YAWL are well estab- Sect. 2 describes the developments related to the Workflow lished and frequently used, Börger seems to misunderstand Patterns Initiative over the last decade thereby clarifying the the goals and contributions of the Workflow Patterns Initia- purpose of the patterns and the role of YAWL. In Sect. 3, tive. Therefore, we put the workflow patterns and YAWL we argue that patterns need to balance between generality in their historic context. Moreover, we address some of the and precision and should not be viewed as a formal speci- criticism of Börger by pointing out the real purpose of the fication of behavioral properties. Patterns should allow for workflow patterns and their relationship to formal languages multiple modeling techniques and not impose very specific (Petri nets) and real-life WFM/BPM systems. semantics, e.g., the well-known design patterns [7]alsodo not impose the use of Java or any other programming lan- Keywords Workflow patterns · YAWL · Petri nets · guage. In Sect. 4, we point out the complexity of the various Business process management interacting workflow perspectives and argue that there is no “Silver Bullet Formalism” that will make everything as simple as suggested in [3]. Finally, in Sect. 5, we acknowledge 1 Introduction the need for empirical validation and discuss our efforts to find out what is happening in real workflow processes and In [3], Egon Börger states that the patterns by the Workflow WFM/BPM tools. Patterns Initiative are “not well founded”, “badly described”, and “no suitable BPM benchmark”. Moreover, he criticizes the semantical foundation of YAWL. Börger makes these 2 History of the workflow patterns claims in a rather provocative and emotional manner. There- fore, the editors encouraged us to write this rebuttal. In the mid-nineties, Wil van der Aalst got involved in the In the remainder, we first sketch the Workflow Patterns Sagitta project when he was working as a part-time work- Initiative in its historic context. In [3], a substantial number flow consultant for Bakkenist Management Consultants. The goal of this project was to realize a workflow system for sup- Communicated by Bernhard Rumpe. porting the key processes within the Dutch Customs [18]. He W. M. P. van der Aalst (B) · A. H. M. ter Hofstede was surprised by the way the Workflow Management (WFM) Eindhoven University of Technology, Eindhoven, The Netherlands system was selected. Stakeholders had problems describing e-mail: [email protected] its desired functionality. For example, they did not distin- guish between—what is called today—the Deferred Choice W. M. P. van der Aalst · A. H. M. ter Hofstede Queensland University of Technology, Brisbane, Australia (Pattern 16) and the Exclusive Choice (Pattern 4). Although e-mail: [email protected] the Dutch Customs clearly had many processes that required 123 320 W. M. P. van der Aalst, A. H. M. ter Hofstede support for the Deferred Choice, they were about to select a the workflow patterns. He talks about “an obvious...group WFM system that did not support this at all [18]. This expe- interest” (Section 4.1 in [3]) suggesting that the patterns were rience and also several similar experiences of the first author used to promote YAWL. However, Börger misunderstands in the second half of the nineties clearly showed the need for the true intentions of YAWL. The development of YAWL describing workflow patterns. started in 2002 and was triggered by frequent discussions However, the real start of the Workflow Patterns Initia- with practitioners that had problems understanding the subtle tive was in 1999 when the authors met at CoopIS 1999 in but important differences between the various patterns. Some Edinburgh. This resulted in the first paper on the workflow vendors doubted that comprehensive patterns support could patterns at the same conference one year later [17]. Initially, be achieved (without adding a lot of complexity). They would the work received little attention, but this changed when often ask “How do you do this?” and wanted to see concrete the http://www.workflowpatterns.com website was launched examples. This instigated the development of YAWL. The and started to attract the attention of people involved in the acronym (Yet Another Workflow Language) clearly shows selection and use of WFM systems. Several reports on the that YAWL was not intended as a commercial effort trying to patterns appeared in Dutch IT journals and these encour- gain competitive advantage by exploiting the patterns. How- aged vendors to study the workflow patterns more closely. In ever, YAWL has had a very positive effect on the Workflow parallel, we refined and extended the patterns and started to Patterns Initiative. When implementing YAWL new ques- systematically evaluate commercial WFM systems [4,5,21]. tions emerged, especially with respect to the interaction of We also evaluated emerging standards such as BPEL4WS, the various patterns. The YAWL environment also helped to BPML, XLANG, WSFL and WSCI. provide a proof-of-concept for various innovative research Initially, the focus was on control-flow only. This changed ideas related to workflow flexibility, work distribution, work- in 2004 when Nick Russell joined the Workflow Patterns list visualization, configurable process models, verification, Initiative. Over time, we added patterns for the other core simulation, and process mining [15]. YAWL is one of the perspectives such as the workflow resource patterns [13], the most visible open source WFM systems (see [22] for a pat- workflow data patterns [14], and the exception handling patterns based analysis of the other main open source WFM sys- terns [12]. Later also patterns related to service interaction tems). YAWL has been downloaded approx. 120,000 times and flexibility were added. Whereas the control-flow pat- and the key paper [20] has been cited more than 800 times terns could be used to evaluate a language or notation, for according to Google Scholar. Overall, we think that YAWL the other patterns it made more sense to consider them in the helped to progress WFM/BPM research and it is unclear why context of a WFM/BPM system. In 2006, the original set of Börger goes to great lengths to denounce it (in [3] no other 20 control-flow patterns was revisited with the goal to pro- WFM systems are singled out for criticism). vide a more precise description of each of the patterns and In our view, the workflow patterns have had a very positive also to identify any potential gaps in the original set of pat- effect on WFM/BPM research and WFM/BPM products. terns. As a result the number roughly doubled. In [3], Börger Today, the patterns are widely used to describe workflow criticizes the growing number of patterns: “an exponential functionality in a language/system-independent manner. In growth can be observed starting with 20 workflow patterns addition, the patterns are also highly visible. The http://www. in 2003 going through 43 WPs in 2006 and reaching a praised workflowpatterns.com website has been one of the most vis- 126 patterns (obtained by adding patterns for the so-called ited websites in the field of BPM averaging more than 300 data and resource perspectives) in 2010. No reason is given visitors per working day over the last decade. Moreover, [21] that this growth has a fundamentum in re and a limit.” (note: is the most cited workflow paper ever (it has more than 2,250 references in this quote were removed). The reason for the citations according to Google Scholar). growing number of patterns was the expanding scope of the Workflow Patterns Initiative and our experiences with a wide range of systems and standards. Moreover, as was clearly 3 Balancing between generality and precision stated in our publications, we do not claim that WFM systems/languages should support all patterns. Users should first In [3] both the informal descriptions of the workflow pat- analyze which patterns they need and then use this short-list terns and the example formalizations using YAWL and CPN of patterns to select a system. Tools are criticized. Surprisingly, despite his criticism, Egon In parallel with the refinement and extensions of the Börger has published various papers on formalizing the con- patterns, we also started the development of the YAWL lan- trol-flow patterns in terms of Abstract State Machine (ASMs) guage and system [15,20]. YAWL is inspired by the work- [2]. On the one hand, he claims that our informal descriptions flow patterns and extends Petri nets to facilitate the use of are vague and ambiguous. On the other hand, Börger does more advanced patterns [19]. Börger is very critical about not like the explanations/formalizations in terms of (colored) the development of YAWL as a reference implementation of Petri nets. In this section, we address these comments. 123 Workflow patterns put into context 321

The notion of patterns as a means of categorizing recurring [15], but other techniques in areas such as process mining problems and solutions in a particular domain is generally [16] and simulation [10] have been developed to support attributed to Christopher Alexander [1]. Later the concept of specific types of analysis relevant for the field of BPM. Such patterns was adopted in software development, resulting in dedicated techniques do not exist for ASMs. In his paper, well-known patterns collections such as the design patterns Börger refers to work on “Software Product Lines” for solu- by the “Gang of Four” [7], the business analysis patterns tions. However, he is not very clear about this as it is just a [6], and the enterprise application integration patterns [8]. side remark in the three page discussion on the “unsuitabil- See http://hillside.net/patterns/patterns-catalog for pointers ity” of Petri nets (cf. Section 4 in [3]). to other collections. All of these collections have in com- mon that they do not impose a particular language and that 4 Make things as simple as possible, but not simpler the description of the pattern is deliberately kept informal. When describing a pattern one needs to resort to concrete In [15], we discuss the semantics of YAWL and in the PhD examples, e.g., the design patterns book by the “Gang of thesis of Nick Russell [11] a formalization of YAWL’s con- Four” [1] includes many examples using C++ and Smalltalk. trol-flow, data, and resource perspectives is given in terms of However, these examples should not be confused with the colored Petri nets. The formal model of YAWL can be exe- actual patterns. We often use Petri nets to explain patterns. cuted using CPN Tools. Börger complains that the CPN Tools However, this does not imply that these Petri nets should be model is not shown in [15]. The model is indeed large: 55 viewed as formal specifications of the pattern. Moreover, as pages, 480 places, and 138 transitions. However, this model our evaluations show, systems not based on Petri nets can covers all three aforementioned perspectives and also the also support any of the patterns if they provide the desired interactions between these perspectives. It is absurd to think functionality. It is quite OK to formulate pattern examples that another formalization technique like ASMs will sud- in terms of ASMs, just like they have been formulated in denly make things simple. The CPN Tools model describes various process algebras (e.g. in [9]). However, this does not many essential and complex mechanisms. touch upon the essence of the workflow patterns which are Egon Börger seems to advocate a “Silver Bullet Formal- representation independent. ism” that will make things simple and manageable. There In Section 4.2 of [3], Börger argues that (colored) Petri is definitely room for improvement. However, making such nets are unsuitable “for the practice of BPM”. As indicated claims without having specified or implemented a WFM sys- in [19,20], we agree that Petri nets are unsuitable as a busi- tem that is able to support a reasonable number of patterns ness process modeling language for end users. (This is the covering the control-flow, data, and resource perspectives is reason for developing more suitable notations like YAWL.) not very convincing. However, ASMs are even less suitable as an end-user language for modeling processes as they are not graphical and very different from mainstream languages such as BPMN, 5 Need for empirical validation UML, EPCs, etc. We do not propose Petri nets as an end- user language. Instead, we use it for: In Section 3.1 of [3], Börger writes “In fact, there is no statis- tical underpinning showing how frequently which patterns • Clarification: Petri nets are graphical, well-known, and appear in real-life business processes. Experimental data like widely used. Therefore, it makes sense to illustrate the that produced for BPMN constructs in [61] should have been basic patterns in terms of small Petri net examples. put in place to validate the pattern selection.” We acknowl- • Semantics: YAWL and various workflow functionalities edge the importance of empirical validation. However, the (e.g., work distribution mechanisms) can be formalized workflow patterns have been validated. First of all, the pat- in terms of colored Petri nets. Moreover, using CPN Tools terns are based on a detailed analysis of various WFM/BPM such models are directly executable and can be used for systems. Moreover, practical experiences obtained through a analysis (simulation and state-space exploration). multitude of evaluations resulted in reformulations and fur- • Analysis: Petri nets allow for different types of analysis, ther refinements of the patterns. Systems such as Staffware, not only for model checking but also for various linear- WebSphere MQ Workflow, FLOWer, COSA, iPlanet, SAP algebraic techniques (invariants and the marking equa- Workflow, Filenet, jBPM, OpenWFE, Enhydra Shark, Web- tion), Markov analysis, simulation, etc. Sphere Integration Developer and Oracle BPEL, and languages/standards such as BPMN, XPDL, BPEL, UML and Clearly, many properties are undecidable unless rigorous EPCs have been analyzed using the patterns. Moreover, we abstractions are used. However, there is a large body of have been interacting with workflow vendors, consultants, knowledge on analyzing workflows using Petri nets. A sys- end-users and analysts regarding the patterns. Second, we tem such as YAWL provides support for checking soundness looked at the frequencies of patterns in real-life projects. See 123 322 W. M. P. van der Aalst, A. H. M. ter Hofstede for example the work done by Kristiaan de Vries and Oscar 5. de Vries, K., Ommert, O.: Advanced workflow patterns in practice Ommert [4,5]. Under the supervision of the first author they (2): experiences based on judicial processes (in Dutch). Bus. Pro- evaluated five workflow projects conducted by ATOS/Ori- cess Mag. 8(1), 20–23 (2002) 6. Fowler, M.: Analysis Patterns: Reusable Object Models. Addison- gin to get quantitative data about the frequency of patterns. Wesley, Reading (1997) Each of these projects involved multiple processes for which 7. Gamma, E., Helm, R., Johnson, R., Vlissides, J.: Design Pat- the number of activities ranged from dozens to hundreds. The terns: Elements of Reusable Object-Oriented Software. Profes- systems Eastman Enterprise Workflow, Staffware, and Dom- sional Computing Series. Addison-Wesley, Reading (1995) 8. Hohpe, G., Woolf, B.: Enterprise Integration Patterns. Addison- ino Workflow were used in these projects. Empirical findings Wesley, Reading (2003) showed that in many of ATOS/Origin’s projects workflow 9. Puhlmann, F., Weske, M.: A look around the corner: the Pi-calcu- designers were forced to adapt the process or had to resort lus. In: Jensen, K., van der Aalst, W.M.P. (eds.) Transactions on to spaghetti-like diagrams or coding because particular pat- Petri Nets and Other Models of Concurrency II. Lecture Notes in Computer Science, vol. 5460, pp. 64–78. Springer, Berlin (2009) terns were not supported by the WFM system. The project 10. Rozinat, A., Wynn, M., van der Aalst, W.M.P., ter Hofstede, also showed a positive correlation between the patterns being A.H.M., Fidge, C.: Workflow simulation for operational decision used and the patterns that were well supported, e.g., the pro- support. Data Knowl. Eng. 68(9), 834–850 (2009) cesses developed using Staffware had much more parallel- 11. Russell, N.: Foundations of Process-Aware Information Systems. PhD thesis, Queensland University of Technology (2007) ism than the processes developed using Eastman Enterprise 12. Russell, N., van der Aalst, W.M.P.,ter Hofstede, A.H.M.: Workflow Workflow. This example shows that patterns tend to be used exception patterns. In: Dubois, E., Pohl, K. (eds.) Proceedings of frequently if they are supported well. This indicates that pro- the 18th International Conference on Advanced Information Sys- cess design is influenced by patterns support. tems Engineering (CAiSE’06). Lecture Notes in Computer Sci- ence, vol. 4001, pp. 288–302. Springer, Berlin (2006) A more elaborate empirical validation of the workflow 13. Russell, N., van der Aalst, W.M.P., ter Hofstede, A.H.M., Edmond, patterns is welcome. Moreover, we would also like to refer D.: Workflow resource patterns: identification, representation and to our work on process mining [16]. In this work, we ana- tool support. In: Pastor, O., Falcão e Cunha, J. (eds.) Proceed- lyze processes based on event logs, i.e., we discover, monitor ings of the 17th Conference on Advanced Information Systems Engineering (CAiSE’05). Lecture Notes in Computer Science, vol. and improve real processes (i.e., not assumed processes) by 3520, pp. 216–232. Springer, Berlin (2005) extracting knowledge from event logs readily available in 14. Russell, N., ter Hofstede, A.H.M., Edmond, D., van der Aalst, today’s information systems. Experience obtained by apply- W.M.P.: Workflow data patterns: identification, representation and ing process mining in over 100 organizations shows that tool support. In: Delcambre, L., Kop, C., Mayr, H.C., Mylopoulos, J., Pastor, O. (eds.) Proceedings of the 24th International Confer- processes are often much more involved than what people ence on Conceptual Modeling (ER 2005). Lecture Notes in Com- would like to think. Typically, 80% of the process instances puter Science, vol. 3716, pp. 353–368. Springer, Berlin (2005) can be described by a rather simple process model. How- 15. ter Hofstede, A.H.M., van der Aalst, W.M.P., Adams, M., Russell, ever, to model the remaining 20% more advanced patterns N.: Modern Business Process Automation: YAWL and its Support Environment. Springer, Berlin (2010) are needed. 16. van der Aalst, W.M.P.: Process Mining: Discovery, Confor- mance and Enhancement of Business Processes. Springer, Berlin Acknowledgments The authors would like to thank all that contrib- (2011) uted to the Workflow Patterns Initiative and the development of YAWL. 17. van der Aalst, W.M.P., Barros, A.P., ter Hofstede, A.H.M., In particular Nick Russell, Nataliya Mulyar, Bartek Kiepuszewski, Ali- Kiepuszewski, B.: Advanced workflow patterns. In: Etzion, O., stair Barros, Petia Wohed, and Michael Adams. Scheuermann, P.(eds.) Proceedings of the 7th International Confer- ence on Cooperative Information Systems (CoopIS 2000). Lecture Open Access This article is distributed under the terms of the Creative Notes in Computer Science, vol. 1901, pp. 18–29. Springer, Berlin Commons Attribution License which permits any use, distribution, and (2000) reproduction in any medium, provided the original author(s) and the 18. van der Aalst, W.M.P., van Hee, K.M.: Workflow Manage- source are credited. ment: Models, Methods and Systems (in Dutch). Academic Ser- vice, Schoonhoven (1997) 19. van der Aalst, W.M.P., ter Hofstede, A.H.M.: Workflow pat- References terns: on the expressive power of (Petri-net-based) workflow languages. In: Jensen, K. Proceedings of the Fourth Workshop on the 1. Alexander, C.: A Pattern Language: Towns, Building and Con- Practical Use of Coloured Petri Nets and CPN Tools (CPN 2002). struction. Oxford University Press, Oxford (1977) DAIMI, vol. 560, pp. 1–20, Aarhus, University of Aarhus, Aar- 2. Börger, E.: Modeling workflow patterns from first principles. hus (2002) In: Parent, C., Schewe, K.D., Storey, V., Thalheim, B. (eds.) Con- 20. van der Aalst, W.M.P., ter Hofstede, A.H.M.: YAWL: Yet Another ceptual Modeling (ER 2007). Lecture Notes in Computer Science, Workflow Language. Inf. Syst. 30(4), 245–275 (2005) vol. 4801, pp. 1–20. Springer, Berlin (2007) 21. van der Aalst, W.M.P., ter Hofstede, A.H.M., Kiepuszew- 3. Börger, E.: Approaches to modeling business processes. a criti- ski, B., Barros, A.P.: Workflow patterns. Distrib. Parallel Dat- cal analysis of BPMN, workflow patterns and YAWL. Softw. Syst. abases 14(1), 5–51 (2003) Model (2012). doi:10.1007/s10270-011-0214-z 22. Wohed, P., Russell, N.C., ter Hofstede, A.H.M., Andersson, B., van 4. de Vries, K., Ommert, O.: Advanced workflow patterns in practice der Aalst, W.M.P.: Patterns-based evaluation of open source BPM (1): experiences based on pension processing (in Dutch). Bus. Pro- systems: the cases of jBPM, OpenWFE, and Enhydra Shark. Inf. cess Mag. 7(6), 15–18 (2001) Softw. Technol. 51(8), 1187–1216 (2009) 123 Workflow patterns put into context 323

Author Biographies A. H. M. ter Hofstede is a Pro- fessor in the Information Sys- W. M. P. van der Aalst is tems School in the Science and a full Professor of Informa- Engineering Faculty, Queens- tion Systems at the Technische land University of Technology, Universiteit Eindhoven (TU/e). Brisbane, Australia, and is Head Currently, he is also an adjunct of the Business Process Man- professor at Queensland Univer- agement Discipline. He is also sity of Technology (QUT) work- a Professor in the Information ing within the BPM group there. Systems Group at Eindhoven He is also a member of the Royal University of Technology, Eind- Holland Society of Sciences and hoven, The Netherlands. His Humanities and the Academy main research interests lie in of Europe. His research inter- the area of business process ests include workﬂow manage- automation. ment, process mining, Petri nets, business process management, process modeling, and process analysis.

123