=Paper=
{{Paper
|id=Vol-489/paper-6
|storemode=property
|title=YAWL: Power through Patterns
|pdfUrl=https://ceur-ws.org/Vol-489/paper6.pdf
|volume=Vol-489
|dblpUrl=https://dblp.org/rec/conf/bpm/AdamsCRH09
}}
==YAWL: Power through Patterns==
https://ceur-ws.org/Vol-489/paper6.pdf
YAWL: Power through Patterns
Michael Adams, Stephan Clemens, Marcello La Rosa
and Arthur H.M. ter Hofstede
BPM Group, Queensland University of Technology, Australia
{mj.adams,stephan.clemens,m.larosa,a.terhofstede}@qut.edu.au
Abstract. Workflow Management Systems (WfMSs) enable the devel-
opment and maintenance of workflow specifications at design time and
their execution and monitoring at runtime. The open source WfMS
YAWL supports the YAWL language – a formally defined language based
on Petri nets which offers comprehensive support for control-flow and re-
source patterns. In addition, the YAWL system provides extensive sup-
port for process flexibility, in particular for process configuration, excep-
tion handling, dynamic workflow and declarative workflow. Due to its for-
mal foundation, sophisticated verification support can also be achieved.
This paper presents the YAWL system and its main applications.
1 Introduction
While workflow automation can be traced back to the seventies, it took until
the mid-to-late nineties to achieve the necessary breakthroughs for it to find
mainstream acceptance. A substantial number of workflow systems were devel-
oped, many using their own vendor-specific language for workflow specification.
In this context the Workflow Patterns Initiative1 emerged in 1999. Through
this initiative a number of control-flow patterns were discovered, which docu-
mented well-known workflow requirements in a language-independent manner.
This allowed an objective benchmarking of workflow functionality and served as
guidance for language extensions and development.
Given the limited support in commercial tools for many of the patterns and
facing potential criticism that realizing comprehensive support for the workflow
patterns would not be feasible, the YAWL language was developed. YAWL, one
of the few formally defined workflow languages, provided comprehensive support
for the workflow patterns, achieved by extending Petri nets with a number of
dedicated constructs.
Over time an open-source implementation of a WfMS that supported the
YAWL language natively has been developed. This system, based on a service-
oriented architecture, incorporates sophisticated solutions for workflow verifica-
tion and flexibility support, notably for process configuration,2 exception han-
dling, dynamic workflow (through the Worklet Service) and declarative workflow
(through the Declare Service).3 The original patterns collection was extended
1
www.workflowpatterns.com
2
www.processconfiguration.com
3
declare.sf.net
with a collection of resource patterns and comprehensive support for these pat-
terns in the YAWL environment was recently realised.
2 The YAWL system
Some of the fundamental goals of the YAWL environment include that it should
be freely available, portable, easy to use and interoperable. By releasing the
environment under an open source license free availability was achieved. Porta-
bility was achieved through the use of JavaTM and by avoiding any operating
system dependencies. Ease of use was realised by the provision of an intuitive
user interface for creating and executing specifications. Finally, interoperability
was supported by a service-oriented architecture, the definition of a common
XML format and a set of API calls for the exchange of workflow specifications
between design time and runtime environment.
YAWL Engine
Persisted
Data
Event
Logs
` Interfaces
Process Designer
Other Custom
Services
Org
Data
Process
Repository Event
Logs
Resource Service
Admin Users Applications
Fig. 1. Simplified architectural overview of the YAWL system.
The three main components of the YAWL system are the Editor, the Engine
and the Resource Service, which communicate through well-defined interfaces
(see Figure 1). They all are connected through the underlying service-oriented
architecture. Based on this architecture users can adapt the YAWL system to
their needs and introduce their own custom services where required.
The Editor provides a tool palette from which modeling elements (such as
tasks or conditions) may be chosen for placement on the design canvas (see
Figure 2). Routing constructs may be attached to tasks, and arcs added to link
tasks and conditions, in order to form a complete workflow graph capturing a
particular business process. At any time a workflow model may be verified using
various algorithms to ensure completeness and soundness, amongst other things.
The Editor communicates with a running Engine to receive a list of YAWL
Services which are registered with the Engine. A workflow designer can then
associate those services with tasks of a workflow specification. Furthermore, from
a running Resource Service, the Editor obtains organizational resources and so-
called codelets. Within the Editor tasks can be associated with those resources,
which are offered the according workitem at runtime. Furthermore, a workflow
designer can automate a task by assigning a codelet, which is a small Java based
application executed for the task at runtime.
Fig. 2. The Editor of the YAWL system.
The Engine deals with control-flow logic and data passing, but is resource-
agnostic. Valid workflow specifications are loaded into the Engine and stored
in a repository from where they may be instantiated to produce cases. The En-
gine is in charge of the execution of cases and determines which workitems are
enabled throughout the process lifecycle. Workflow data can be updated through
the execution of workitems and be passed between workflow elements or e.g. be
used for the evaluation of routing conditions.
The Resource Service ensures the correct routing of workitems to resources.
It consists of the following four sub-services: A Resource Manager, which man-
ages the allocation of resources to workitems; a Worklist Handler – a web-form
based user interface that provides users with the ability to interact with and
to process workitems; a Forms Connector to visualize tailor-made and/or dy-
namically generated web-forms for displaying and editing workitem data; and
a Codelet Service that maintains and executes codelets selected for automated
tasks.
3 Maturity of YAWL
The YAWL system has been down-
loaded from SourceForge more than
80,000 times4 . Being open source,
and thus freely available, and hav-
ing a solid theoretical base, makes the
YAWL environment suitable for ter-
tiary education — at least 24 univer-
sities worldwide have used it in their
teaching. The original YAWL paper5
has attracted over 400 citations ac-
cording to Google Scholar and is the
second most cited paper to appear
in Information Systems according to
Scopus.
The Australian Film, Television
& Radio School (AFTRS) and the
Queensland University of Technology
(QUT) started the YAWL4Film ini- Fig. 3. A dynamically generated form.
tiative in 2007 in the context of the
ARC Centre of Excellence for Creative Industries and Innovation. As part of
this collaboration a solution was developed for film shoot production data man-
agement and report generation which uses YAWL. This solution was trialed in
two student projects at the AFTRS and it was further adapted for the shooting
of the commercial feature film “Prime Mover” in the Australian outback.6
first:utility and first:telecom in the UK, both part of the Impello plc group
of companies, providing energy and telecoms services respectively, have been
collaborating with QUT since 2005 on the YAWL initiative. These companies
have built software around the YAWL system providing a novel approach to
page navigation for web based systems together with the more traditional use
of choreographing long-lived business processes.7
4 Outlook
Future work for the YAWL system is anticipated to include sophisticated work-
flow monitoring, conceptual process integration primitives, improved support for
forms generation and templating, and system integration.
4
https://sourceforge.net/projects/yawl
5
W.M.P. van der Aalst and A.H.M. ter Hofstede. YAWL: Yet Another Workflow
Language. Information Systems, 30(4):245–275, 2005
6
http://www.primemovermovie.com
7
Quoted almost verbatim from http://www.yawlfoundation.org/about/adoption.html.