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16 W. van der Aalst et al.
evolve over time due to changes in a business and/or its environment. As in differ-
ent contexts different solutions may be desirable, YAWL not only provides services
supporting various flexibility requirements but also allows these services to inter-
operate. This leads to a powerful approach in dealing with such requirements. For
example, it is possible to combine the procedural YAWL language with worklets
(cf. Chap. 4) and Declare (cf. Chap. 6). Worklets can be used to select process frag-
ments at runtime based on rules. Declare can be used to enact processes based on
(temporal) constraints.
The YAWL support environment is open source and is therefore not only freely
available but can also be extended as desired, thus avoiding vendor lock-in. Its
service-oriented architecture, with a rich set of interfaces, allows the system to
interact with other systems and to be extended in a variety of ways.
Through a link with the ProM environment, YAWL logs can be analyzed in a
number of ways and YAWL specifications can be simulated. For example, it is pos-
sible to do a simulation that starts with the current state and that looks into the
near future, for example, to decide whether it is beneficial to hire more resources
or not. The log analysis can reveal, among others, what the typical walk-throughs
are through a process or what the probabilities are of the process choosing one
alternative path over another.
Finally, one can leverage the benefits of BPMN through the BPMN2YAWL plug-
in, where BPMN models can be mapped onto YAWL specifications. Contrary to
BPEL, the resulting YAWL models remain readable and maintain a close link with
the original BPMN models. This is a consequence of the fact that in YAWL arbi-
trary cycles can be specified and that YAWL offers comprehensive support for the
control-flow patterns in general. The BPMN2YAWL plug-in is discussed in detail
in Chap. 13.

1.9 Overview of the Book

This book is about YAWL, both the language and the open source support environ-
ment. It discusses the foundations of the language, its unique support for flexibility,
the support environment, services that can interoperate with this environment, its
relationship to other well-established approaches in the field of BPM, and sev-
eral applications. There are many technical papers that focus on various aspects
of YAWL. This book aims to make this work accessible to a wider audience and is
therefore less technical in nature. Where possible, references are provided so that the
interested reader can deepen her understanding of specific topics. Another objective
of this book is to bring the main material concerning YAWL together in one place
and to properly integrate this material. To facilitate the understanding of the reader,
a running example is provided in the area of supply chain management.
The book is divided into nine parts. Part II provides the conceptual foundation of
YAWL. It explains the concepts required for workflow specification and introduces
the running example.