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24 N. Russell and A. ter Hofstede

Application Application Client Device Client Device Client Device
User Interface User Interface User Interface
User Interface User Interface Application Application Application
Application Logic Application Logic Application Logic Application Logic ServiceServiceService
Business Rules Business Rules Business Rules Business Rules
Rules Engine
Control Flow Control Flow Control Flow Business Rules
Data
Workflow BPMS
Control Flow Control Flow

Database System Database System Database System Database System
Data Data Data Data
1960s 1970s 1980s 1990s 2000s

Fig. 2.1 Evolution of BPM technology

interaction schemes. Moreover, individual workflow vendors were reluctant to
commit to a common operational platform that would leave them with minimal
opportunity for product differentiation. The net result was that the Workflow Ref-
erence Model and the associated standards proffered by the WfMC essentially
constituted the lowest common denominator of workflow concepts acceptable to
all parties rather than laying a foundation for the workflow domain more generally.
Nonetheless, the issues identified remain unaddressed and there is a marked
absence of a common conceptual foundation for workflow technology or for the
area of business process management more generally. Furthermore, there are a
plethora of competing approaches to business process modeling and enactment, and
the lack of an agreed set of fundamentals in the domain means that direct compar-
isons between them and integration of their functionality is extremely difficult. In
light of these issues, in 1999, the Workflow Pattern Initiative was conceived as an
empirical means of identifying the core functionality required for workflow systems.
During the past 10 years, over 100 patterns have been identified that are rele-
vant to workflow technology and to the various perspectives of business processes
more generally. One of the criticisms that the patterns faced early on was that they
represented isolated process concepts and did not give a guide as to the form that
a process language should take. In response to this, YAWL (Yet Another Work-
flow Language) was developed. Initially, it sought to show the manner in which the
original 20 control-flow patterns should be operationalized in a workflow language.
More recently, it has been expanded to encompass a broader range of the overall
set of workflow patterns. In tandem with the language effort, the YAWL System has
also been developed with the aim of providing a reference implementation for the
YAWL language and workflow technology.
In this chapter, we will explore the fundamental underpinnings of the YAWL
language, looking at the precursing workflow patterns, then examining the formal
foundations on which the language is based and finally reviewing the language
constructs of which it is comprised.