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7 The Architecture 209

Fig. 7.2 The path of a manual work item through the YAWL services

work item by plugging self-contained subprocesses (worklets) drawn from a Worklet
Repository, thus providing dynamic flexibility to otherwise static process instances.
The Worklet Service is also called upon to handle expected and unexpected excep-
tions and to store information, allowing users to better deal with such exceptions in
future occasions. The Worklet Service provides such capabilities through a Graphi-
cal User Interface (GUI) component that allows users to create rules and exception
handling processes (or exlets) and link them to tasks through a Ripple-Down Rule
strategy.
The YAWL Editor allows users to create and edit workflow specifications. The
Editor relies on the process validator to handle the validation of the workflow
specifications, both syntactically and semantically.
Developers can extend the YAWL System by introducing custom services.A cus-
tom service is any service that interacts with the Engine or with the task-related
YAWL services presented above (e.g., resource manager, worklist handler). By
default, the YAWL system is shipped with a number of custom services. For
example, the execution of a task can also be delegated to the declare engine,the
notification service, the WS-invoker,and the digital signature service. These ser-
vices are grouped under the label custom services in Fig. 7.1. The list of custom
services included in the YAWL System is intended to be extended. One could think,
for example, of a custom services providing RSS feeds to track the creation and
execution of work items. Another example of a YAWL Custom Service could be a
special type of worklist visualizer, which would display work items based on spatial
attributes as described in de Leoni et al.
Having provided an overview of the YAWL System and its underlying services,
the following section introduces the interfaces that these services offer and consume.

7.3 YAWL Services and Interfaces

The interfaces of the YAWL Architecture are inspired by those defined in the Work-
7
flow Reference Model (WRM) of the Workflow Management Coalition (WfMC).
The WRM describes a core Workflow Enactment Service (or Engine) interacting
with a number of generic components via a defined set of standardized interfaces and

7 www.wfmc.org/standards/docs/tc003v11.pdf