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Check constraints events, which notify the service at the launch and comple-
tion of each process instance, and at the enabling and completion of each work
item – thus, four unique constraint type events are required. The service uses
these events to check whether constraint rules have been violated, using avail-
able contextual data, and if so, launch the corresponding exlet for that constraint
type. Therefore, the constraint event types do not notify of exception per se,but
are checkpoints that allow the service to determine if a constraint violation has
occurred.
A time-out event, which would notify the service when a deadline has been
reached, allowing the service to take the appropriate action, if necessary.
Unavailable resource, item abort,and constraint violation during execution
events. For these event types, the enactment engine must determine if and when
these exceptions occur, as opposed to those above where the determination rests
with the Worklet Service.
Case canceled event, required so that the service can take the appropriate action if
a worklet instance, or a parent instance for which a worklet is currently executing
as a compensation, is canceled. Note that a case canceled event is different from
a case completed event described in Chap. 4.
The following methods are specifically required to be available to the interface
for the exception handling service (existing methods of other interfaces are also
used):
Suspend work item: To pause or prevent the execution of a work item, until
such time as it is continued (unsuspended). Through this method, entire process
instances may also be suspended, by suspending each “active” work item of the
process instance as a list.
Continue work item: To unsuspend a previously suspended work item, or unsus-
pend each suspended work item for a previously suspended process instance.
Update work item data: For those situations where a worklet has been run as
a compensation process for a work item that has generated an exception, this
method would enable any data gathered during the compensation execution to be
passed back to the original work item.
Update case data: As above, but for case-level exceptions as opposed to work
item-level exceptions.
Restart, cancel, force-complete, fail work item: To perform an action as specified
in an exlet definition.

Figure 5.8 shows the interface requirements for the exception handling proce-
dures of the Worklet Service (see also Fig. 4.4).

5.4.3 Exception Handling Types and Primitives

To recap, there are several different types of exception that may occur during the
life of a YAWL process instance. At various points, constraints may be checked