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13 The Business Process Modeling Notation 363

Fig. 13.12 Mapping of exceptions

that sets a boolean variable (say e)to True, followed by an output condition. In
YAWL, when a token reaches the output condition of the (sub-)net, the entire exe-
cution of the subnet is stopped. This matches the behavior of the end error event in
BPMN, which effectively stops the execution of all activities in the encompassing
(sub-)process instance. The error variable e is initially set to False. Setting it to True
signifies that an exception has occurred. The value of the error variable is passed to
the parent net. Recall that a subprocess in BPMN is mapped to a composite task in
YAWL. Accordingly, a subprocess in BPMN with an attached error event is mapped
to a composite task in YAWL with an XOR-split decorator. This XOR-split tests
whether the error variable is set to True or False.If it is set to True, it means that the
error event has occurred. Thus, the exception flow should be taken (in the example
at hand, task C should be executed). Meanwhile, if the error variable is set to False,
it means that the error event has not occurred. Thus, the normal flow should be taken
(in the example, task B should be executed).
The mapping of the second type of error events is illustrated in Fig. 13.12(ii)
and 13.12(iii). Figure 13.12(ii) illustrates the mapping of a timer event attached to
an activity, regardless of whether this activity is an atomic activity or a subprocess
invocation. A timer event in BPMN maps to a task with an attached timer in YAWL,
cf. Task e in Fig. 13.12(ii). Meanwhile, the BPMN activity to which the timer event
is attached (task A in the figure) is also mapped to a YAWL task (which may be
simple or composite). These two YAWL tasks are enabled in parallel, but they can-
cel one another, that is, the firing of task e cancels task A and vice-versa. Thus, the
expiration of the timer causes task A to be canceled and the corresponding “excep-
tion path” to be taken. Reciprocally, completion of task A causes the timer task to
become irrelevant and the “normal path” to be pursued. A message event attached
to a BPMN activity is mapped to YAWL in a similar way, except that the main task
(i.e., task A in the example) is mapped to a task that listens for the corresponding
message type.
Figure 13.12(iii) shows a mapping of a rule event attached to a subprocess. A rule
is evaluated immediately after completion of each activity in the subprocess. Specif-
ically, every task in the generated YAWL subnet has an XOR-split attached to it with