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350 G. Decket et al.

Payment
+
Carrier Freight
Ordering
Appointment Delivered
+ + Freight in +
Transit
+
Fig. 13.2 Example of a business process model in BPMN

represented using a star symbol. Figure 13.1 features an event-based exclusive gate-
way. When the execution of the process arrives at this point (in other words, when a
token arrives at this gateway), the execution of that thread stops until either the mes-
sage event or the timer event occur. Whichever occurs first will determine which
way the execution will proceed. If the timer event occurs first, a shipment status
inquiry is initiated and the execution flow comes back to the event-based exclusive
gateway. If the message signaling the freight delivery is received first, the execu-
tion flow proceeds along the sequence flow that leads to the AND-join. Note that in
Fig. 13.1, the event-based exclusive gateway is also a join, as it has two incoming
flows. A token may arrive to this gateway from either of these incoming flows.
Events are classified along two criteria. First, an event can be a start event, an
end event, or an intermediate event. The sample BPD contains a single start (left-
most element) and a single end event (rightmost element), and two intermediate
events in the middle of the BPD. Second, events are classified according to their
trigger. In this chapter, we consider four types of triggers: messages, timers, condi-
tions, and errors. A message event is represented using an envelope icon. Such an
event is triggered by the receipt of a message, and can also be used to represent the
event of sending a message. Figure 13.1 shows a message event that is triggered by
the receipt of a message (presumably from a system located in the warehouse) rep-
resenting the arrival of the freight. A timer event is represented using a clock icon.
Figure 13.1 shows a timer event that is triggered when 24 h have elapsed. A rule
event is represented by an icon corresponding to a sheet of lined paper. It has a con-
dition attached to it and the event is triggered when this condition becomes true. An
error event is represented by a lightning icon. It is triggered when an error occurs,
but it can also represent the event of throwing an error. 2
The process model shown in Fig. 13.1 is “flat,” meaning that all the activities in
the model correspond to atomic units of work – also called atomic tasks in BPMN.
BPMN also supports activities that correspond to the execution of entire subpro-
cesses – like composite tasks in YAWL. Figure 13.2 shows a fragment of a BPD
consisting of a number of subprocesses, one of which corresponds to the Freight
In Transit process shown in Fig. 13.1. Subprocesses are distinguished from atomic
tasks using the C marker.

2
Specifically, an end error event corresponds to throwing an error, while an intermediate error
event corresponds to catching an error.