Extraction, Transformation, and Loading - Data Warehouse Systems: Design and Implementation - page 283

any combination can be chosen to continue the flow. A parallel gateway
allows the synchronization between outgoing and incoming flows as follows.
A splitting parallel gateway is analogous to an AND operator: the incoming
flow triggers one or more outgoing parallel flows. On the other hand, a
merging parallel gateway synchronizes the flow merging all the incoming
flows into a single outgoing one. Finally, complex gateways can represent
complex conditions. For example, a merging complex gateway can model that
when three out of five flows are completed, the process can continue without
waiting for the completion of the other two.

Start event Intermediate

event

End event

Time Message Compensation Cancel Terminate

Fig. 8.3 Examples of events

Activity Activity

Canceled Compensated

Send
Message

Correct
Error

Fig. 8.4 Error handling: canceled and compensated activities

Events (see Fig. 8.3 ) represent something that happens that affects the
sequence and timing of the workflow activities. Events may be internal or
external to the task into consideration. There are three types of events, which
can be distinguished depending on whether they are drawn with a single, a
double, or a thick line. Start and end events indicate the beginning and
ending of a process, respectively. Intermediate events include time, message,
cancel, and terminate events. Time events can be used to represent situations
when a task must wait for some period of time before continuing. Message
events can be used to represent communication, for example, to send an
e-mail indicating that an error has occurred. They can also be used for
triggering a task, for example, a message may indicate that an activity can
start. Cancel events listen to the errors in a process and notify them either by
an explicit action like sending a message, as in the canceled activity shown
in Fig. 8.4 , or by an implicit action to be defined in the next steps of the
process development. Compensation events can be employed to recover