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82 N. Russell and A. ter Hofstede

Fig. 2.40 Chained execution initiation in the Resource Service’s worklist

the associated work items are placed in the user’s worklist with a started status.
There are two parts to the configuration and use of chained execution: (1) the
user must have the chained execution privilege (see Fig. 2.43 for details of how
this occurs) and (2) the user must choose to enter the chained execution mode via
their worklist handler (the option for this is shown in Fig. 2.40). Once initiated,
chained execution continues until the user disables it or the case completes.
Piled execution: Piled execution is another operating mode; however, it oper-
ates across multiple process instances. It is enabled for a specified user-task
combination, and once initiated, it overrides any routing directive for the nomi-
nated task and ensures that any work items associated with the task in any process
instance are routed to the nominated user. There are two steps in enabling piled
execution for a task: (1) it needs to be specified in the YAWL model that the task
can be subject to execution in piled mode, as is the case for the Book Accommo-
dation task in Fig. 2.38 and (2) piled execution mode needs to be initiated by a
user for a given task using the option for this available in their worklist handler
as shown in Fig. 2.41. Once initiated, piled execution continues until the user
disables it.

2.6.5 Privileges

YAWL provides support for a number of privileges that can be enabled on a per-
task or per-user basis that affect the way in which work items are distributed and the
various interactions that the user can initiate to otherwise change the normal manner
in which the work item is handled. Table 2.8 summarizes the privileges that can be
set for individual users.