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8 The Design Environment 223
the starting net Overall of the Order Fulfillment process model (see Appendix A)
displayed on the canvas.
The control-flow of a YAWL specification can be defined by using elements
from the palette, located at the top of the left toolbar. The palette Wf-Elements &
Tools contains seven buttons, which assist with creation, selection, and positioning
of workflow elements on the canvas (see Fig. 8.2). The first five buttons are used
to place workflow elements on the net, such as an atomic task or a condition. The
marquee button allows the selection of individual or multiple elements that can be
moved within the net, while the Net Drag button (indicated by a cross) is used to
drag or modify single workflow elements on the canvas.
The palette Task Icons contains a set of predefined icons to decorate tasks (see
Fig. 8.2). For example, icons can be used to distinguish tasks that are executed by
a human resource from those executed by an application, as in the Order Fulfill-
ment process model. This set of standard icons can be enriched with custom-made
icons, added through a plug-in mechanism. The use of icons can increase the overall
understanding of a process, but it will not influence the task’s behavior.
A YAWL workflow specification is composed of one starting net and zero or
more subnets. The starting net captures the behavior of the overall process and is
the first net to be executed when a case is launched. Each subnet captures the behav-
ior of a composite task and is executed once the respective task fires. For example, in
the Order Fulfillment process model, the net labeled Overall is the starting net, while
nets Ordering, Payment,and Freight in Transit are subnets mapped to the homony-
mous composite tasks (see Fig. 8.2). The division of a process model into smaller
parts by means of subnets can facilitate maintenance and readability, especially in
the case of complex process models such as the Order Fulfillment example.
Each net features two mandatory elements: the input and the output condition,
which cannot be removed from the canvas. Tasks can be arranged between these two
nodes to describe the control-flow of the process and are connected to each other by
means of arrows, which represent order dependencies.
While a composite task is a placeholder for a subnet, an atomic task captures
a standalone piece of work, namely a work item. There are three types of atomic
task: Manual, automated, and routing task. A manual task is executed by a human
resource, for example, a particular employee or an organization participant with a
certain role or position. In the subnet Ordering (Fig. A.3)ofthe OrderFulfillment
process model, tasks Create Purchase Order and Confirm Purchase Order are man-
ual tasks that are performed by the Purchase Order Manager. The Editor supports
the workflow designer in setting up resource related aspects, for example, which
manual task will be performed by which resource or what strategy to use when allo-
cating a work item. This can be done via the Manage Resourcing... dialogue, which
is available from the context menu that pops up by right-clicking on the task. More
details on resource assignments are provided in Sect. 8.4.
An atomic task associated with the default worklist handler is manual by default.
If set as automated, a designer may assign a codelet or an external application to
the task via the Update Task Decomposition dialogue available from the task’s con-
text menu. Codelets are code snippets that are executed internally by the Engine.