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YAWL Reset net YAWL Reset net
p1 p1
t ts pt te
AND-split pt te
t
pN pN
p1 p1
AND-join ts pt
t p1
pN pN te p1 p1
XOR-split pt
t
pN te pN pN
p1 p1 ts p1
XOR-join
t pt
pN pN ts pN
te p1 p1
p1
OR-split pt te p1N
t
p t p ts pt te pN
pN
te pN
Fig. 3.4 Mapping various YAWL elements to reset net transitions and places
The following steps are taken to convert a YAWL net to a corresponding reset net
in the context of a specific OR-join:

1. The YAWL elements that do not affect the OR-join evaluation such as multiple
instances and composite tasks are abstracted.
2. All OR-joins in the net (except the one under consideration) are converted into
XOR-joins. This is called an optimistic approach and it is used to prevent the
premature firing of OR-joins. (See the next section for the reasoning behind this
conversion.)
3. The resulting YAWL net is transformed into a reset net using the mappings given
in Fig. 3.4.
After this transformation, a coverability analysis is carried out on the reset net to
determine whether the OR-join should be enabled at a given marking.
In this section, we have described the informal semantics of an OR-join using
some example YAWL nets. We have also seen how the existence of cancelation
regions in the net can complicate the analysis. Finally, we have briefly explained
how the formal semantics is defined in the YAWL language using reset nets. In the
next section, we motivate the particular OR-join semantics chosen for YAWL.

3.3 Motivation

In a workflow language like YAWL that supports many complex constructs such
as cancelation and loops without restrictions, there are a number of complicating
factors when it comes to defining a general approach to OR-join semantics. First,
for workflows with multiple OR-joins, it is an open issue how a state space analysis