Information Technology Reference
In-Depth Information
Sequence
AND split and join
p
2
p
3
t
2
p
6
p
1
p
2
p
3
p
1
t
1
t
2
t
1
t
4
p
4
t
3
p
5
OR split and join
Iteration
t
2
p
3
p
4
t
4
p
3
p
4
p
1
t
1
p
2
p
1
t
1
p
2
t
2
t
4
t
3
t
3
Figure 2.1
Basic workflow structures expressed in Petri nets.
iteration that is a special case of OR split and join. First
t
1
and
t
2
are
fired in sequence; when
t
2
completes its firing, either
t
3
or
t
4
is executed;
if
t
3
executes,
t
2
will fire afterward, which achieves an iteration of
t
2
;
otherwise
t
4
fires and the iteration is completed.
In a Petri net model of a workflow, the aforementioned reachability,
boundedness, and liveness properties have the following practical
meanings. Consider an order processing workflow of an online mer-
chant, for example, Amazon.com. Reachability concerns whether some
state of processing can be reached. For example, a customer order should
eventually be put into either
completed
or
canceled
state. Boundedness
usually concerns whether or not some activity will be executedmore than
expected. For example, the place that enables a payment activity should
never have more than one token such that the payment only carries
through once. Liveness concerns the absence of deadlock and possibility
to execute an arbitrary activity involved in a workflow. For example,
regardless of the execution sequence, the order processing should end
properly and does not hang in the middle (absence of deadlock). Also, for
an arbitrary activity in a workflow, it should be able to be executed in
some circumstances. That is to say, if there is an activity that is not able to
execute in any situation, then it makes no sense to include it in the
workflow. To address the aforementioned concerns, Aalst further defined
soundness
as the property of a correct WF-net.
