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precedence relation. This step yields a graph with all nodes representing actions, while
the precedence relation defines directed edges between them. Second, control nodes
(split and join nodes) that realize the behavior routing in the process model have to be
introduced whenever a node has more than one predecessor or successor.
Starting with split nodes, our approach inserts nodes that implement either AND-,
XOR-, or OR-logic. The routing semantics depends on the precedence conditions for
the edges to succeeding nodes. If all precedences originating at an action are uncondi-
tioned, an AND-split node is inserted. If all precedences are conditioned and those con-
ditions do not overlap, an XOR-split node is inserted and each outgoing edge inherits
the respective precedence condition. Similarly, an OR-split is applied if the conditions
are overlapping.
The case of join nodes, nodes with multiple predecessors, is not straightforward. We
distinguish the following cases.
-
All precedences of an action
a
are conditioned, we use an AND-join to synchronize
these conditions.
-
Only a proper subset of precedences of an action
a
is conditioned, we use an OR-
join to synchronize any subset of these conditions.
-
All precedences of an action
a
are unconditioned. If
a
is mandatory, we apply either
an OR-join or an AND-join. The former is used if at least one of the preceding
actions is optional; the latter is used in all other cases. If
a
is optional, we proceed
as follows. An AND-join is applied to synchronize all precedences. Moreover, for
all combinations of results of preceding actions of
a
, we check for a state in which
the execution of
a
is observed as well. In other words, we identify all combinations
of results of preceding actions under which
a
can occur. The disjunction of these
result combinations is then used as a precedence condition for the edge between
the AND-join and action
a
.
Applying these steps yields a process template. Still, our approach to model synthesis
is rather naive and may create OR-joins for which the synchronization behavior could
be implemented using solely AND- and XOR-joins. However, existing methods for
restructuring a process model are used to replace these OR-joins with a semantically
corresponding structure of AND- and XOR-joins, see [22].
Example.
After we adapted the set of constraints
for our running example as discussed above, we
derive the basic graph structure for the template
based on the precedence relation. Fig. 2 visual-
izes this structure in a BPMN-like notation. Here,
the start and end actions are represented by start
and end events. Activities depicted with a dashed
border are optional. After inserting control nodes
(aka gateways in BPMN) into the graph, the com-
plete process template is derived. The first ver-
sion of the generated process template is shown in Fig. 3a. Application of the restruc-
turing according to [22] yields the process template shown in Fig. 3b.
bl
ef
edd
ep
og
rl
ra
rh
od
Fig. 2.
Precedence among actions
