xoEPC - Jan Mendling

3.4. EPC Semantics 61
Even though the approach by Kindler provides semantics for a large subclass of EPCs,
i.e. clean EPCs, there are some cases like the EPCs of Figure 3.6(b) and 3.7 that do not
have semantics. The theorem by Kindler proves that it is not possible to calculate these
EPCs semantics as long as the transition relation is defined with a self-reference under
negation. Furthermore, such a semantics definition may imply some unexpected results,
e.g. the EPC of Figure 3.9(a) behaves differently than its refinement in Figure 3.9(b).
While it is argued that unclean EPCs only have theoretical relevance, there actually
are unclean EPCs in practice. Figure 3.10 shows the Test Equipment Management EPC
from the Quality Management branch of the SAP Reference Model (cf. [KT98]). The
marking of this EPC in the figure can be produced by firing the OR-split on the right-
hand side of the EPC. Both XOR-joins are on a loop resulting in an unclean marking.
This illustrates the need in theory and practice to also provide semantics for EPCs that
are unclean, according to Kindler’s definition [Kin06].
Van Hee, Oanea, and Sidorova discuss a formalization of extended EPCs as they are
implemented in the simulation tool of the ARIS Toolset (see [IDS03a]) based on a tran-
sition system [HOS05]. This transition system is mapped to colored Petri nets in order to
do verification with CPN Tools (see [JKW07]). The considered EPC extension includes
data attributes, time, and probabilities which are used for the simulation in ARIS. The
essential idea of this formalization and the ARIS implementation is that process folders
can have timers, and that these timers are used at an OR-join for synchronization pur-
poses. 2 If a folder arrives at an OR-join it has to wait until its timer expires. Since the
timers are only reduced if there are no folders to propagate, the OR-join can synchronize
multiple incoming folders. A problem of this approach is that once the timer of a folder
is expired, there is no way to synchronize it once it has passed the OR-join. If there
are several OR-joins used in sequence, only the first one will be synchronized. There-
fore, this formalization – though being elaborate – provides only a partial solution to the
formalization of the OR-join.
Van der Aalst and Ter Hofstede defined a workflow language called YAWL [AH05]
which also offers an OR-join with non-local semantics. As Mendling, Moser, and Neu-
2 Note that this general approach can be parameterized in ARIS with respect to sychronization and
waiting semantics (see [HOS05, p.194]).