xoEPC - Jan Mendling

70 3. Event-driven Process Chains (EPC)
Initial and Final Marking
The initial marking is the starting point for applying an iteration of the four-phase cycle.
In [Rum99], the initial marking of an EPC is specified as an assignment of tokens to one,
some, or all start events. While such a definition contains enough information for verifi-
cation purposes, for example by the bundling of start and end events with OR-connectors
as proposed in [MMN06a], it does not provide executable semantics according to the
original definition of EPCs. As pointed out in [Rit99], it is not possible to equate the trig-
gering of a single start event with the instantiation of a new process. This is because EPC
start events do not only capture the creation of a process instance, but also external events
that influence the execution of a running EPC (cf. [CS94]). This observation suggests an
interactive validation approach as presented by [DAV05], where the user makes explicit
assumptions about potential combinations of start events. In our approach, we assume
that in the initial marking, all start arcs as ∈ As have either a positive or a negative token
with the matching context 5 . A respective formalization of initial and final marking is
given later in Definitions 3.14 and 3.15. In the following sections, we describe the tran-
sition relations of each node n ∈ E ∪ F ∪ C in the phases of dead context, wait context,
negative and positive token propagation.
Phase 1: Dead Context Propagation
The transition relation for dead context propagation defines rules for deriving a dead
context if one input arc of a node has a dead context status. Note that this rule might
result in arcs having a dead context that could still receive a positive token. Those arcs
are reset to a wait context in the subsequent phase of wait context propagation (Phase 2).
Figure 3.12 gives an illustration of the transition relation. Please note that the figure
does not depict the fact that the the rules for dead context propagation can only be ap-
plied if the respective output arc does not hold a positive or a negative token. Concrete
tokens override context information, for isntance, an arc with a positive token will always
5 The context of non-start arcs is derived when the four propagation phases are entered the first time.
We choose to initialize all non-start arcs with a wait context (cf. Figure 3.11). Note that this context might
be changed in the dead context propagation phase before any token is moved.