BPMN21 — BPMN 2.0 Choreography issues page 338 of dtc/2010-06-05 about Sub-choreographics

On Page 338 of /dtc/2010-06-05/, about Sub-choreographies:

“The Participant Band of the Participant that does not initiate the
interaction MUST be shaded with a light fill.”

This wording does not cover some corner cases. Consider the example
depicted in the attached image Example Issue Initiating Participant
in Sub-Choreographies.png. In the choreography above it is unknown
until the enactment of Task 1which participant is the initiator of
Sub-choreography 1. But this is only the symptom of a wider-reaching
problem. When there is no choreography task that dominates of all
the others (or worse, there is a race condition!), and the various
choreography tasks that may be executed as first have different
initiators, modelers have no way to pick which participant is marked
as the initiator of the sub-choreographies.

The same issue with initiators of sub-choreographies affects also
the messages sent by them. In fact, Page 93 of /dtc/2010-06-05/reads:

“Any Message sent by the non-initiating Participant or
Sub-Choreography MUST be shaded with a light fill.”

We propose two possible, mutually exclusive solutions:

1. Additional constraints are specified for choreographies so that
no such corner case can occur. However, this is very likely to
result in not being able to model with BPMN 2.0 choreographies
some inter-organizational processes that can instead be modeled
with BPMN 2.0 Orchestrations.
2. Drop the differentiation between initiator and non-initiator
participants in sub-choreographies. We see no real shortcoming
resulting from this approach. In particular, with respect to the
enactability of choreographies, knowing which participant is the
first to act in a sub-choreography gives no guarantees as to the
fact that the same participant will also be involved in the
“last” choreography activities to be executed in that
sub-choreography. Therefore, we can extract no useful
information from it with respect to the enactability of what
follows that sub-choreography. This is particularly true in the
case of collapsed sub-choreographies.