BPMN21 — BPMN 2.0 Choreography issues page 327 of dtc/2010-06-05

Page 327 of /dtc/2010-06-05/ runs as follows (emphasis in the last
sentence added):

“To leverage the familiarity of flow charting types of Process
models, BPMN Choreographies also have “activities” that are ordered
by Sequence Flows. These “activities” consist of one (1) or more
interactions between Participants. These interactions are often
described as being message exchange patterns (MEPs). A MEP is the
atomic unit (“Activity”) of a Choreography.

Some MEPs involve a single Message (e.g., a “Customer” requests an
“Order” from a “Supplier”). Other MEPs will involve two (2) Messages
in a request and response format (e.g., a “Supplier” request a
“Credit Rating” from a “Financial Institution,” who then returns the
“Credit Rating” to the “Supplier”). There can be even more complex
MEPs that involve error Messages, for example.”

This wording may result confusing to practitioners. First of all,
there is no way in BPMN 2.0 Choreographies to mark a message as an
“error message”. What the specification seems to hint at is that
some of the messages in the choreography may deliver information
over errors. However, since BPMN 2.0 Choreographies (and, more
generally, BPMN 2.0 as a whole) does not differentiate among the
possible “typologies” of messages (e.g. error, acknowledgement,
invocation, or response), there is no immediate way to map WSDL
1.1/2.0 MEPs to single Choreography Taskswithout losing some
information (e.g. the distinction between output messages and fault
messages). Even using the structureRefof the message(see e.g. Page 7
of /dtc/2010-06-05/) to point to a WSDL 1.1/2.0 message, this would
provide no information about what “type” of message it is. In fact,
in WSDL 1.1/2.0 a message is “labelled” as input, output or fault
only in the WSDL operations, so much that a given WSDL message
could, for example, be input in one operation and a fault in another.

Secondly, despite the fact that the text mentions Choreography
Activities as the realization of MEPs (i.e. also
Sub-choreographies), the reference to MEPs as atomic units sounds
intuitively related to Choreography Tasks. Given the fact that it is
not possible to (1) specify error messages and (2) specify the
sequencing in a single Choreography Taskfor more than two messages,
a single Choreography Taskallows the specification of only the WSDL
1.1 “one-way messaging” and “notification” MEPs. In fact, the
specification of “request/reply” and “solicit/response” MEPs is not
possible because their faults cannot be represented in the same
Choreography Tasktogether with the input and output messages.

To clarify this issues, the specification should address clearly and
in detail the relation between BPMN 2.0 Choreographies and MEPs in
WSDL 1.1 and WSDL 2.0, in particular with relation to the
granularity of the MEPs (Choreography Task-level, Sub-choreography
level, etc.). Moreover, it is still an open issue how to map WSDL
2.0 Complex MEPs to BPMN 2.0 Choreographies.