How do I illustrate both sides of a transaction in BPMN?

Question

We have a number of processes that rely on interactions between two people/groups. I am trying to figure out the best way to illustrate this in BPMN.

CONSIDER: Using the example of a pizza order, I call a pizzeria to order, an order-taker answers the phone and then we discuss my order. I am trying to accurately capture the "we discuss the order" portion of the process. Here is how I envision the diagram playing out. I call, they answer, then there's branching for a simultaneous exchange, which converges at the end of the call and my order is finished. Is this illustrated correctly? or are there better ways to show that two different entities communicating with each other at the same time to accomplish a task?

Answer 1

The moment you want to show interaction in greater detail than high level bird view, you typically run into troubles when remaining within the paradigm of using "one pool with several lanes". In such a case you need to draw a so called "collaboration diagram", which means you make use of several pools and hence several process definitions interacting with each other by means of message exchange. I give you an example here:

You may use those envelope symbols attached to the message flows, but you don't need to.

The big advantage of that approach is that you can now show that those processes are dependent on each other, yes, but each participant also wants to remain in drivers seat of his/her own process, e.g. by deciding what to do if the other side doesn't provide the desired answer, doesn't do anything within a reasonable time and so on. Furthermore you can look at that diagram from both perspectives and people will actually see "their own process" - and not something mixed with the concerns of others involved.

For that same reason BPMN also offers the concept of "collapsed pools" to be able to look just at the communication from one side and treat the internal details of the other side as a kind of "black box":