Frank's Blog

BPMN and the Event-based Gateway 

Today I would like to tell you something about BPMN's Event-based Gateway. If you don't remember it by now, maybe you do by taking a closer look:



The shape of the Event-based gateway changed from BPMN 1.0 to BPMN 1.1. Both types of Gateways (Event-based or Exclusive) make a decision, i.e. only one of the outgoing Sequence Flows is taken. Only one of the Gateways, however, is used for joining Sequence Flows. That one is the Exclusive Gateway (please keep this in mind when modeling).

But what are the differences between the two types of Gateways, which make a decision leading to the activation of exactly one of the outgoing Sequence Flows? Here are the golden rules:

1. An Exclusive Gateway is always used when the decision is internally (up to you, based on locally available information).
2. An Event-based Gateway must be used when the decision is externally (up to others, based on distributed events).

Consider for instance a simple business process of getting in touch with a friend:



You could either choose to write a letter or make a phone call. The decision on how to get in touch is made in the Exclusive Gateway numbered with (1). Please note, that the decision on how to get in touch is completely up to you. The Sequence Flows are joined with Exclusive Gateway numbered with (2).

The corresponding business process of your friend, however, can not anticipate how you will get in touch with him. He needs to support both cases, where the decision is triggered externally:



The decision is made in the Event-based Gateway (3), based on the directly following Event that is triggered first. The flows are joined, once again, by an Exclusive Gateway, in (4).

The difference becomes immediately visible when the two business processes are brought together to form a choreography:



It can be clearly seen that the decision is made in the upper Pool, whereas the lower Pool can only react to that decision.

Now, hopefully, I will never see a BPD with a wrong use of the Event-based Gateway anymore (and I've seen way too much).

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