Describe the BPMN Event-Based Exclusive Gateway and how it is used in process modeling?

The more common BPMN gateways that you might be familiar with are usually data-based gateways such as the inclusive and exclusive gateways. Here is the list of all the BPMN Gateways.

When a data-based exclusive gateway is used, the value of a data element is evaluated by a condition statement to determine which path the process should proceed along. 

In the following example, when the process reaches the gateway it already has data regarding the distance that must be traveling.  The gateway condition evaluates the data to determine which path should be taken.

BPMN Data-Based Gateway

BPMN also provides event-based gateways. Event-based gateways do not evaluate data.  Instead, when the Event-Based Exclusive Gateway is reached it waits.  In particular it is waiting for an event which it will use to determine which path the process should proceed along.  An event-based gateway is required any time a decision is made by another participant based on data that is not visible to our process.

If you examine the icon used for the BPMN Event-Based Exclusive Gateway you will see that it inherits elements from several other BPMN icons.  First, there is the diamond which clearly shows it’s a gateway.  Per the BPMN specification, this gateway is only used to branch the process and is not used for merging or synchronization.  Within the diamond shape, there is a thin double circle representing an intermediate event shape.  Finally, there is a pentagon shape which represent multiple events.  So the Event-Based Exclusive Gateway is a gateway that deals with multiple intermediate events.

In the following example, the Event-Based Exclusive Gateway waits for an event to arrive, passes the flow of control along the correct path, and then doesn’t permit any other tokens to be passed along. Any other events that arrive as part of the same process instance will be ignored. 
Event-Based Exclusive Gateway Example

The event-based gateway also has the ability to be used to instantiate a process.  When this is the case the Event-Based Exclusive Gateway icon has only a single circle within the diamond.  When used to start a process, the Event-Based Exclusive Gateway allows the process to start in a number of ways based on the event that triggers it. 

BPMN Event-Based Exclusive Start

posted @ Saturday, July 6, 2013 4:46 PM by Chris Adams