Lesson 2: BPMN Events and Gateways
This is the second lesson of our BPMN tutorial, which explains how to describe business processes using the following graphical elements: Events and Gateways (exclusive, inclusive and parallel).
Apart from the Start Event and the End Event, you can also use Intermediate Events to describe a business process. Intermediate Events usually occur in between Activities and show where something happens after a process has started and before it has ended. Examples of Intermediate Events are: arrival of a message, an e-mail or an event that occurs at a certain time.
An Intermediate Event is shown as an open circle with a double line.
Fig.2.1. Intermediate Event
In BPMN, there are different types of Intermediate, Start and End Events, such as Message, Timer, and Escalation. You can find their complete list in the BPMN 2.0 specification. The event types are distinguished by markers – specially designed icons that appear in the center of the circle that represents the Event. These markers define how an Event is triggered.
Here are some examples of Start Event types:
|– basic Start Event.|
|– Timer. It triggers a business process at a specific time (consider a meeting that must start at 9 a.m.) and specifies the circumstances in which the process is started.|
|– Message. A process is started when a message arrives from another process participant. For example, when a customer pays for their order, a corresponding message is sent, triggering the process of delivery.|
Examples of End Event types:
|– basic End Event. The result of the process is not defined.|
|– Message. A process participant sends a Message to another process participant at the end of the process.|
Examples of Intermediate Event types:
|– basic Intermediate Event.|
|– Timer. When a Timer Event triggers, the process is suspended for a certain period of time.|
|– Message. The envelope icon stands for a Message, which must be sent to or received from another participant in the process. An Intermediate Event of this type can initiate a Start Event of another process.|
The most commonly used type of Intermediate Event is the Timer. It allows you to specify a certain time, a date-time cycle or a time-out. Intermediate Events of this type are graphically marked by a clock icon.
Take a look at the process used in Lesson 1. We have extended it with additional activities, events and gateways.
Fig. 2.2 “Process order” process
Suppose, the bicycle that the customer has chosen is not in stock. In this case, it has to be ordered from the supplier. The process must be suspended until the bicycle is delivered to the store’s warehouse. Until then, the store’s employees cannot proceed to the following tasks. The Timer Event allows us to specify this point of the process in the process diagram..
Fig. 2.3 Timer Intermediate Event in process diagram
Another example of using the Timer Intermediate Event within our process is the delivery to the customer: once the purchased goods are delivered, the responsible employee has to contact the customer to receive feedback or to ask them to write a review on the store’s website..
Fig. 2.4 Timer Intermediate Event in process diagram
Another BMPN element, which we have mentioned before is the Gateway. Gateways are also differentiated by type. In Lesson 1, we used an Exclusive Gateway, which is represented as a diamond shape with no internal marker.
Fig.2.5 Exclusive Gateway
Exclusive Gateways split the process flow into several paths, allowing the process to take only one of them. The path is chosen according to conditions. A condition can be defined as a question asked at a certain point in a process. It has several possible answers; depending on the answer, the process follows one of the paths.
Take a look at our example process, where we used two Exclusive Gateways. The first one is defined by the question “Will the customer buy or rent the bicycle?”. Depending on the answer, the process goes to one of the following activities: “Fill in purchase form” or “Fill in rental form”.
The second Gateway that we used is defined by the question “Is the product in stock?”. Depending on the answer, the store’s employees either book the order at the warehouse or place an order with the supplier. These two tasks cannot be executed simultaneously.
Fig.2.6 Exclusive Gateway in process diagram
The second commonly used type of Gateways is a Parallel Gateway. A Parallel Gateway is used to visualize concurrent execution of activities. Graphically, it is shown as a diamond shape with a + marker.
Fig. 2.7 Parallel Gateway
A Parallel Gateway does not evaluate any conditions. It splits the process flow by sending it down all outgoing paths. For merging (synchronizing) different paths, the Gateway waits for all the incoming flows to be completed. Only then, the Gateways sends the process down the outgoing sequence flow.
Take a look at our example process in Figure 2.6. We used one Parallel Gateway to split the process into two parallel paths: “Place an order with supplier” and “Inform the customer”. These tasks are both executed at the same time: the responsible employee contacts the supplier to place the order and informs the customer about the possible delay in delivery. We used another Parallel Gateway to merge these paths: until both of the abovementioned tasks are completed, the process will not follow the sequence flow.
Another widely used type is the Inclusive Gateway, graphically identified by an O marker.
Fig.2.8 Inclusive Gateway
An Inclusive Gateway is a decision point of a process that breaks the flow into one or more paths. If only one condition evaluates to true, the process follows the path that meets the condition. If two conditions are true, then the process will take two parallel paths.
In our example, we used an Inclusive gateway to split the process flow into several possible paths, either alternative or parallel.
Fig.2.9 Inclusive Gateway in process diagram
There can be two possible alternative paths:
- If the ordered bicycle costs more than 500 USD, a gift is added to the order, even though the customer did not order any additional goods.
- If the ordered bicycle costs less than 500 USD, and the customer did not order any additional goods, a gift is not added to the order.
There can be two possible parallel paths:
- If the bicycle costs more than 500 USD and the customer has purchased additional goods for more than 100 USD, two gifts are added to the order. The process takes two parallel paths.
- If the bicycle costs less than 500 USD, and the customer has purchased additional goods for more than 100 USD, one gift is added to the order. The process takes two parallel paths.
These parallel paths are then merged in another Parallel Gateway.
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