Activity Diagram - Accept Event Notation

UML Tutorials - Herong's Tutorial Examples

∟Activity Diagram - Accept Event Notation

This section describes the Accept Event Notation used in a UML Activity Diagram. An Accept Event Notation represents an action that waits for an event to arrive from outside and processes it.

An Accept Event Notation is a graphical notation used in a UML Activity Diagram to represent an accept event action that waits for a certain event to arrive from outside of the activity.

There are two types of accept event actions in an UML activity diagram with different notations:

1. Recurring Accept Event Action - Waits for multiple events to arrive and processing events and passing the execution control to the next action. Once a recurring accept event is started, it never stops. A Recurring Accept Event Action Notation is drawn as a concave pentagon with on incoming flow control, which starts immediately when the parent activity starts and remains to accept events all the time.

2. One-Time Accept Event Action - Waits for one event to arrive and stops after processing that event and passing the execution control the next action. A One-Time Accept Event Action Notation is drawn as a concave pentagon with one incoming flow control, which connects to the previous action that triggers the start of this Accept Event action.

The following picture gives a good example of using Accept Event Notations in an Activity Diagram.

The first action in the diagram is a Recurring Accept Event Action called "Listen Order Request", which continuously waits and accepts new order requests. Each time a new order request is accepted, it will invoke the "Process Order" action, which in turn invokes the "Request Payment" action.

The "Request Payment" sends a payment request signal to the outside and invokes the "Wait for Payment" action, which is a One-Time Accept Event Action. It waits for the payment event to arrive, invokes the "Ship Order" action, then ends itself until it is invoked again by the "Request Payment" action.