The extend relationship specifies that the incorporation of the extension use case is dependent on what happens when the base use case executes. The extension use case owns the extend relationship. You can specify several extend relationships for a single base use case.
While the base use case is defined independently and is meaningful by itself, the extension use case is not meaningful on its own. The extension use case consists of one or several behavior sequences (segments) that describe additional behavior that can incrementally augment the behavior of the base use case. Each segment can be inserted into the base use case at a different point, called an extension point.
The extension use case can access and modify the attributes of the base use case; however, the base use case is not aware of the extension use case and, therefore, cannot access or modify the attributes and operations of the extension use case.
You can add extend relationships to a model to show the following situations:
- A part of a use case that is optional system behavior
- A subflow is executed only under certain conditions
- A set of behavior segments that may be inserted in a base use case
Extend relationships do not have names.
As the following figure illustrates, an extend relationship is displayed in the diagram editor as a dashed line with an open arrowhead pointing from the extension use case to the base use case. The arrow is labeled with the keyword «extend».
Example
You are developing an e-commerce system in which you have a base use case called Place Online Order that has an extending use case called Specify Shipping Instructions. An extend relationship points from the Specify Shipping Instructions use case to the Place Online Order use case to indicate that the behaviors in the Specify Shipping Instructions use case are optional and only occur in certain circumstances.