Use case

How to read it

UML 2.5.1 draws a use case as an ellipse with its name inside or below it. Each use case is a unit of useful functionality that the subject, usually the system, provides to its users, and yields an observable result of value to its actors or other stakeholders. Solid lines join it to the actors involved. A rectangle drawn around a group of use cases, with a name in its top-left corner, is the subject, often called the system boundary: the use cases inside apply to it. The specification notes that the rectangle doesn't mean the subject owns them, and Scott Ambler, in The Object Primer, describes the boundary box as optional. A use case can also be drawn as a class rectangle with a small ellipse icon in its top-right corner.

Also searched as: uml use case, use case diagram, use case oval, use case symbol.

«include» and «extend»

Dashed arrows with open heads between use cases carry a keyword. An «include» arrow points from a base use case to an included one whose behaviour is inserted into it, typically behaviour that several use cases have in common. An «extend» arrow points the other way, from an extending use case to the one it extends, for behaviour added at a named extension point, possibly only under a condition. Ambler compares include to a procedure call and extend to a hardware interrupt, because the extending use case may or may not be invoked.

What the oval doesn't show

The ellipse names the use case; its steps are written somewhere else. The specification says a use case's detailed behaviour is set out in a separate diagram or a text document, and Ambler notes that a use case diagram doesn't show the information passing back and forth: an activity diagram can.

Look-alikes

An ellipse in an entity-relationship diagram is an attribute, from a different notation.