Building Use Case Diagrams

A use case diagram is a UML diagram that provides a graphical view of the requirements of your system, and helps you identify how users interact with it.

With a use case diagram, you immediately see a snapshot of the system functionality. Further details can later be added to the diagram if you need to elucidate interesting points in the system behavior.

A use case diagram is well suited to the task of describing all of the things that can be done with a database system by all the people who might use it. However, it would be poorly suited to describing the TCP/IP network protocol because there are many exception cases, branching behaviors, and conditional functionality (what happens when the connection dies, what happens when a packet is lost?)

In the following example, the actor "photographer" does two things with the camera: take pictures and change the film. When he takes a picture, he has to switch the flash on, open the shutter, and then close the shutter but these activities are not of a high enough level to be represented in a use case.

Use Case Diagram Objects
You can create the following objects in ause case diagram:
Creating a Use Case Diagram
You can create a use case diagram in an existing OOM in any of the following ways:
Use Cases (OOM)
A use case is an interaction between a user and a system (or part of a system). It defines a discrete goal that a user wants to achieve with the system, without revealing the system's internal structure.
Actors (OOM)
An actor is an outside user or set of users that interact with a system. Actors can be humans or other external systems. For example, actors in a computer network system may include a system administrator, a database administrator and users. Actors are typically those entities whose behavior you cannot control or change, because they are not part of the system that you are describing.
Use Case Associations (OOM)
An association is a unidirectional relationship that describes a link between objects.