The Next Stage in Analysis: Systems Use Case Diagrams

Creating Systems use case diagrams Last stage of Requirements Gathering Satisfies the need to describe the interaction between users of the system and the system itself Describes what the system is to do (functions), not how it is going to do it (features)

Systems Use Case Diagram “Visual representation of the dialog between the actor and the system The system and its actors interact by sending signals or messages to one another”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 25

Dialog Expresses that the User and Computer Interact by Sending Messages

Systems Use Case “Each use case delivers something of value to at least one of the actors. The concepts of actor goals and the delivery of value to the actors are fundamental to the successful discovery, definition, and application of use cases.”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p.

Systems Use Case “The actors use the system only if it enables them to do something that they want to do The actors perform a use case only if doing so helps them achieve one of their goals. The physical manifestation of the goal is the value that the use case delivers to the actor.”* Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 23

Systems Use Case A concrete value can be put on the successful performance of a use case. Every use case should have an easily understandable and clearly identifiable value* * Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 23

Systems Use Case Diagram “The use case starts when an actor does something, causing the system to do something in response. This dialog continues…until the system has done something useful for at least one actor.”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 24

Systems Use Case Diagram “To indicate such interactions, we use a communicate association between the use case where the interaction occurs and the actors involved in the interaction”. * * Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 25

System Use Case Diagram “The communicate association represents a dialog between the actor and the system, a kind of communication channel over which data flows in both directions during the dialog.”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 26

Systems Use Case The ellipse is a graphical representation of a use case It is a placeholder for a description of how the system and its actors interact”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 23

Actors “Can represent humans or other systems Define the roles that users or other systems play when interacting with the system Are outside the system, and usually outside the control of the system Impose requirements on what the system being built must do”* * Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 22

Examples of Actors User Applications Devices Time Events Somebody who maintains the data, uses the data or generates reports Applications External processes or software systems (email interface) Devices External sensors (printers, scanners) Time Events System clock

UML Notation Actors are represented in UML by a ‘stick’ person “I am an actor. I play a role that involves using the system. I am outside of the system.” “My name indicates my role.” Order Clerk “I can be a person, a department, a system, hardware, scheduler, and so on”.

Actors communicate with the system “To start the use case To ask for some data stored in the system, which the use case then presents to the actor To change data stored in the system by means of a dialog with the system To report that something special has happened in the system’s surroundings that the system should be aware of”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 26

Use cases communicate with the Actor “To report that something special has happened in the system that the actor should be aware of To ask an actor for help in making a decision needed to achieve a goal To delegate responsibility to an actor”* Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 26

Systems Use Case Diagram “A use case has at most one communicate association to a specific actor, and an actor has…one communicate association to a specific use case, no matter how many interactions there are”* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 25

Systems Use Case diagrams One actor initiates a use case. However, after the use case has started, the use case can communicate with several actors.* *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 26

Systems Use Case Diagram “The complete network of such associations provides a static picture of the communication between the system and its environment.”* (the boundaries of the system) *Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, pp. 25 -26

Derive Use cases from Project Initiation Document When creating use cases, look to the product position statement for help Ask yourself for whom the system is being built, what problems the system is expected to solve and make sure that the system provides use cases that deliver this value. * *Bitner and Spence, page 109

Scope of the system The system use case diagram shows the scope of the system to be developed and is the foundation for your design If the use case diagram and the use case specifications are not correct, your design will not work.

System Use Case Diagram System Boundary