the next stage in analysis: systems use case diagrams 1 sys366

The Next Stage in Analysis: Systems Use Case Diagrams

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SYS366

Identifying Actors and Systems Use Cases

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Requirements Gathering Need to find out what the user requires in the

system (user’s needs)Allows the Analyst to clearly understand the

user’s requirementsNeed to describe the interaction between

users of the system and the system itselfDescribes what the system is to do, not how

it is going to do it (features)

Systems Use Case Diagrams & Systems Use Case Descriptions

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Based on the dialog metaphor

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Dialog Expresses that the User and ComputerInteract by Sending Messages

Documenting Dialogs

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Many methods exist for documenting DialogsSystems Use Case diagrams: the dialog

between the actor and the systemWritten descriptions such as systems use case

descriptions or scenariosSketches of screens, i.e. storyboards (PRJ566

for games design)Dialog charts

Systems Use Case

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“Visual representation of the dialog between the actor and the system

The ellipse is a graphicalrepresentation 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

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“Can represent humans or other systemsDefine the roles that users or other systems

play when interacting with the systemAre outside the system, and usually outside the

control of the systemImpose requirements on what the system

being built must do”*

* Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 22

Actors

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SystemBoundary

Examples of Actors

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UserSomebody who maintains the data, uses the data or

generates reportsApplications

External processes or software systems (email interface)

DevicesExternal sensors (printers, scanners)

Time EventsSystem clock

UML Notation

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“I can be a person, a department,

a system, hardware, scheduler, and so on”.

“I am an actor. I play a rolethat involves using the system. I

am outside of the system.”

“My name indicates my role.”

OrderClerk

Actors are represented in UML by a ‘stick’ person

Systems Use Case

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“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

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“The use case should reflect the goals of the actors and enable, at least in part, their achievement”*

* Use Case Modeling, Kurt Bittner & Ian Spence, Addison Wesley, 2003, p. 23

Systems Use Case

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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*


Systems Use Case

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“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 Diagram

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“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”*



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“To indicate such interactions, we use a communicate association

between the use case where the interaction occurs and the actors involved in the interaction”. *


System Use Case Diagram

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“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.”*



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“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”*


System Use Case Diagram

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SystemBoundary


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“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


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“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.”*


Actors communicate with the system

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“To start the use caseTo ask for some data stored in the system,

which the use case then presents to the actorTo change data stored in the system by means

of a dialog with the systemTo report that something special has happened

in the system’s surroundings that the system should be aware of”*


Systems Use Case diagrams

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One actor initiates a use case. However, after the use case has started, the use case can communicate with several actors.*


Use cases communicate with the Actor

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“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

the next stage in analysis: systems use case diagrams 1 sys366

Documents