SYS366

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SYS366

Last Lecture

System Use Descriptions

created through Use case

Authoring

Systems Use Case Descriptions Systems Use Case Authoring

Systems Use Case Diagrams and Descriptions

Based on the dialog metaphor

Dialog Expresses that the User and ComputerInteract by Sending Messages

Designing Dialogs

The process of designing the overall sequences that users follow to interact with an information system

the sequence in which information is displayed to and obtained from the user

Sequence

understanding how the user will interact with the system– clear understanding of user, task,

technological and environmental characteristics

Systems Use Case Descriptions

The (systems) use case descriptions provide the substance of the (systems) use case model and they are the basis for most of the …modeling work…More than 90% of the (systems) use-case model lies beneath the surface, in the textual use-case descriptions themselves. *

*Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, p. 30

“(Systems) use cases are more than just a named ellipse and a brief description. For each (systems) use case there will also be a (systems) use-case description where the full story of the use case is told.”*

“The use case description tells a story of how a system and its actors collaborate to achieve a specific goal

This collaboration takes the form of a dialog between the system and its actors

It is a step-by-step description of a particular way of using a system”*

Systems Use Case Description

Not a complete description of all possible ways that some task is performed

Does not say how the system is designed or implemented

Describes typical ways (or cases) of using the system*

*Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, pp. 24-25

Systems use Case descriptions are required to define, in detail, the processing that needs to happen in each use case

The systems use case description must include:

– Who the actors are and how many of them are interacting with the system at any point in time

– What data is used and how– All normal logic– All exception and error logic

Systems Use Case Descriptions Systems Use Cases Authoring

Systems Use Cases Authoring

The Systems Use Cases and its description evolves through the authoring process.*

*Systems Use Cases Modeling by Bittner & Spence, Page 152

The Authoring Life Cycle

Discovered Briefly Described

EssentialOutline

DetailedOutline

BulletedOutline

Systems Use CaseDescription

State 1: Discovered *– Through the Actor/Function/Use Case Table – Through experience– Shown on a Systems Use Case diagram

Place holder for the Systems Use Case description A visual index, providing a context for the descriptions

*Systems Use Cases Modeling by Bittner & Spence, Pages 153 - 154

State 2: Briefly Described– Once the Systems Use Case has been identified, it

should be described

State 3: Essential Outline– Focuses on only the most important behaviour of the system– Presents a “black box” view of the system in order to focus on

getting right what the system must do– Emphasizes usability

Helps describe user intent and actions, along with the observable response of the system

Does not describe what is happening inside the system

The Essential Outline

Stripped down essential outlines capture the essence of the required dialog without forcing the designers into any particular technology or mode of interaction

The Detailed Description

State 4: Detailed Description– Start adding to the (Essential) outline the detail

required to complete the Description of the system– Here the use case is evolving as more and more

detail is added to flesh out the outline

The Use Case Description

State 6: Fully Described– The final state in the evolution of a use case– The use case has a complete flow of events– Unambiguously defines all of the inputs and outputs

involved in the flow of events.

Use Case Descriptions

“The use case Description tells a story of how a system and its actors collaborate to achieve a specific goal

This collaboration takes the form of a dialog between the system and its actors

It is a step-by-step description of a particular way of using a system”*

“Just like a story, every use case should have – a clear beginning (how the actor starts the use

case)– Middle (how the system and actors work together)– End how the use case is concluded”*

The major sections of a use case Description are:

– Brief Description– Precondition statement– Main flow– Subflows– Exceptions– Successful post conditions

The Main Flow

The main flow describes:– How the use case starts– What events initiate system activity– What system activity each event initiates

The main flow describes the logic that controls the use case activity

Subflows

Describe a piece of system activity Only some subflows are initiated from the

main flow Subflows can initiate other subflows or

they can initiate another use case

Subflows

Must be specific. You must describe how each ends.

A subflow might:– Restart itself, another subflow or the use case– Continue the use case from the insertion point– End the use case– Launch another use case

Exceptions

Describe how the system handles user errors and unexpected system responses

Use Case Description

Not a complete description of all possible ways that some task is performed

Does not say how the system is designed or implemented

Describes typical ways (or cases) of using the system*

*Use Case Modeling, Kurt Bittner & Ian Spence, Addison-Wesley, 2003, pp. 24-25

Fully Described– One of the best checks of whether a use case

Description is finished is to ask if you could use the use case to derive system tests.

Fully Described– The best way to tell if the use cases fit the purpose

is to pass them along to the …test team for test design.

– If the team is satisfied that they can use the use cases to support this activity, then the use case Description contains sufficient levels of detail.

State 6: Fully Described– One of the best checks of whether a use case

Description is finished is to ask if you could use the use case to derive system tests.

– The best way to tell if the use cases fit the purposed is to pass them along to the

Use Case Description

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