Ch.9 Requirements Modeling ： Scenario-Based
Methods
Requirements Analysis
• objectives
– Describe what the customer requires
– Establish a basis for the creation of a software design
– Define a set of requirements that can be validated
• Requirements analysis allows the software engineer (called an analyst or modeler in this role) to:
– elaborate on basic requirements established during earlier requirement engineering tasks
– build models that depict user scenarios, functional activities, problem classes and their relationships, system and class behavior, the flow of data as it is transformed, constraints that software must meet.
A Bridge
system description
system description
design model design
model analysis
model
Rules of Thumb
• The model should focus on requirements that are visible within the problem or business domain. The level of abstraction should be relatively high.
• Each element of the analysis model should add to an overall understanding of software requirements and provide insight into the information domain, function and behavior of the system.
• Delay consideration of infrastructure and other non-functional models until design.
• Minimize coupling throughout the system.
• Be certain that the analysis model provides value to all stakeholders.
• Keep the model as simple as it can be.
Domain Analysis
Goal: Software domain analysis is the
identification, analysis, and specification of
common requirements from a specific application domain, typically for reuse on multiple projects within that application domain . . .
DOMAIN ANALYSIS
SOURCES OF DOMAIN KNOWLEDGE
DOMAIN ANALYSIS MODEL technical literature
existing applications customer surveys
expert advice current/future requirements
class taxononmies reuse standards functional models domain languages
Software requirements
Scenario-based models
Use-cases (text) Use-case diagrams Activity diagrams Swim lane diagrams
Class-based models
Class diagrams Analysis packages CRC models
Collaboration diagrams
Flow-oriented models
Data flow diagrams control flow diagrams Processing narratives
Behavioral models
State diagrams
Sequence diagrams
Use-cases are simply an aid to defining what exists outside the system (actors) and what should be
performed by the system
(1) What should we write about?
(2) How much should we write about it?
(3) How detailed should we make our description?
(4) How should we organize the description?
Scenario-Based Modeling
Use-Cases
• a scenario that describes a “thread of usage” for a system
• actors represent roles people or devices play as the system functions
• users can play a number of different roles for a given scenario
Developing a Use-Case
• What are the main tasks or functions that are performed by the actor?
• What system information will the actor acquire, produce or change?
• Will the actor have to inform the system about changes in the external environment?
• What information does the actor desire from the system?
• Does the actor wish to be informed about unexpected changes?
Reviewing a Use-Case
• Use-cases are written first in narrative form and mapped to a template if formality is needed
• Each primary scenario should be reviewed and refined to see if alternative interactions are
possible
– Can the actor take some other action at this point?
– Is it possible that the actor will encounter an error condition at some point? If so, what?
– Is it possible that the actor will encounter some other behavior at some point? If so, what?
Use-Case Diagram
homeowner
Access camera surveillance via the
Internet
Configure SafeHome system parameters
Set alarm
cameras SafeHome
Activity and Swim Lane Diagrams
• Activity diagram supplements the use-case by providing a diagrammatic representation of procedural flow
• Swim lane diagram allows the modeler to represent the flow of activities described by the use-case and at the same time indicate which actor (if there are multiple actors involved in a specific use-case) or analysis class has responsibility for the action
described by an activity rectangle
Enter password and user ID
Select surveillance
Prompt for reentry
Select specific Camera thumbnails
Select Camera icon Select major function
View camera output in labeled window Prompt for another view
Other functions
Valid passwords/ID Invalid passwords/ID
No input Tries remain
Input tries remain Thumbnail views Select a specific camera
Exit See another camera
Activity diagram for access camera
surveillance — display camera views function
