Transcript Lecture for Chapter 4, Requirements Elicitation

Using UML, Patterns, and Java
Object-Oriented Software Engineering
Requirements Elicitation
Chapter 4
Object-Oriented Software Engineering:
Using UML, Patterns, and Java, 2nd Edition
By B. Bruegge and A. Dutoit
Prentice Hall, 2004.
Requirements Elicitation Activities
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Identifying Actors
Identifying Scenarios
Identifying Use Cases
Refining Use Cases
Identifying Relationships between Actors and Use Cases
Identifying Initial Analysis Objects
Identifying Nonfunctional Requirements
Modified from originals of Bruegge &. Dutoit
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Identifying Actors
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Actors – person or machine using the system in a particular role
Actors usually correspond to existing roles within the client
organization
Related roles can be grouped together according to viewpoints
Guide Questions
 Which user groups are supported by the system to perform their
work?
 Which user groups execute the system’s main functions?
 Which user groups perform secondary functions, such as
maintenance and administration?
 With what external hardware or software system will the system
interact?
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Watch out for confusion between actors and objects
Modified from originals of Bruegge &. Dutoit
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Identifying Scenarios
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Scenario
 “A narrative description of what people do and experience as they
try to make use of computer systems and applications” [Carrol,
Scenario-based Design, 1995]
 Informal description of a single feature from the viewpoint of a
single actor
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Types of Scenarios
 As-is scenarios – describes current situation
 Visionary scenarios – describes future system
 Evaluation scenarios – describes user tasks for evaluating the
system (acceptance criteria)
 Training scenarios – introduces new users to the system
Modified from originals of Bruegge &. Dutoit
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Heuristics for Identifying Scenarios
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Ask yourself or the client the following questions:
 What are the primary tasks that the system needs to perform?
 What data will the actor create, store, change, remove or add in the
system? Who else can modify this data?
 What external changes does the system need to know about?
 What changes or events will the actor of the system need to be
informed about?
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However, don’t rely on questionnaires alone.
Insist on task observation (ethnography) if the system already
exists
 Ask to speak to the end user, not just to the software contractor
 Expect resistance and try to overcome it
Modified from originals of Bruegge &. Dutoit
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Scenario Example: Warehouse on Fire
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Bob, driving down main street in his patrol car notices smoke coming out of
a warehouse. His partner, Alice, reports the emergency from her car.
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Alice enters the address of the building, a brief description of its location
(i.e., north west corner), and an emergency level. In addition to a fire unit,
she requests several paramedic units on the scene given that area appear to
be relatively busy. She confirms her input and waits for an acknowledgment.
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John, the Dispatcher, is alerted to the emergency by a beep of his
workstation. He reviews the information submitted by Alice and
acknowledges the report. He allocates a fire unit and two paramedic units to
the Incident site and sends their estimated arrival time (ETA) to Alice.
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Alice received the acknowledgment and the ETA.
Modified from originals of Bruegge &. Dutoit
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Observations about Warehouse on Fire Scenario
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Concrete scenario
 Describes a single instance of reporting a fire incident.
 Does not describe all possible situations in which a fire
can be reported.
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Participating actors
 Bob, Alice and John
Modified from originals of Bruegge &. Dutoit
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Identifying Use Cases
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Use Case
 Specifies all possible scenarios for a given functionality
 Initiated by an actor
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Motivations for use cases
 Generalizing related scenarios help developers define the scope of
the system
 The role of each user of the system is clarified
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Use Case Descriptions
 Entry and exit conditions
 Flow of events
 Quality requirements
Modified from originals of Bruegge &. Dutoit
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Heuristics: How do I find use cases?
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Select a narrow vertical slice of the system (i.e. one scenario)
 Discuss it in detail with the user to understand the user’s preferred
style of interaction
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Select a horizontal slice (i.e. many scenarios) to define the
scope of the system.
 Discuss the scope with the user
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Use illustrative prototypes (mock-ups) as visual support
Find out what the user does
 Task observation (Good)
 Questionnaires (Bad)
Modified from originals of Bruegge &. Dutoit
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Order of steps when formulating use cases
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First step: name the use case
 Use case name: ReportEmergency
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Second step: Find the actors
 Generalize the concrete names (“Bob”) to participating actors
(“Field officer”)
 Participating Actors:
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Field Officer (Bob and Alice in the Scenario)
Dispatcher (John in the Scenario)
Third step: Then concentrate on the flow of events
 Use informal natural language
Modified from originals of Bruegge &. Dutoit
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Use Case Example: ReportEmergency
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Use case name: ReportEmergency
Participating Actors:
 Field Officer (Bob and Alice in the Scenario)
 Dispatcher (John in the Scenario)
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Exceptions:
 The FieldOfficer is notified immediately if the connection between
her terminal and the central is lost.
 The Dispatcher is notified immediately if the connection between
any logged in FieldOfficer and the central is lost.
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Flow of Events: on next slide.
Special Requirements:
 The FieldOfficer’s report is acknowledged within 30 seconds. The
selected response arrives no later than 30 seconds after it is sent by
the Dispatcher.
Modified from originals of Bruegge &. Dutoit
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Use Case Example: ReportEmergency
Flow of Events
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The FieldOfficer activates the “Report Emergency” function of her
terminal. FRIEND responds by presenting a form to the officer.
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The FieldOfficer fills the form, by selecting the emergency level, type,
location, and brief description of the situation. The FieldOfficer also
describes possible responses to the emergency situation. Once the form is
completed, the FieldOfficer submits the form, at which point, the
Dispatcher is notified.
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The Dispatcher reviews the submitted information and creates an Incident in
the database by invoking the OpenIncident use case. The Dispatcher selects
a response and acknowledges the emergency report.
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The FieldOfficer receives the acknowledgment and the selected response.
Modified from originals of Bruegge &. Dutoit
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Identifying Use Cases
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Writing Guide
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Choose proper name – use verb phrases; indicate user’s objective
Name actors with noun phrases
Clearly distinguish actors’ actions from system’s actions
Use active voice to phrase steps in flow of events
The causal relationship between steps should be clear
Describe complete user transaction
Describe exceptions separately
Do not describe the user interface
Use cases should not exceed 2-3 pages – break up using <<include>>
and <<extends>> relationships
Modified from originals of Bruegge &. Dutoit
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Refining Use Cases
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Goal: completeness and correctness
Refining use case descriptions leads to other use cases and
clarifies system boundaries
 Entry and exit conditions – additional use cases are identified as
entry and exit conditions are refined
 Flow of events – discussing flow of events clarifies system
boundaries
 Quality requirements – elicit nonfunctional requirements in the
context of this particular functionality
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Refinements
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Details of objects in the system
Low-level interactions between actors and system
Access rights
Missing exceptions
Common functionality among use cases
Modified from originals of Bruegge &. Dutoit
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Refining Use Cases
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Heuristics
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Use scenarios to communicate with users and validate functionality
Refine a single scenario to understand user’s assumptions
Define many high-level scenarios to determine scope of the system
Use mock-ups or prototypes for visual support
Present user with a range of alternatives
Detail a broad vertical slice when scope of system and user
preferences are well-understood
Modified from originals of Bruegge &. Dutoit
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Relationships Between Actors and Use Cases
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Relationships between actors and use cases
 <<initiate>>
 <<participate>>
 Determines access rights
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Who can initiate a functionality
Who else is involved in this functionality
Relationships between use cases
 Heuristics for making use cases shorter and simpler to understand
 <<include>>
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For factoring out common functionality
Explicitly invoked from the including use case
 <<extend>>
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For specifying exceptions
Entry conditions of the extending use case determine when it is used
 Caveat: use discretion when applying these decompositions (a few longer
use cases are sometimes easier to understand than many short ones)
Modified from originals of Bruegge &. Dutoit
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<<Include>>: Functional Decomposition
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Problem:
 A function in the original problem statement is too complex to be
solvable immediately
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Solution:
 Describe the function as the aggregation of a set of simpler
functions. The associated use case is decomposed into smaller use
cases
ManageIncident
<<include>>
CreateIncident
Modified from originals of Bruegge &. Dutoit
HandleIncident
CloseIncident
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<<Include>>: Reuse of Existing Functionality
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Problem:
 There are already existing functions. How can we reuse them?
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Solution:
 The include association from a use case A to a use case B indicates
that an instance of the use case A performs all the behavior
described in the use case B (“A delegates to B”)
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Example:
 The use case “ViewMap” describes behavior that can be used by
the use case “OpenIncident” (“ViewMap” is factored out)
<<include>>
OpenIncident
Base Use
Case
ViewMap
<<include>>
AllocateResources
Supplier
Use Case
Note: The base case cannot exist alone. It is always called with the
supplier use case
Modified from originals of Bruegge &. Dutoit
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<Extend>> Association for Use Cases
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Problem:
 The functionality in the original problem statement needs to be
extended.
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Solution:
 An extend association from a use case A to a use case B indicates that use
case B is an extension of use case A.
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Example:
 The use case “ReportEmergency” is complete by itself , but can be
extended by the use case “ConnectionDown” for a specific scenario in
which the user cannot communicate with the dispatcher
ConnectionDown
FieldOfficer
f
<<extend>>
ReportEmergency
Note: The base use case can be executed without the use case extension
in extend associations.
Modified from originals of Bruegge &. Dutoit
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Identifying Initial Analysis Objects
Level 1
Level 2
Level 3
Modified from originals of Bruegge &. Dutoit
Level 2 Use Cases
Level 2
Level 3
Level 3
Level 4
A
Top Level Use Case
Level 3 Use Cases
Operations
Level 4
B
Object-Oriented Software Engineering: Using UML, Patterns, and Java
A and B
are called
Participating
Objects
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Use Cases can be used by more than one object
Level 1
Level 2
Level 3
Level 2 Use Cases
Level 2
Level 3
Level 3
Level 4
A
Top Level Use Case
Level 3 Use Cases
Operations
Level 4
B
Participating
Objects
Modified from originals of Bruegge &. Dutoit
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Identifying Initial Analysis Objects
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Identify the participating objects to create the initial analysis object model
Maintaining glossary of objects minimizes potential confusion in
terminology between users and developers
Heuristics
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Terms the needed clarification (by developer or user)
Recurring nouns in use cases
Real-world entities and resources that system must track
Use cases
Data sources or sinks
Artifacts with which user interacts
Use application domain terms
Cross-check
 Eliminate ambiguity: verify that objects with the same name refer to the
same concept
 Maintain consistency: verify that objects do not refer to the same concept
using different names
 Eliminate objects not involved in any use cases
Modified from originals of Bruegge &. Dutoit
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Identifying Nonfunctional Requirements
(FURPS+ Classification Scheme)
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Quality Requirements
 Usability
 Reliability/Dependability
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Safety
Security
Survivability
 Performance
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Pseudo Requirements
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Implementation
Interface
Operations
Packaging
Legal
Response Time
Throughput
Availability
Accuracy
 Supportability
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Adaptability
Maintainability
Portability
Modified from originals of Bruegge &. Dutoit
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Identifying Nonfunctional Requirements
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Heuristics
 Use a taxonomy (e.g., FURPS+) to generate checklists
 Give different checklists to users in appropriate roles
 Checklists vary depending on application domain
Modified from originals of Bruegge &. Dutoit
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Nonfunctional Requirements: Trigger Questions
User interface and human factors
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What type of user will be using the system?
Will more than one type of user be using the system?
What sort of training will be required for each type of user?
Is it particularly important that the system be easy to learn?
Is it particularly important that users be protected from making errors?
What sort of input/output devices for the human interface are available,
and what are their characteristics?
Documentation
 What kind of documentation is required?
 What audience is to be addressed by each document?
Hardware considerations
 What hardware is the proposed system to be used on?
 What are the characteristics of the target hardware, including memory size
and auxiliary storage space?
Modified from originals of Bruegge &. Dutoit
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Nonfunctional Requirements, ctd
Performance characteristics
 Are there any speed, throughput, or response time constraints on
the system?
 Are there size or capacity constraints on the data to be processed by
the system?
Error handling and extreme conditions
 How should the system respond to input errors?
 How should the system respond to extreme conditions?
System interfacing
 Is input coming from systems outside the proposed system?
 Is output going to systems outside the proposed system?
 Are there restrictions on the format or medium that must be used
for input or output?
Modified from originals of Bruegge &. Dutoit
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Nonfunctional Requirements, ctd
Quality issues
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What are the requirements for reliability?
Must the system trap faults?
What is the maximum time for restarting the system after a failure?
What is the acceptable system downtime per 24-hour period?
Is it important that the system be portable (able to move to different hardware
or operating system environments)?
System Modifications
 What parts of the system are likely candidates for later modification?
 What sorts of modifications are expected?
Physical Environment
 Where will the target equipment operate?
 Will the target equipment be in one or several locations?
 Will the environmental conditions in any way be out of the ordinary (for
example, unusual temperatures, vibrations, magnetic fields, ...)?
Modified from originals of Bruegge &. Dutoit
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Nonfunctional Requirements, ctd
Security Issues
 Must access to any data or the system itself be controlled?
 Is physical security an issue?
Resources and Management Issues
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How often will the system be backed up?
Who will be responsible for the back up?
Who is responsible for system installation?
Who will be responsible for system maintenance?
Modified from originals of Bruegge &. Dutoit
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Constraints (Pseudo Requirements)
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Constraint:
 Any client restriction on the solution domain
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Examples:
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The target platform must be an IBM/360
The implementation language must be COBOL
The documentation standard X must be used
A dataglove must be used
ActiveX must be used
The system must interface to a papertape reader
Modified from originals of Bruegge &. Dutoit
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How to Specify a Use Case (Summary)
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Name of Use Case
Actors
 Description of Actors involved in use case)
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Entry condition
 “This use case starts when…”
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Flow of Events
 Free form, informal natural language
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Exit condition
 “This use cases terminates when…”
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Exceptions
 Describe what happens if things go wrong
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Special Requirements
 Nonfunctional Requirements, Constraints
Modified from originals of Bruegge &. Dutoit
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