1Spring 2005Specification and Analysis of Information Systems
Session 4:
Specifying Requirementswith Use Case Diagrams
Winter 2008
Analysis and Specification of Information Systems
Eran Tochhttp://www.technion.ac.il/~erant
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Outline
• Introduction • Use Case Diagrams• Writing Use Cases• Linking Use Cases• Guidelines for Effective Use Cases
Introduction | Diagrams | Writing | Linking | Guidelines
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Where are we?
Introduction | Diagrams | Writing | Linking | Guidelines
PhaseActionsOutcome
InitiationRaising a business needBusiness documents
AnalysisInterviewing stakeholders, exploring the system environment
Organized documentation
SpecificationAnalyze the engineering aspect of the system, building system concepts
Logical System Model
ImplementationProgram, build, unit-testing, integrate, documentation
Testable system
Testing & Integration
Integrate all components, verification, validation, installation, guidance
Testing results, Working sys
MaintenanceBug fixes, modifications, adaptationSystem versions
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Source of Requirements
• Initial requirements come from the customer, by:– Documents, such as RFI/RFP– Meetings, reports
• Advanced requirements come from the analysts, after studying:– Scope and price– Feasibility (technological, organizational etc)– Prototypes
• Final requirements are stabilized in an iterative process.
Introduction | Diagrams | Writing | Linking | Guidelines
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Requirements vs. Design
• Requirements:– What the system
should do– More abstract
• Design:– How the system
should do it– More detailed
Introduction | Diagrams | Writing | Linking | Guidelines
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Types of Requirements
• Visible Functional Requirements– “The system will deliver cash to the
customer”– “Cash will be delivered after card
was taken out”
• Qualitative Requirements– “The authorization process will take
no more than 1 sec”– “The user interface will be easy to
use”
• Hidden Requirements– “Database maintenance processes
will occur every night”
QualitativeRequirements
HiddenFunctional
Requirements
Functional Visible
Requirements
Introduction | Diagrams | Writing | Linking | Guidelines
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Outline
• Introduction • Use Case Diagrams• Writing Use Cases• Linking Use Cases• Guidelines for Effective Use Cases
Introduction | Diagrams | Writing | Linking | Guidelines
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Illustration
Use Cases
• A use case is a contract of an interaction between the system and an actor.
• A full use-case model comprise of:– A diagram, describing relations between use-cases and
actors.– A document describing the use case in details
Use Case
Use case in diagram Use Case in scriptActor
Introduction | Diagrams | Writing | Linking | Guidelines
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Objective
1. Create a semi-formal model of the functional requirements
2. Analyze and define:– Scope– External interfaces– Scenarios and reactions
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What Makes Good Use-Case Specification?
• Lack of ambiguity– Each requirement must be interpreted in a single manner.
• Completeness– They should cater for all current demands of the system.
• Consistency– Requirements should not conflict with each other. If there
are, tradeoffs must be detected and discussed.
• Avoid design– Requirements should raise a need, not answer it. (Why?)
Introduction | Diagrams | Writing | Linking | Guidelines
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Use Cases as Means of Communication
The use case should stimulate a discussion about what the system should do, mainly with people who are outside of the development team.
Customers Designers Users
Introduction | Diagrams | Writing | Linking | Guidelines
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Example
A Simple Example
Handle Message
Cellular Phone
Customer
Bill Management
Handle CallExternal Phone
Company
ActorsUse CaseSystem
boundaryAssociation
Introduction | Diagrams | Writing | Linking | Guidelines
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Finding Actors
• External objects that produce/consume data:– Must serve as sources and destinations for data– Must be external to the system
Humans MachinesExternal systems Sensors
Database PrinterOrganizational Units
Introduction | Diagrams | Writing | Linking | Guidelines
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Example
Actors can be generalized
The child actor inherits all use-cases associations
Should be used if (and only if), the specific actor has more responsibility than the generalized one (i.e., associated with more use-cases)
Register ClientSales Person
Institutional Sales Person
Perform Sale
Perform Business Sale
Sales Manager
Cancel Sale
Introduction | Diagrams | Writing | Linking | Guidelines
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Outline
• Introduction • Use Case Diagrams• Writing Use Cases• Linking Use Cases• Guidelines for Effective Use Cases
Introduction | Diagrams | Writing | Linking | Guidelines
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Structure of a Use Case Specification
Name
Actors
Preconditions
Post conditions
Success Scenario
Alternatives flows
Alistair Cockburn “Writing Effective Use Cases”
Trigger
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Triggers
• What starts the use-case?• Examples:
– Customer reports a claim– Customer inserts card– System clock is 10:00
Introduction | Diagrams | Writing | Linking | Guidelines
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Preconditions
• What the system needs to be true before running the use-case.
• Examples– User account exists– User has enough money in her account– There is enough disk space
Introduction | Diagrams | Writing | Linking | Guidelines
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Post-Conditions• A post-condition is the outcome of the use-case.• Examples
– Money was transferred to the user account– User is logged in– The file is saved to the hard-disk
• Minimal guarantee– The minimal things a system can promise, holding even when
the use case execution ended in failure– Examples: Money is not transferred unless
authorization is granted by the user• Success guarantee
– What happens after a successful conclusion of the use-case.– Examples: The file is saved; Money is transferred
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Success Scenario• The success scenario is the main story-line of the
use-case• It is written under the assumption that everything is
okay, no errors or problems occur, and it leads directly to the desired outcome of the use-case
• It is composed of a sequence of action steps • Example:
1. User enters product name, SKU and description2. System validates product SKU3. System adds the product to the DB and shows a confirmation message
Interaction step
Validation Step
Internal Change Step
(plus) Interaction Step
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Success Scenario Example
1. User enters a keyword2. System presents a set of search results, and
sponsored products, all of which include name, short description and price
3. User selects a product4. System displays a product page, including product
information, reviews and related products 5. User adds the product to the shopping cart
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Guidelines for Effective Writing
• Use simple grammar• Only one side (system or actor)
is doing something in a single step
• Write from an “objective” point of view– Bad: “Get the amount form the
user and give him the money”
• Any step should lead to some progress– Bad: “User click the enter key”
System ActorActor asks for money
System asks for amount
Actor gives the amount
System produce the money
Introduction | Diagrams | Writing | Linking | Guidelines
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Steps – cont’d
• Branches:– If the user has more than 10000$ in her account, the
system presents a list of commercials– Otherwise…
• Repeats:1. User enters the name of the item he wishes to buy2. System presents the items3. User selects items to buy4. Systems adds the item to the shopping cart5. User repeats steps 1-4 until indicating he is done
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Use-Cases – Common Mistakes
• Complex diagram• No system• No actor• Too many user interface details
– “User types ID and password, clicks OK or hits Enter”
• Very low goal details– User provides name– User provides address– User provides telephone number– …
Introduction | Diagrams | Writing | Linking | Guidelines
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Alternative Flows
Success Scenario
ShortcutsExceptions
Endpoints
• Used to describe exceptional functionality
• Examples:– Errors– Unusual or rare
cases– Failures– Starting points– Endpoints– Shortcuts
Starting points
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Alternative Flows - Example
• Errors:– “Case did not eject properly”– “Any network error occurred during steps 4-7”– “Any type of error occurred”
• Unusual or rare cases– “Credit card is defined as stolen”– “User selects to add a new word to the dictionary”
• Endpoints– “The system detects no more open issues”
• Shortcuts:– “The user can leave the use-case by clicking on the
“esc” key
Introduction | Diagrams | Writing | Linking | Guidelines
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Outline
• Introduction • Use Case Diagrams• Writing Use Cases• Linking Use Cases• Guidelines for Effective Use Cases
Introduction | Diagrams | Writing | Linking | Guidelines
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Linking Use-Cases
• Linking enables flexibility in requirements specification– Isolating functionality– Enabling functionality sharing – Breaking functionality into manageable chunks
• Three mechanism are used:– Include– Extend– Inheritance
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The “Include” Construct
• Include is used when:– Decomposing complicated behavior– Centralizing common behavior
• The base use case explicitly incorporates the behavior of another use case at a location specified in the base.
Perform Sale
Fill-in billing info
<<include>>
Example
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Writing Include
• If a base use-case include another use-case, we will add a reference as a step:1. System presents homepage2. User performs login to the system
OR<include: login to the system>
Introduction | Diagrams | Writing | Linking | Guidelines
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Extend – Graphical Representation
• The base use case can incorporate another use case at certain points, called extension points.
• Note the direction of the arrow– The base use-case does not know which use-case
extends it
Perform SaleAfter checkout
Gift wrap Products
<<extend>>Product is a gift
Example
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Writing Extend
• Scenarios do not include direct references• Instead, they include extension points, such as:
User enters search stringSystem presents search resultsExtension point: results presentations
OR<extension point: results presentations>
• The extension use-case includes conditions in which the extension is being committed– Example: if the user belongs to the “rich clients” group– If more than two commercials were found
Introduction | Diagrams | Writing | Linking | Guidelines
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Example: Amazon
Product Page
Review Writing Page
Shopping Cart
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Example – cont’d
Search Product
Navigate Deals
Checkout
Handle Order Status
Login Register
View Product Details Write
Review
Rank Supplier«include»
«include»
«include»
«include»
«extend»user is not a member
«extend»
«extend»
After page generation
Add to cart
«extend»
Customer
Introduction | Diagrams | Writing | Linking | Guidelines
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Generalization between Use-Cases• The child use case inherits the behavior parent use case:
– The interaction (described in the textual description)– Use case links (associations, include, extend, generalization)
• Child use-case can substitute parent Use case• Overriding occurs through the textual description
Example
Handle Sale Call
Customer Representative
Tech Assistant Representative
Handle Call
Handle Technical Assistance Call
1. Transfer call to available representative2. Mark representative as busy3. Start record call4. Stop record call 5. Log call details6. Mark representative as free
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Generalization Hazards
• Combining generalizations of actors and use-cases can be dangerous
Undergrad Student
Graduate Student
Submit Exam
Submit Thesis
Undergrad Student
Graduate Student
Submit and Get Grade
Submit Thesis
Submit Exam
Bad: Undergrad can submit thesis
Good: Only graduate student can submit thesis
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Example: Phone Company Operational System
The Cellular Phone
Who are the actors?
External Phone companies
Orange’s objective: Build a system that handles SMS messages, handles calls (for 2 and 3 generation phones), including conference calls and multiple calls from a single phone. The system must support users on the move.
Introduction | Diagrams | Writing | Linking | Guidelines
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Example: Cell Company System
3G Phone
Handle SMS Message
Cellular Phone
while talking
Handle Call
External PhoneCompany
Handle Video Call
Handle Cell Migration
<<include>> <<include>>
Handle Multiple Calls
Handle Conference Call
<<extend>>{incoming call}
<<extend>>{phone initiate call}
Handle Voice Call
Introduction | Diagrams | Writing | Linking | Guidelines
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Outline
• Introduction • Use Case Diagrams• Writing Use Cases• Linking Use Cases• Guidelines for Effective Use Cases
Introduction | Diagrams | Writing | Linking | Guidelines
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How to Model?
saveBullets format
loadSave
as
preview
File action
s
Formatting actions
Viewing Actions
Font forma
t
Paragraph format
Bottom-up Process Top-down Process
Starting with throwing all scenarios on the page, and then combining them:
Starting with an overview ofthe system, and then splittingUse-cases
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How to Model – cont’d
• Most of the analysis process are actually
Combined
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Combining Processes
• Number Limit:– The diagram should have between 3 to 10 base use-case. No
more than 15 use cases (base + included + extending).
• Abstraction:– All use-cases should be in similar abstraction levels.
• Size:– Use cases should be described in half a page or more.
• Interaction: – Use-cases which are carried out as part of the same interaction.
UCUC UC
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Dividing Processes
• Size:– If a use-cases takes more than a page, consider
include/extend
• Weak dependency:– If the dependency between two parts of a use-case is weak,
they should be divided.
UC
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More Guidelines
• Factor out common usages that are required by multiple use cases– If the usage is required use <<include>>– If the base use case is complete and the usage may be
optional, consider use <<extend>>
• A use case diagram should:– contain only use cases at the same level of abstraction– include only actors who are required
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Scope
• A good way to decide on the scope is by in/out lists:
Alistair Cockburn “Writing Effective Use Cases”
TopicInOut
Any non-software parts of the systemStatistical analysis of logsInterfacing with credit card systemsDatabase cleaning processesBackup of logs…
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When Are we Done?
• When every actor is specified.• When every functional requirement has a use-case
which satisfies it.• A tractability matrix can help us determine it:
Use Cases
Requirements
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Moving on
• Data entering and exiting the system is represented by data entities in structural diagrams.
• Behavior induced by use cases can be captured in behavioral diagrams.
Class AClass C
Class BClass D
Use Case 1
Use Case 2Use Case 3
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Summary Introduction
to the Unified Modeling Language (UML)To Use Case Diagram
Use Case DiagramsDual presentation of use-casesInclude, Extend, Inheritance
Writing Use CasesPreconditions & Post-conditionsMain scenario vs. Alternative Flow
Guidelines for Effective Use Cases
Introduction | Diagrams | Writing | Linking | Guidelines