Requirements Modeling with Use Cases CEN5016: Software Engineering © Dr. David A. Workman School of Computer Science University of Central Florida February

Requirements Modelingwith

Use Cases

CEN5016: Software Engineering

© Dr. David A. Workman

School of Computer Science

University of Central Florida

February 1, 2005

April 19, 2023

January 30, 2007 (c) Dr. David A. Workman 2

Requirements Capture• Input

Client approaches Developer with a problem and or product concept. This may be expressed verbally or in the form of a document (Statement of Work (SOW)) (Request for Proposal (RFP))

• ActivitiesDeveloper interacts with Client and Users to elicit product requirements. This

involves face-to-face meetings and possibly the exchange of technical documents. The Developer must determine as completely and precisely as possible the following information:

– cost and time constraints – target system platform and operational environment– user groups– functional capabilities– non-functional constraints: quality and performance– Client and User's needs (as opposed to "wants")

• OutputsA complete understanding of the problem the Client and Users need to have solved.Client should be in agreement with the Developer’s assessment of the problem. This

shared view of the system is captured in the form of a UML Use Case Model.


Use Case Model• Definitions1

A Use Case is a sequence of actions that the system performs to offer some results of value to a User.

– Use cases drive the whole development process. “They offer a systematic and intuitive means of capturing functional requirements from the user’s perspective.”

– A system has many types of users. Each type of user is define by an actor. Actors may be people or external systems. Actors interact with the product via one or more Use Cases. An actor role is defined by a particular set of use cases performed by that actor to accomplish a particular goal or objective.

– “All actors and uses cases make up a Use Case Model.”

– A good collection of use cases is central to understanding what your users want. Use Cases also present a good vehicle for project planning, because they control iterative development, … it gives regular feedback to users about where the software is going.

– Use cases provide the basis of communication between the client and the developers in planning the project.

1The Unified Software Development Process, by Rumbaugh, Jacobson, and Booch, Addison-Wesley, 1999, 0-201-57169-2.


Use Case Model

• Definition2 (Fowler)A Use Case captures a typical interaction between a user and a computer system.

– A use case captures some user-visible function.

– A use case may be small or large.

– A use case achieves a discrete objective for the user.

In its simplest usage, you capture a use case by talking to typical users and discussing the various things they might want to do with the system. Take each discrete task or action they want to do, give it a name, and write up a short description.

During the Elaboration phase, this is all you need to do to get started.

2UML Distilled, by Scott & Fowler


Use Case Diagram

• Actors: perform user roles • Actions: identify use cases• Connectors: identify actors

that participate in an action.

Actor3

Actor4

SelectCatalog Pay by

Credit Card

Place Order

ArrangePayment

The system to be developed.

association

extends

includes

specialization/ generalization

<extends>

<include>

Seenotes

OrderProduct

<include>

* *

* *

* * * *

Actor1

Actor2

<specializes>


Use Case ModelActors

An actor is a role that a user plays with respect to the system. An actor is a role that requires the system to perform some function or task on its behalf, and not the converse.

Since a user may play more than one role at different times, it is important to focus on roles, rather than users (job titles ).

Actors carry out Use Cases; conversley, a Use Case may have several actors performing it.

– For large systems, it is easier to identify the actors first and then define the Use Cases each actor would perform.

– Actors do not have to be human, they could be external systems that must interface with the system in question.

– User groups can be characterized or profiled in terms of the actor roles they play.

– Different actors may share some set of Use Cases. This may suggest a need for assigning priorities to actors and/or defining some type of security mechanism to resolve access conflicts; the system may therefore need a mechanism for identifying a particular actor with which it is interacting.

– Actors generally receive value from a Use Case.


Use Case DiagramsConnector: Association

Bank

Manager

Pay byCredit Card

OrderProduct

* *

* * * *

Customer

Association defines a communication link between actors and the use cases they participate in, or between two or more use cases that have to interact (or share data) to accomplish their task. Associations are not directional, suggesting that interactions are generally bi-directional. However, UML does permit an “initiates” annotation to identify the actor or use case that initiates the interaction. Association is the default relationship when others do not apply.

« initiates »

« initiates »


Use Case DiagramsConnector: Includes

Bank

Manager

ArrangePayment

BrowseCatalog

* *

* *

* *

Customer

Includes is analogous to “whole-part” relationship between objects. One use case (the “whole”) may require several subordinate processing actions or steps (the “parts”).The subordinate actions are generally use cases themselves, but need not be. The Includesrelation should always be used if the subordinate action defines a necessary step to ensure success of the superior use case, or if the superior use case has sole responsibility for ensuring that the subordinate action is performed.

« initiates » BuyProduct

OrderProduct

« includes »

« includes »

« includes »


Use Case DiagramsConnector: Extends

FinancialInstitutions

* *

* *

In-houseDealer

Extends implies that one use case is a functional extension of the capabilities of another use case. If A extends B, then A does everything B does and more. Extension usually suggests extending either the potential user base, or extending the functional capability to an existing client base – offering more services – or both.

« initiates »

ExchangeMoney

Play CardGames

Electronically

PlayBlack Jack

« extends »

In-houseGamblers

* *

« initiates »

Casino System

* *

RemoteGamblers

See Notes


Use Case DiagramsConnector: Generalizes/Specializes

Customer

Generalization is analogous to the superclass – subclass relationship amongobject classes. A generalizes B if B is a special case of A. Specialization is the converse of generalizes, B specializes A.NOTE: the arrow dictates how the relation should be read.

« initiates »Ship

Package

Ship byUPS

Ship byUS Mail

US MailSystem

« specializes »

« generalizes »

Mail Boxes, Etc.

UnitedParcel Service

See Notes

* *

* *

* *


Use Case DiagramsConnector: Uses

Bank

* *

* *

Customer

Uses defines a relationship where two or more use cases (or actions) share a common function or action. Shared actions need not be use cases per se, but denote significant system-level operations that can be factored to improve system performance. Also,uses should be used to relate a major use case and an optional minor system action orsub- use case.

« initiates »

NYSE

* *

« uses »

e-PayBills

BuyStock

EstablishTelecomm

Link

« uses »


Use Case Model OutlineTitle (System Name, Author Name, Assignment, Course, Pub. Date)

– TOC– List of Figures (optional for small documents)

1. System Summary– Overview of System Purpose and Context– Business Case ( business need and how this system will address this need )– System Operation

• Use Case Diagram• Supporting Narrative (explains diagram: operational flow, actor roles )

– System Interfaces ( External Interfaces with Actors )

2. Use Case Specifications1. Purpose ( function from user’s perspective )

• Collaboration diagram (flow of interactions between actors and interface objects )• Narrative summary of use case purpose or function

2. Precondition (system states & triggering events )3. Flow of Events (nominal flow of interaction events between actor and interface objects)4. Alternative Paths (error processing flows; special case flows )5. Post Condition (system states & completion events )6. Special Requirements (non-functional : performance and quality )

3. Requirements Traceability4. Glossary


Use Case Model: Detail

Title Page and FormatDocument type, System Title, Author, Publication Date, Course, Assignment

Table of Contents (TOC)

Table of Figures (optional)

1. System Summary1. Scope and purpose of document ( Document content, Intended audience )

2. Business case: motivation for building the system; how it meets needs of users and organization

3. Concept of Operation• Use Case Diagram

• Narrative explaining flow of use cases and their relationships. Identifies all actors (and perhaps key internal agents) and introduces their roles with respect to the use case(s) they engage in. Should also identify all major problem data elements consumed, manipulated, or produced by the system.

4. System Interfaces (See Next Slide)• For each interface: identify the actors and use cases that exercise that interface, the

problem data content, flow direction, medium and/or mechanism.

• For each interface: supporting diagrams illustrating actor interface (optional)


Use Case Model: System Interfaces

Actor UseCase Actor Inputs

(Response)

System Outputs(Requests)

Interface

Mechanism

Vehicle at

Toll Booth

Pay Toll with Cash

Money (Coins | Bills)

Transaction Receipt

& Access Arm Raised

[& Change]

Interacts withToll Clerk and

Lane AccessArm

On-Line

Transaction

System

Pay Toll with Cash

(TransactionAccepted)

(Cash Transaction

Request)

(Amount Paid,

Change, Toll Clerk, Date)

TCP/IP

… … … …


Use Case Model: Detail

2. Use Case SpecificationsThis section should include a subsection (2.xx) for each use case identified in section 1.3. Each

use case should have the following format and organization. An introductory section should be included giving a diagram and narrative describing system states and their transitions.

1. Purpose: Specify the purpose of the use case – what it accomplishes from the view of the actors involved and the service the system provides these actors. Support with a collaboration diagram .

2. Preconditions: Specify the system states that must hold before the use case is defined or can begin, AND the triggering event(s) that mark the beginning of the use case.

3. Interaction Scenario: In conjunction with collaboration and state diagrams , present the primary or normal flow of interaction events that accomplish the purpose of the use case. The collaboration diagram identifies the analysis classes involved in the scenario.

4. Alternative Scenarios: In conjunction with collaboration and state diagrams , present possible alternative flows of interaction events. This subsection can be used to describe error handling scenarios and/or alternative sub-use cases.

5. Post conditions: Specify the system states that result after the use case has completed, AND the event(s) that mark the end of the use case.

6. Other Requirements: resource constraints, other qualify factors (e.g. reliability).


Use Case Modeling: Activity Diagram

Initial

?? [over payment]

[exact Amt]

AcceptMotoristPayment

ReturnChange

DeliverReceipt

ReleaseToll Arm


Use Case Modeling: Communication Diagram

Toll Clerk

Toll Arm

1: Pay Cash

2a: Return Change2b: Return Receipt 3: Raise Access Arm

4: Leave Gate

NOTE: Communication Diagram is UML 2.0 terminology.In earlier versions of UML, these were called Collaboration Diagrams.


Use Case Model: Detail3. Requirements Traceability

In this section you identify the sources of requirements and enumerate allrequirements statements. A source must be associated with each requirement. This is best presented in the form of a table.

1. List all sources of requirements statements. A source can be a document, an interview with some individual (customer or user or expert), or a web site, etc. Each source should be uniquely identified (by number or acronym ).Ref[1] Customer Requirements for Jiffy Stop Simulation SystemRef[2] Personal interview with Dr. David Workman, CEO of CEN 5016..

2. A Table should be constructed, such as the one shown below, where a statementof the requirement is given (“shall” statement) and a reference to thesource containing or implying the requirement statement.

Requirements Statments Requirements

Source

“The system shall simulate a convenience store customer engaged in the activity of purchasing gasoline from the time the customer leaves the automobile to the time the customer returns to the automobile upon completing the scenario.”

Ref[1] pp12, line 4.

“The pay-by-credit scenario shall support credit cards only – no debit cards.” Ref[2].


Use Case Model: Detail4. Glossary

The Glossary presents an organized list of problem space terms and their definitions as elicited from the client and system users. This provides the common vocabulary by which all system stake holders communicate. All common synonyms should be included, but the most common synonym should be used consistently through the rest of the document. For example, “clerk” is the most preferred term for the person that processes purchases and interacts with the customer to collect payment for gasoline purchased – this term should be used in the body of the document. However, one should list acceptable synonyms in a format as shown below.

Clerk: the convenience store agent (store employee) who processes each cash transaction for gasoline; configures the gas pump to dispense the appropriate fuel amount, and to produce a sales receipt of the transaction. Synonyms: cashier.

The Glossary should capture all terms that denote problem objects and system operations or use cases. It should capture all actors that externally interact with the system and the terms that pertain to actor interfaces. The Glossary should be used as a central repository of all information that is known about a particular term.


Use Case Modeling Process

List ofNoun Phrases

List ofInput Data

List ofOutput Data

List ofUse Cases

List ofActors

Actor/Use CaseRelationships

Inputs:Problem Statement (SOW)Requirements DocumentInitial Customer/User Interviews

Activity DiagramOf Dynamic

Use Case Flows

1.1Analyze RQ forNoun Phrases

1.2Classify Data asInputs/Outputs

1.3Analyze RQ forUse Cases (Verb

Phrases) & Actors

1.4Allocate Inputs/Outputs/Actors

to Use cases

1.5Determine

relationships amongUse cases

1.6Review with

customer/users

1.0 StartUse CaseModeling

Use CaseDiagram

1.7Revise Work

Products[need changes]

List of ActorInputs to Use Case

List of Use CaseOutputs to Actor

Use CaseInteraction

Scenario 1.8Select a

Use Case

1.9For this use case

determineInputs/Outputsfor each Actor

1.10Decide on

Boundary Objectsfor each Input

& Output

1.11Develop

InteractionScenarios forthis Use Case

Use CaseCommunication

Diagram

[ more use casesneed modeling ]

1.12Write

Use CaseSpecification

Nextslide

Use CaseSpec


Use Case Modeling Process

Use CaseModel

1.17Revise Work

Products

1.13Construct Requirements

Mapping TableFor Functional

And Non-FunctionalRequirements

1.14Write

Glossary

1.15ProduceUse Case

Model

1.16Review with

customer/users

[need changes]

StartAnalysisModeling

Documents

Requirements Modeling with Use Cases CEN5016: Software Engineering © Dr. David A. Workman School of Computer Science University of Central Florida February