04-usecasetutorial1

Use Case - 1 © Minder Chen, 1994-2004

UML

Use Case Modeling


Objectives

• Identify actors from a problem statement.

• Identify use cases from a problem statement.

• Learn how to create a use-case diagram that accurately models the system.

• Learn how to write use-case specification, glossary, and supplementary specification document.


The Purpose of the Requirements Discipline

• The Requirements discipline intends to: • Find agreement on what the system should do.

• Provide a better understanding of the system requirements.

• Define the boundaries of the system.

• Provide a basis for planning the technical contents of iterations.

• Provide a basis for estimating cost.

• Define a user-interface for the system.


Requirements in the Context of SDLC or RUP


SupplementarySpecification

Glossary

Use-Case Specifications

...

Use-Case Model

Actors

Use Cases

Relevant Requirements Deliverables or Artifacts


• A model that describes a system’s functional requirements in terms of use cases.

• A model of the system’s intended functionality (use cases) and its environment (actors).

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What Is a Use-Case Model?


What is Use Case Modeling?

A a view of a system that emphasizes the behavior as it appears to outside users. A use case model partitions system functionality into transactions (i.e., use cases) that are meaningful to users (i.e., actors).

• System behavior is how a system acts and reacts.

– It is an outwardly visible and testable activity of a system.

• System behavior is captured in use cases.– Use cases describe the system, its environment,

and the relationship between the system and its environment.


Usages of a Use Case Model


The software development life cycle


Use Case Granularity

Source: http://www-106.ibm.com/developerworks/rational/library/content/RationalEdge/jul02/TopTenWaysJul02.pdf


• An actor actor represents anything that interacts with the system.

• A use caseuse case defines a set of use-case instances, where each instance is a sequence of actions a system performs that yields an observable result of value to a particular actor.

Use Case

Actor

Major Concepts in Use-Case Modeling


• Used to communicate with the end users and domain experts

– Provides buy-in at an early stage of system development

– Insures a mutual understanding of the requirements

• Used to identify– Who interacts with the system and what the system should do

– The interfaces the system should have

• Used to verify– All requirements have been captured

– The development team understands the requirements

What Are the Benefits of Use-Case Models?


System Surroundings and Actors

• Users who execute the system’s – Main functions

– Secondary functions, such as system administration

• External hardware that the system uses

• Other systems interacting with the system


• Who will supply, use, or remove information?

• Who will use this functionality?

• Who is interested in a certain requirement?

• Where in the organization is the system used?

• Who will support and maintain the system?

• What are the system’s external resources?

• What other systems will need to interact with this one?

Actor

Useful Questions in Finding Actors


Actors and Roles

• An actor represents a role that a human, hardware device, or another system can play.

• The difference between an actor and an individual system user is that an actor represents a particular class of user rather than an actual user.

• Actor names should clearly denote the actor’s role

• Actor description:– Area of responsibility

– Dependency of the actor on the system

Minder asStudent Student

Minder asProfessor

Professor

A User May Have Different Roles


Use Case Modeling: Core Elements

Construct Description Syntax

use case A sequence of actions, including variants, that a system (or other entity) can perform, interacting with actors of the system.

actor A coherent set of roles that users of use cases play when interacting with these use cases.

system boundary

Represents the boundary between the physical system and the actors who interact with the physical system.

UseCaseNam e

ActorNam e



association The participation of an actor in a usecase. i.e., instance of an actor andinstances of a use case communicatewith each other.

extend A relationship from an extension usecase to a base use case, specifyinghow the behavior for the extensionuse case can be inserted into thebehavior defined for the base usecase.

generalization A taxonomic relationship between amore general use case and a morespecific use case.

Use Case Modeling: Core Relationships

<<extend>>



include Or use

An relationship from a base use case to an inclusion use case, specifying how the behavior for the inclusion use case is inserted into the behavior defined for the base use case.

Use Case Modeling: Core Relationships (cont’d)

<<include>>


Example: Online HR System

Online HR System

LocateEm ployees

UpdateEm ployee

Profile

Update Benefits

Access TravelSystem

Access PayRecords

Em ployee

M anager

Healthcare P lan System

{if currentMonth = O ct.}

{readOnly}

Insurance P lan System


Case Study: Course Registration Problem Statement

Course Registration Problem Statement (From Visual Modelling with Rational Rose and UML, Terry Quatrani 1998)The university will require a new Course Registration system. At the beginning of each semester, professors at the university will register the courses that they are going to teach with the system. The system then will allow students to request a course catalogue containing a list of course offerings for the coming semester. Information about each course such as professor, department and prerequisites are included. The student then can select four-course offering. Once the registration process is completed for a student, the registration system sends information to the billing system so the student can be billed for the semester. For each semester, there is a period of time that students can change their schedule through adding or dropping courses. Professors must be able to access the system to indicate which courses they will be teaching and to see which students signed up for their course offerings.


• In the Course Registration System Requirements document, read the Problem Statement for the Course Registration case study.

• As a group, identify the following:– Actors

– Description of the actor

Practice: Find the Actors


Finding Use Cases: Focus on the Actor

• The system exists only for its users• Use cases should be based on the user’s needs

• Answer the following questions to find use cases.

– For each actor you have identified, what are the tasks the system would be involved in?

– Does the actor need to be informed about certain occurrences in the system?

– Will the actor need to inform the system about sudden, external changes?

– What information must be modified or created in the system?


MOT Analysis Example: Credit Card • Pri to MOT

– Recognition– Information gathering– Comparison

• MOT – Applying for Credit Card– Receiving Credit Card– Using Credit Card– Providing Information – Changing and Upgrading – Gifts giving– Emergency Assisting

• After MOT– No usage follow-up– Stop membership follow-up

MOT: Moment of Truth


Practice: Solution

Billing System

Registrar

Professor Course Catalog

Student

A person who is registered to take courses at the University

The unabridged catalog of all courses offered by the University

The external system responsible for student billing

A person who is teaching classes at the University

The person who is responsible for the maintenance of the course registration system


• The name indicates what is achieved by its interactions with the actor(s).

• The name may be several words in length.

• No two use cases should have the same name.

Register forCourses

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Maintain StudentInformation

Name Use Cases


Practice: Finding Use Cases

• In the Course Registration System Requirements document, read the Problem Statement for the Course Registrations case study.

• As a group, using the actors identified in the earlier practice session, identify the following:

– Use Cases

– Use-Case names


Practice: Solution

Register forCourses

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MaintainProfessor Information

Close Registration

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Login Select CoursesTo Teach

SubmitGrades


• A use case models a dialog between actors and the system.

• A use case is initiated by an actor to invoke a certain functionality in the system.

Use Cases and Actors

a use case:. . .describes a sequence of actions, performed by a system, that yields a result of value to the user.


Concept: Communicate-Association

• Use cases and actors interact by sending signals to one another.

• To indicate such interactions, use a communicate-association.

Actor Use Case

Communicate-Association


Practice: Use Case and Actor Communication

• In the Course Registrations System Requirements document, read the Problem Statement for the Course Registration case study.

• As a group, using the actors and use cases identified in the earlier practice session, identify:

– Communicate-associations between the actors and the use cases.


Course Catalog

View Report Card

Register for Courses

Submit Grades

Select Courses to Teach

Student

Professor

Billing System

Maintain Student Information

Maintain Professor Information

Login

Close Registration

Registrar

Use Case and Actor Communication


Develop a Library System Use Cases Exercise

• Identify actors

• Determine major use major use cases

• Draw a use case diagram

The library system is designed for a local public library.


• Name

• Brief description

• Flows of Events

• Relationships

• Activity diagrams

• Use-Case diagrams

• Special requirements

• Pre-conditions

• Post-conditions

• Other diagrams


...

Use-Case Model

Actors

Use Cases



Use Case Description: Change Flight

Actors: traveler, client account db, airline reservation system Preconditions:

Traveler has logged on to the system and selected ‘change flight itinerary’ option

Basic course System retrieves traveler’s account and flight itinerary from client

account database System asks traveler to select itinerary segment she wants to

change; traveler selects itinerary segment. System asks traveler for new departure and destination information;

traveler provides information. If flights are available then … System displays transaction summary.

Alternative courses If no flights are available then …


Use Case Flow of Events• Has one normal, basic flow Happy Case

• Several alternative flows

– Regular variants of the basic flow

– Odd cases

– Exceptional flows handling error situations


• A scenario is an instance of a use case

What Are Scenarios?


Glossary

Course Registration System Glossary

1. Introduction

This document is used to define terminology specific to the problem domain, explaining terms, which may be unfamiliar to the reader of the use-case descriptions or other project documents. Often, this document can be used as an informal data dictionary, capturing data definitions so that use-case descriptions and other project documents can focus on what the system must do with the information.

2. Definitions

The glossary contains the working definitions for the key concepts in the Course Registration System.

2.1 Course: A class offered by the university.

2.2 Course Offering: A specific delivery of the course for a specific semester – you could run the same course in parallel sessions in the semester. Includes the days of the week and times it is offered.

2.3 Course Catalog: The unabridged catalog of all courses offered by the university.

Glossary


SupplementarySpecification

Supplementary Specification

• Functionality

• Usability

• Reliability

• Performance

• Supportability

• Design constraints

Online Resource: http://www-106.ibm.com/developerworks/rational/library/3975.html#N10071


Verify a Use Case Model

• Is the use-case model understandable?

• By studying the use-case model, can you form a clear idea of the system's functions and how they are related?

• Have all functional requirements been met?

• Does the use-case model contain any superfluous behavior?

• Is the division of the model into use-case packages appropriate?


• Have all the actors been identified?

• Is each actor involved with at least one use case?

• Is each actor really a role? Should any be merged or split?

• Do two actors play the same role in relation to a use case?

• Do the actors have intuitive and descriptive names? Can both users and customers understand the names?

Verify Actors


• Is each use case involved with at least one actor? • Is each use case independent of the others? • Do any use cases have very similar behaviors or flows

of events? • Do the use cases have unique, intuitive, and

explanatory names so that they cannot be mixed up at a later stage?

• Do customers and users understand the names and descriptions of the use cases?

Verify Use Cases


• Is it clear who wishes to perform a use case? • Is the purpose of the use case also clear? • Does the brief description give a true picture of

the use case? • Is it clear how and when the use-case's flow of

events starts and ends? • Does the communication sequence between

actor and use case conform to the user's expectations?

• Are the actor interactions and exchanged information clear?

• Are any use cases overly complex?

Verify Use-Case Specifications


• Does each term have a clear and concise definition?

• Is each glossary term included somewhere in the use-case descriptions?

• Are terms used consistently in the brief descriptions of actors and use cases?

Verify Glossary


Library System Use CaseA library contains books and journals. The task is to develop acomputer system for borrowing books. In order to borrow a bookthe borrower must be a member of the library. There is a limit on thenumber of books that can be borrowed by each member of the library.The library may have several copies of a given book.It is possible to reserve a book.Some books are for short term loans only. Other books may beborrowed for 3 weeks. Users can extend the loans.1. Draw a use case diagram for a library system.2. Give a use case description for two use cases:• Borrow a copy of a book• Extend the loan of a book

http://www.cs.nott.ac.uk/~sxp/OOMethods/Exer1.pdf


Step Wise Refinement of Use Case Model

The eight basic steps to generate use cases model for each business process area:

Step 1: Confirm actors and goals. Have all actors and their goals been identified? Which actors can be generalized (combined)? Which goals are potential use cases?

Step 2: Develop an outline of the use case(s). For the goals identified as potential use cases, what are the key pieces? For each outline level, what are key data? Outline all use cases. Prioritize the use-case flows. Decide on a final use-case list (for initial pass).

Step 3: Write a brief description of the use case(s). What two or three sentences describe all actors and the basic flow? Generate content first, and worry about wordsmithing it later.

Step 4: Detail the basic flow. What event starts the use case? How does the use case end? How does the use case repeat some behavior? What is the "happy" (best case) path? There is one and only one basic flow.


Step Wise Refinement of Use Case Model

Step 5: Detail the alternate flows.Are there optional situations for the use case? What might go wrong? What might not happen? Which resources might be blocked? Which alternate flows are special — perhaps nonfunctional — requirements (i.e., they apply to this use case only)?

Step 6: Review the use case(s). Are there more use cases? Should some use cases be redefined? Which ones can be combined?

Step 7: Record pre- and post-conditions. What was the previous state before this use case comes into play? What happens once the use case is complete?

Step 8: Develop generalizations for all use cases. Determine shared content and process for the use cases. What items have been noted for the glossary or as global business rules? Who has the most recent and accurate source document? Where is it located?


Reminders• Write something readable.

– Casual, readable use cases are still useful whereas unreadable use cases won’t get read.

• Work breadth-first, from lower precision to higher precision.– Precision Level 1: Primary actors name and goal– Precision Level 2: The use case brief; or the main success scenario– Precision Level 3: The extension conditions– Precision Level 4: The extension handling steps

• For each step:– Show a goal succeeding.– Capture the actor’s intention, not the user interface details.– Have an actor pass information, validate a condition, or update state.– Write between-step commentary to indicate step sequencing (or lack

of).– Ask “why” to find a next-higher level goal

• For data descriptions (only put Precision Level 1 into the use case text):

– Precision Level 1: Data nickname– Precision Level 2: Data fields associated with the nickname– Precision Level 3: Field types, lengths, and validations

Source: Writing Effective Use Cases by Alistair Cockburn, 2001, Addison-Wesley


The Writing Process

1. Name the system scope and boundaries.Track changes to this initial context diagram with the in/out list.

2. Brainstorm and list the primary actors.Find every human and non-human primary actor, over the life of the system.

3. Brainstorm and exhaustively list user goals for the system. The initial Actor-Goal List is now available.

4. Capture the outermost summary use cases to see who really cares. Check for an outermost use case for each primary actor.

5. Reconsider and revise the summary use cases. Add, subtract, or merge goals.Double-check for time-based triggers and other events at the system boundary.

6. Select one use case to expand.Consider writing a narrative to learn the material.

Source: Writing Effective Use Cases by Alistair Cockburn, 2001, Addison-Wesley


Continued…7. Capture stakeholders and interests, preconditions and

guarantees. The system will ensure the preconditions and guarantee the interests.

8. Write the main success scenario (MSS). Use 3 to 9 steps to meet all interests and guarantees.

9. Brainstorm and exhaustively list the extension conditions. Include all that the system can detect and must handle.

10.Write the extension-handling steps.Each will end back in the MSS, at a separate success exit, or in failure.

11.Extract complex flows to sub use cases; merge trivial sub use cases. Extracting a sub use case is easy, but it adds cost to the project.

12.Readjust the set: add, subtract, merge, as needed. Check for readability, completeness, and meeting stakeholders’ interests.


• What are the main artifacts of requirements?

• What are the requirements artifacts used for?

• What is a use-case model?

• What is an actor?

• What is a use case?

• What is the difference between a scenario and a use case?

Review Questions


Exercise

Create a payroll system for restaurants and hotels. Make certain thatthe usual deductions are taken into consideration. The payroll mustaccommodate both salaried and hourly employees. The waiters aresalaried employees, but the busboys are hourly employees. Thepayroll system must print checks weekly. The system will produce apayroll register which will be turned over to auditors monthly.Income and tax reports (such as W-2s* in the U.S.) must be preparedaccording to legal requirements. Reports concerning voluntarydeductions will be prepared for various agencies on a quarterly basis.? The payroll must treat part-time employees as hourly employees.Full-time employees and salaried employees may take advantage ofthe various company benefits, part-time employees may not.Restaurant employees will be able to eat meals at their restaurant butwill have the cost of the meals deducted from their paycheck. Hotelemployees will have room costs deducted if they live in the hotel.There are voluntary deductions and mandatory governmentdeductions that must be taken into account.*W-2 = U.S. annual income statement

Continued…


Exercise

• Identify for the Payroll System– Actors

– Use cases

• Name the use cases

• Produce the use-case model for the Payroll System.

• Provide:– Actor descriptions

– Brief use-case descriptions

• Compare your results to other groups.– Are there differences? Why? How would you

resolve these differences?


UML Notation Guide

Customer

Supervisor

SalespersonPlace

Establishcredit

Check

Telephone Catalog

F ill orde rs

Shipping Clerk

status

order

Use Case Diagram


Use Case Relationships

additional requests :

OrderProduct

SupplyArrange

«include»«include»«include»

RequestCatalog

«extend»Extension points

PaymentCustomer Data

after creation of the order

Place Order

1 * the salesperson asks forthe catalog

<<includes>> and <<extends>>


Actor Relationships

EstablishCredit

PlaceOrder

Salesperson

Supervisor

1 *

1 *


Online HR System: Use Case Relationships

Update M edicalP lan

Update DentalP lan

Update Benefits______________Extension pointsbenefit options:

after required enrollm ents

UpdateInsurance P lan

Em ployee

<<include>> <<include>> <<include>>

ElectReim bursem entfor Healthcare

Elect StockPurchase

<<extend>>em ployee requestsstock purchase option

<<extend>>em ployee requestsreim bursem ent option

extensioncondition

extension pointname andlocation


Online HR System: Update Benefits Use Case

Actors: employee, employee account db, healthcare plan system, insurance plan system

Preconditions: Employee has logged on to the system and selected

"update benefits" option Basic course

System retrieves employee account from employee account db

System asks employee to select medical plan type; include Update Medical Plan.

System asks employee to select dental plan type; include Update Dental Plan.

… Alternative courses

If health plan is not available in the employee’s area the employee is informed and asked to select another plan...


When to model use cases

• Model user requirements with use cases.

• Model test scenarios with use cases.

• If you are using a use-case driven method– start with use cases and derive your structural and

behavioral models from it.

• If you are not using a use-case driven method– make sure that your use cases are consistent with

your structural and behavioral models.


Use Case Modeling Tips

• Make sure that each use case describes a significant chunk of system usage that is understandable by both domain experts and programmers

• When defining use cases in text, use nouns and verbs accurately and consistently to help derive objects and messages for interaction diagrams

• Factor out common usages that are required by multiple use cases– If the usage is required use <<includes>> or

<<uses>>– If the base use case is complete and the

usage may be optional, consider use <<extends>>

• A use case diagram should– contain only use cases at the same level of abstraction– include only actors who are required

• Large numbers of use cases should be organized into packages


Set Display Options


A Use Case Diagram Created in Visio

Customer

OPS

Place order___________

Handle Rush Order

Track shipment

Log in

«uses»

«uses»

Handle Rush Order

«extends»

Order Operation Clerk

Manage inventory

AdministrationLogin

Manage Shipment


Display Options for Communications


Model Explorer

Linking an Linking a model element to another diagram


Exercise: Automated Teller Machine (ATM) • Brief Statement of Purpose: • An ATM is an electronic device designed for automated

dispensing of money. A user can withdraw money quickly and easily after authorization. The user interacts with the system through a card reader and a numerical keypad. A small display screen allows messages and information to be displayed to the user. Bank members can access special functions such as ordering a statement

• Additional Notes: • Users shall be able to access the ATM by punching in their

account number and PIN. Once the system has verified that the account is active and the PIN matches with the account number, the system offers the users four choices. Users can withdraw money, deposit money, check balance or quit the session.

• The user must have a minimum of $100 in his / her account. At the end of any transaction a printed copy of the transaction is provided to the user. A transaction could be - withdraw money, deposit money or check balance. Once the user has completed a transaction, the system offers the user the same four choices, until the user decides to quit.


Requirements• Brief Summary of Requirements: • The ATM is required:

1. To allow authorized card holders to make transactions 1. Card holders shall view and/or print account balances 2. Card holder shall make cash withdrawals 3. Card holder shall make cash or check deposits 4. Card holder shall quit session

2. To allow bank members to access additional, special services 1. A bank member shall be able to order a statement 2. A bank member shall be able to change security details (e.g. PIN number)

3. To allow access to authorized bank staff 1. Authorized staff can gain access to re-stock the machine 2. Authorized staff are able to carry out routine servicing and maintenance

4. To keep track of how much money it contains and alert bank staff when stocks are getting low.


Exercise Requirements

• Identify the actors

• Identify the uses cases

• Draw the top level Use-Case diagram

• Develop a detailed Use-Case analysis for a use case.

• Your analysis should contain a brief description of the use case, the basic and alternative flows, pre and post conditions.


The authoring life cycle of Use Cases


Context of a System

• The whole is more than the sum of the parts

• The whole determines the sum of the parts

• The parts cannot be understood if considered in isolation from the whole

• The parts are dynamically interrelated and interdependent

-- Fredrick Hagel (1770-1831)

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