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An Introduction to Use Case Modeling

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Page 1: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

An Introduction to Use Case Modeling

Page 2: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Agenda

Requirements Use Cases Use Case Diagrams Use Case Scenarios Use Case Modeling with UML

Page 3: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Requirements

Look at how the traditional approach to requirements has severely limited our ability to satisfy our customer and stabilize our work effort.

Establish a requirement structure that will help ask the right questions and capture the information the Architects, Developers, Testers, and others will need.

Recognize that requirements strategies for categorizing, relating and agreeing requirements can greatly minimize the risk for building the wrong product.

Embrace the critical nature of a user-centric approach and employ techniques which keep user satisfaction at the center of our effort.

Page 4: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Who needs to understand the system?

Owners To make sure they are getting

the value they paid for Developers

To make sure they know what to develop

Testers To make sure they know what

to test Managers

To know where the system will be deployed

Technical Writers To know how to document the

system IT Operations/Support

To know how to install and maintain the system

Users To know how to use the

system

Page 5: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Exercise: Requirements Review

Situation You have been handed these requirements, a deadline, and a

budget.Exercise Break into groups of 3-4 people. Review the sample requirements document to become familiar

with what the business wants to achieve. As you review, think about the requirements in the context of

communication between the owners, developers, managers, testers, technical writers, IT operations, and users.

As a group, mark problems in the requirements.

Page 6: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Cases Defined

In general, use cases are high-level, user-centric requirements. [Fowler]

A use case specifies a sequence of actions, including variants, which the system can perform and that yields an observable result of value to a particular actor. [Jacobson]

Use Cases are: One of many techniques to elicit and elaborate user

requirements. A common presentation of requirements for different roles. A handy unit of planning and estimation.

Page 7: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Advantages of Use Cases

Avoid analysis paralysis Use cases help break up the problem so it can be solved

incrementally. Do just enough analysis to get started but we don’t have to worry that it’s hard to come back later and add more.

Avoid gold plating Using use cases to derive functional requirements avoids

stating a functional requirement that is not directly tied to a user task needed to accomplish a business goal.

Page 8: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case Diagrams

A cashier uses the POS system to scan an item.

A cashier uses the POS system to total items.

Page 9: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

System Boundary

Marks off the system as separate from its environment

Actors are outside

When no system boundary is shown, the system is assumed

Page 10: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Actor

Someone or something outside the system that interacts with it

Actors represent “roles” not individuals

Page 11: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case

A use case achieves a goal of value to an actor

System does these things for actors

What system is for not how it does it

Starts with an active verb from the point of view of the system

Page 12: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Communications

Lines between Actor and Use Case summarize interactions graphically

Actors and use cases exchange information to achieve the goal but the details of interaction are not shown

Page 13: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Considerations for Use Case Diagrams

Do not represent the flow of information or sequence of events.

Do not represent communication between actors. Keep the focus on putting the system in context, not the actors. Actors may collaborate through a use case.

Actors are not always roles played by a person. Actors may represent the role played by anything that acts on the system such as another system.

Page 14: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case Diagram Summary

Show the system in its environment Show what a system is used for From a behavioral perspective:

For capturing functional requirements For enabling incremental specification To understand who the system is for

Details of interactions are not shown

Page 15: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Levels of Use Cases

Use Case Diagrams can be at different levels of granularity: One diagram can show a system-wide view with every actor This diagram can be later refined into design-level use cases

Page 16: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case Analysis

When do we start use case analysis? During requirements elicitation

Obtain agreement on major concepts Actors, system boundaries, business use cases

Maybe as a sketch, transfer to tool later During requirements analysis

Insure the right questions have been asked and answered Supplement complex text with pictures Disambiguate text with precise models

When do we stop doing use case analysis? When the use cases meet the communication needs

Stakeholders reach consensus Less is more

Page 17: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Exercise: Use Case Diagrams

Situation You want to put requirements into context with use cases.

Exercise Break into groups of 3-4 people. Using the sample requirements document sketch an initial use

case diagram. As you create the diagram, think about the interactions

between the system and actors. As a group, consider how this level of use case analysis adds

value to the existing requirements.

Page 18: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Scenarios

Scenario is another name for a particular flow of events.

A use case covers a range of situations – a scenario is just one.

Each use case typically has: a main flow describing the

“happy path” alternate flows describing

major exceptions Several alternatives exist for

specifying the use case scenarios.

Page 19: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Describing Scenarios Textually

Write text to describe the interaction of the actor(s) and the system.

Simple and easy approach May be limiting:

Numerous alternate flows make it hard to understand where normal flow can branch.

Long alternate flows need to be broken out as steps too.

Use Case: Checks out item1. Customer sets item on counter.2. Sales clerk swipes UPC reader across UPC code on

item.3. System looks up UPC code in database procuring item

description and price.4. System emits audible beep.5. System announces item description and price over

voice output.6. System adds price and item type to current invoice.7. System adds price to correct tax subtotal.

Error case 1: UPC code unreadableIf after step 2, the UPC code was invalid or was not properly read, emit an audible ‘bonk’ sound.

Error case 2: No item in databaseIf after step 3 no database entry is found for the UPC flash the ‘manual entry’ button on the terminal. Accept key entry of price and tax code from Sales Clerk. Set Item description to “Unknown item”. Go to step 4.

Page 20: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Describing Scenarios Graphically

Create an Activity Diagram to graphically show the interaction of the actor(s) and the system.

Requires a little UML savvy Easy to slip into too much detail

Create a Sequence Diagram to graphically show the interaction of the actor(s) and the system.

Requires more UML savvy Great start for design activities

Page 21: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Exercise: Use Case Scenarios

Situation You want to elaborate an initial use case sketch with a

scenario for one of the use cases.

Exercise Break into groups of 3-4 people. Using the initial use case diagram, create a use case scenario. As a group, try both approaches. One person can try to capture

the flow textually and another graphically.

Page 22: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case Modeling with UML

In combination with plain text Large volume of requirements difficult to consume as a whole Modeling provides constructs to help organize ideas Visualization helps clarify complex ideas Standards bring focus to key abstractions

In contrast to ad-hoc diagrams Standards facilitate precise communication Common language for all members of the team

Reap the benefits of Model-Driven Development (MDD)Get ready for Model-Driven Architecture (MDA)

Page 23: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case Diagrams as the Hub of Architectural Viewpoints

StructuralView

ImplementationView

BehavioralView

EnvironmentView

User View

Activity DiagramsSequence DiagramsCollaboration DiagramsState Machine DiagramsInteraction Overview Diagrams

Composite Structure DiagramPackage DiagramsObject DiagramsClass Diagrams

Component Diagrams

Deployment Diagrams

Use Case Diagrams

Adapted from [Kruchten]

Page 24: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Formal UML Use Case Diagrams

Page 25: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Association

The line between an actor and a use case is an association.

The association means the actor is participating and/or communicating with the system via that use case.

The meaning of the association can be refined with:

Multiplicity Direction Labels

Page 26: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Multiplicity

Typically, An actor will have up to one interaction with a given use case. A use case instantiation interacts with one and only one of a given

actor. Previous to UML 2.0 this was the default association.

There are cases where we have multiple interactions simultaneously.

Page 27: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Direction

Indicates who initiates the communication Warning: does not necessarily indicate flow of information!

Page 28: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Labels

UML allows: Labels for the association Labels for roles on each end

Usage is uncommon and is not recommended

Page 29: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Specialization/Generalization of Actors

One actor can be a specialization of another.

Arrow points to the more general (base) actor.

Page 30: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Using Generalization

Page 31: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

How Use Cases are Related to Each Other

Generalization One use case is a special case of another

Includes Included use case embodies common behavior

Extends Extending use case adds behavior

Page 32: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Generalization

Shows inheritance and specialization One use case is simply a special kind of another

Page 33: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Includes

“Factor out” of a use case commonly used behavior Allows reuse of functionality by multiple use cases

Page 34: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Extends

Indicates that one use case adds or replaces behavior of another Must have a an associated extension point May have a condition

Page 35: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Exercise: Use Cases in UML

Situation You want to formalize the initial use case sketch as UML.

Exercise Break into groups of 3-4 people. Using the initial use case diagram, create a formal UML use

case diagram. As a group, try determine if there are opportunities for reuse

with generalization, extends, or includes.

Page 36: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Getting More from Modeling

Modeling maturity levels: Level 0: No specification Level 1: Textual Level 2: Text with Diagrams Level 3: Models with Text Level 4: Precise Models Level 5: Models only

But getting more means putting more in

GAPModel Code

Level 0

Level 1

Level 2

Level 3

Level 4

Level 5

Page 37: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Use Case Behavior

Adapted from the Final Adopted Specification for the UML 2.0 Superstructure

Page 38: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Activity

Activity diagrams are an easy way to represent the high-level flow of activity.

Show how activities connect to one another in a process.

Sequential or concurrent activities Often used to:

model the flow of events in a use case

model business processes model internal system

processes

Not easy to tell who is responsible for the behavior.

Page 39: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Interaction

Interaction diagrams help capture “who does what when”

Most popular way to show dynamic aspects of models.

Reveal the details of message passing – how objects respond to messages by delegating to others.

Page 40: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

State Machine

Use case analysis often reveals state-based behavior.

When you hear “status”, think of a state machine.

Exception flows may occur based on state.

The user may operate directly on the state of something in the system.

Page 41: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Quotable Quotes on Using UML

The UML is a large and growing beast, but you don’t need all of it. – Martin Fowler

“…when learning the UML, you need to be aware that certain constructs and notations are only helpful in detailed design while others are useful in requirements analysis …UML needs to be used together with an effective process…” – Brian Henderson-Sellers

Page 42: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

A Parting Thought

The term “use-case driven” is a wonderful marketing term but the reality is that use cases aren’t sufficient to drive much of anything. Use cases are a good technique to document behavioral requirements but that’s only a small part of the functional requirements picture and an even smaller part of the total requirements picture – they aren’t very good at documenting business rules, user interface requirements, constraints, or non-functional requirements. [Ambler]

Page 43: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

References

[Fowler] Fowler, Scott, UML Distilled 2nd Edition, Addison-Wesley

[Jacobson] Jacobson, Booch, Raumbaugh, The Unified Software Development Process, Addison-Wesley

[Ambler] Scott Ambler, www.agile-modeling.com

[Kruchten] Philippe Kruchten, Architectural Blueprints—The “4+1” View Model of Software Architecture, IEEE Software 1995

Page 44: An Introduction to Use Case Modeling. Agenda  Requirements  Use Cases  Use Case Diagrams  Use Case Scenarios  Use Case Modeling with UML

Recommended Reading

Armour, Frank, and Granville Miller, Advanced Use Case Modeling: Software Systems, Addison-Wesley

Charbonneau, Serge, Modeling Use Cases with the Borland Suite of Tools, BDN

Cockburn, Alistair, Writing Effective Use Cases, Addison-Wesley

Miller, Granville, Gathering Requirements: Use Cases, BDN

Miller, Randy, Practical UML™: A Hands-On Introduction for Developers, BDN

Miller, Randy, What's New in UML 2? The Use Case Diagram, BDN

Overgaard, Gunnar, and Karin Palmkvist, Use Cases: Patterns and Blueprints, Addison-Wesley

BDN content can be found at http://bdn.borland.com/