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A
use case diagram is “a diagram that shows the relationships
among actors and use cases within a system.”
Use case diagrams are often used to:
- Provide
an overview of all or part of the usage requirements for a system or
organization in the form of an
essential model or a business model
- Communicate
the scope of a development project
- Model
your analysis of your usage requirements in the form of a
system use case
model
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A use case model is comprised of one or more use case diagrams and any
supporting documentation such as use case specifications and actor definitions.
Within most use case models the use case specifications tend to be the primary
artifact with use case diagrams filling a supporting role as the “glue” that
keeps your requirements model together. Use case models should be
developed from the point of view of your project stakeholders and not from the
(often technical) point of view of developers. There are guidelines for:
-
Use Cases
-
Actors
-
Relationships
-
System Boundary Boxes
A use case describes a sequence of actions that provide a
measurable value to an actor. A use
case is drawn as a horizontal ellipse on a UML use case diagram, as you see in
Figure 1.
- Use Case Names Begin With a Strong Verb
- Name Use Cases Using Domain Terminology
- Place Your Primary Use Cases In The Top-Left Corner Of The Diagram
- Imply Timing Considerations By Stacking Use Cases. As you see in
Figure
1, the use cases that typically occur first are shown above those that appear
later.
Figure 1. Implying timing considerations between use
cases.
An actor is a person, organization, or external system that
plays a role in one or more interactions with your system (actors are typically drawn as
stick figures on UML Use Case diagrams).
Figure 2. Online shopping.
- Place Your Primary Actor(S) In The Top-Left Corner Of The Diagram
- Draw Actors To The Outside Of A Use Case Diagram
- Name Actors With Singular, Business-Relevant Nouns
- Associate Each Actor With One Or More Use Cases
- Actors Model Roles, Not Positions
- Use <<system>> to Indicate System Actors
- Actors Don’t Interact With One Another
- Introduce an Actor Called “Time” to Initiate Scheduled Events
There are several types of relationships that may appear on
a use case diagram:
- An
association between an actor and a use case
- An
association between two use cases
- A
generalization between two actors
- A
generalization between two use cases
Associations are depicted as lines connecting two modeling
elements with an optional open-headed arrowhead on one end of the line
indicating the direction of the initial invocation of the relationship.
Generalizations are depicted as a close-headed arrow with the arrow pointing
towards the more general modeling element.
Figure 3. Enrolling students in a university.
Indicate An Association Between An Actor And A Use Case If The Actor
Appears Within The Use Case Logic
Avoid Arrowheads On Actor-Use Case Relationships
Apply <<include>> When You Know Exactly When To Invoke The
Use Case
Apply <<extend>> When A Use Case May Be Invoked Across
Several Use Case Steps
Introduce <<extend>> associations sparingly
Generalize Use Cases When a Single Condition Results In Significantly New
Business Logic
Do Not Apply <<uses>>, <<includes>>, or
<<extends>>
Avoid More Than Two Levels Of Use Case Associations
Place An Included Use Case To The Right Of The Invoking Use Case
Place An Extending Use Case Below The Parent Use Case
Apply the “Is Like” Rule to Use Case Generalization
Place an Inheriting Use Case Below The Base Use Case
Apply the “Is Like” Rule to Actor Inheritance
Place an Inheriting Actor Below the Parent Actor
The rectangle around the use cases is called the system
boundary box and as the name suggests it indicates the scope of your system –
the use cases inside the rectangle represent the functionality that you intend
to implement.
- Indicate Release Scope with a System Boundary Box. In
Figure 2 you see that three system boundary boxes are
included, each of which has a label indicating which release the various use
cases have been assigned to.
- Avoid Meaningless System Boundary Boxes.
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The Elements of UML 2.0 Style describes a collection
of standards, conventions, and
guidelines
for creating effective
UML diagrams. They are based on sound, proven
software engineering principles that lead to diagrams
that are easier to understand and work with. These
conventions exist as a collection of simple, concise
guidelines that if applied consistently, represent an
important first step in increasing your productivity as
a modeler. This book is oriented towards
intermediate to advanced UML modelers, although there
are numerous examples throughout the book it would not
be a good way to learn the UML (instead, consider
The Object Primer). The book is a brief 188
pages long and is conveniently pocket-sized so it's easy
to carry around. |
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The Object Primer 3rd Edition: Agile Model Driven
Development with UML 2 is an
important reference book for agile modelers,
describing how to develop 35
types of agile
models including all 13
UML 2 diagrams.
Furthermore, this book describes the techniques
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Full Lifecycle Object Oriented Testing
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agile software development. The book also
shows how to move from your agile models to
source code (Java examples are provided) as well
as how to succeed at implementation techniques
such as
refactoring and
test-driven development
(TDD). The Object Primer also includes a
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development techniques (database refactoring,
object/relational mapping,
legacy analysis, and
database access coding) from my award-winning
Agile Database Techniques
book. |
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Agile Modeling: Effective Practices for Extreme
Programming and the Unified Process is the seminal
book describing how agile software developers approach
modeling and
documentation. It describes principles and
practices which you can tailor into your existing
software process, such as
XP, the
Rational Unified Process (RUP), or the
Agile Unified Process (AUP), to streamline your
modeling and documentation efforts. Modeling and
documentation are important aspects of any software
project, including agile projects, and this book
describes in detail how to
elicit requirements,
architect, and then
design your system in an agile manner. |
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I actively work with clients around the world to
improve their information technology (IT) practices as
both a mentor/coach and trainer. A full description of
what I do, and how to contact me, can be
found here.
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