Intro: Use Case and Use Case Diagram Documentation

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Intro Use Case and Use Case DiagramDocumentation
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Use Case
A use case is a contract of an interaction
between the system and an actor. Use Case
Diagram an integration of use cases
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Use Case Diagram
A use case diagram illustrates a set of use cases
for a system, the actors, and the interactions
between actors and use cases. A graphical
overview of the functionality provided by a
system in terms of actors, their goals
(represented as use cases), and any dependencies
between those use cases.
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Use Case Diagram Objectives

• 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 a good Use Case Diagram?
Lack of ambiguity - Each requirement must be
interpreted in a single manner. Completeness -
The collection of all use cases is everything
that can be done to/with 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.
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Construct a Use Case Diagram
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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 Machines External systems Sensors
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Actor Relationships Generalization/Specializatio
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Define hierarchy for actors Notation 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)
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Association Actor and Use Case

• Solid line
• Interaction between actors and use case
• Arrowhead (optional)
• Control flow
• Initial invocation, primary actor

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Use Case Relationships

• Goal enable flexibility in requirements
  specification
• Isolating functionality
• Enabling functionality sharing
• Breaking functionality into manageable chunks
• Relationships
• Include
• Extend
• Generalization

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Include

• Goal
• Decomposing complicated behavior
• Centralizing common behavior
• the behavior of the included use case is inserted
  into the behavior of the including use case -
  The first use case often depends on the outcome
  of the included use case.

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Extend
the behavior of the extension use case may be
inserted in the extended use case under some
conditions Note the direction of the arrow The
base use-case does not know which use-case
extends it
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Generalization
use case may have common behaviors, requirements,
constraints, and assumptions with a more general
use case.
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Example Cellphone Company System
Hint - Actors Phones, Phone Companies
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Writing Use Cases

• Name
• Actors
• Descriptions
• Precondition
• Main flow
• Sub flow
• Alternative flow

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Precondition

• What the system needs to be true before running
  the use-case.
• User account exists
• User has enough money in her account
• There is enough disk space

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Main flow
The success scenario is the main story-line of
the use-case Assumption 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 subflows
Example Step 1 Administrator enters course
name, code and description (interaction) Step 2
System validates course code Step 3 System adds
the course to the db and shows a confirmation
message (interaction)
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Sub flow
Branches If the user has more than 10000 in
her account, the system presents a list of
commercials Otherwise Repeats User enters
the name of the item he wishes to buy System
presents the items User selects items to buy
Systems adds the item to the shopping cart User
repeats steps 1-4 until indicating he is done
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Alternative flows

• Used to describe exceptional functionality
• Examples
• Errors
• Unusual or rare cases
• Failures
• Starting points
• Endpoints
• Shortcuts

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Write Include in User Case
Reference
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Write Exclude in Use Case
Extension Point
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Use Case sample
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Include Concept
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Extend Concept
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Generalization Concept
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Effective Use Cases

• Only one side (system or actor) is doing
  something in a single step
• Write from an objective point of view using
  active tense
• Any step should lead to some progress

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Effective Use Cases
ATM
Get the amount form the user and give him the
money User click the enter key
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Effective Use Cases Common Mistakes

• 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

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From Use-Case to Use-Case Diagrams

• Top down ?
• Starting with an overview of the system, and
  then splitting Use-cases
• Bottom up ?
• Starting with throwing all scenarios on the
  page, and then combining them
• Most of the analysis process are actually
  combined

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Common Rules

• Number Limit The diagram should have between 3
  to 10 base use-cases. No more than 15 use cases
  (base included extending).
• ---- If the dependency between two parts of a
  use-case is weak, they should be divided.
• Abstraction All use-cases should be in similar
  abstraction levels.
• Size Use cases should be described in half a
  page or more.
• - split using include/exclude

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When we are 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

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Use Case and Use Case Diagram a Summary

• When to use
• What is Use Case and Use Case Diagram
• How to Construct Use Case Diagram
• How to Write Use Case
• Questions?