An Introduction to Use-Case Modeling

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An Introduction to Use-Case Modeling
The hardest single part of building a software
system is deciding precisely what to build. No
other part of the conceptual work is a difficult
as establishing the detailed technical
requirements, including all the interfaces to
people, to machines, and to other software
systems. No other work so cripples the resulting
system if done wrong. No other part is more
difficult to rectify later.
-Fred Brooks
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Requirements Process

• Requirements Elicitation Definition of the
  system in terms understood by the customer
• Requirements Analysis Technical specification
  of the system in terms understood by the
  developer.

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Requirements Elicitation

• Very challenging activity
• Requires collaboration of people with different
  backgrounds
• User with application domain knowledge
• Developer with implementation domain knowledge
• Bridging the gap between user and developer
• Scenarios Example of the use of the system in
  terms of a series of interactions with between
  the user and the system
• Use cases Abstraction that describes a class of
  scenarios

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Requirements Validation

• Critical step in the development process,
• Usually after requirements engineering or
  requirements analysis. Also at delivery
• Requirements validation criteria
• Correctness
• The requirements represent the clients view.
• Completeness
• All possible scenarios through the system are
  described, including exceptional behavior by the
  user or the system

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Requirements Validation

• Consistency
• There are functional or nonfunctional
  requirements that contradict each other
• Realism
• Requirements can be implemented and delivered
• Traceability
• Each system function can be traced to a
  corresponding set of functional requirements

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User-Centered Development and Use-Case Modeling

• User-centered development a process of systems
  development based on understanding the needs of
  the stakeholders and the reasons why the system
  should be developed.
• Use-case modeling the process of modeling a
  systems functions in terms of business events,
  who initiated the events, and how the system
  responds to those events.
• Use-case modeling has roots in object-oriented
  modeling.
• Gained popularity in nonobject development
  environments because of its usefulness in
  communicating with users.
• Compliments traditional modeling tools.

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Benefits of Use-Case Modeling

• Provides a tool for capturing functional
  requirements.
• Assists in decomposing system scope into more
  manageable pieces.
• Provides a means of communicating with users and
  other stakeholders concerning system
  functionality in a language that is easily
  understood.
• Provides a means of identifying, assigning,
  tracking, controlling, and management system
  development activities, especially incremental
  and iterative development.
• Provides an aid in estimating project scope,
  effort, and schedule.
• Provides a baseline for testing in terms of
  defining test plans and test cases.
• Provides a baseline for user help systems and
  manuals as well as system development
  documentation.
• Provides a tool for requirements traceability.
• Provides a starting point for the identification
  of data objects or entities.
• Provides functional specifications for designing
  user and system interfaces.
• Provides a means of defining database access
  requirements.
• Provides a framework for driving the system
  development project.

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System Concepts

• Use-case diagram a diagram that depicts the
  interactions between the system and external
  systems and users.
• It graphically describes who will use the system
  and in what ways the user expects to interact
  with the system.
• Use-case narrative a textual description of
  the business even and how the user will interact
  with the system to accomplish the task.
• Use case a behaviorally related sequence of
  steps (a scenario), both automated and manual,
  for the purpose of completing a single business
  task.
• Description of system functions from the
  perspective of external users in terminology they
  understand.

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Sample Diagram
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Basic Use-Case Symbols

• Use case subset of the overall system
  functionality
• Represented graphically by a horizontal ellipse
  with the name of the use case appearing above,
  below, or inside the ellipse.
• Actor anything that needs to interact with the
  system to exchange information.
• Could be a human, an organization, another
  information system, an external device, or even
  time.
• Actors communicate by sending and receiving
  stimuli to and from the system. Each actor has a
  name.
• Temporal event a system event triggered by
  time.
• The actor is time.

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Four Types of Actors

• Primary business actor
• The stakeholder that primarily benefits from the
  execution of the use case.
• e.g. the employee receiving the paycheck
• Primary system actor
• The stakeholder that directly interfaces with the
  system to initiate or trigger the business or
  system event.
• e.g. the bank teller entering deposit information
• External server actor
• The stakeholder that responds to a request from
  the use case.
• e.g. the credit bureau authorizing a credit card
  charge
• External receiver actor
• The stakeholder that is not the primary actor but
  receives something of value from the use case.
• e.g. the warehouse receiving a packing slip

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Scenarios

• A narrative description of what people do and
  experience as they try to make use of computer
  systems and applications
• A concrete, focused, informal description of a
  single feature of the system used by a single
  actor.
• Scenarios can have many different uses during the
  software lifecycle

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Types of Scenarios

• As-is scenario
• Used in describing a current situation. Usually
  used during reengineering. The user describes
  the system.
• Visionary scenario
• Used to describe a future system.
• Can often not be done by the user or developer
  alone
• Evaluation scenario
• User tasks against which the system is to be
  evaluated
• Training scenario
• Step by step instructions designed to guide a
  novice user through a system

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How do we find scenarios?

• Dont expect the client to be verbal if the
  system does not exist
• Dont wait for information even if the system
  exists
• Engage in a dialectic approach (evolutionary,
  incremental)
• You help the client to formulate the requirements
• The client helps you to understand the
  requirements
• The requirements evolve while the scenarios are
  being developed

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Heuristics for Finding Scenarios

• Ask yourself or the client the following
  questions
• What are the primary tasks that the system needs
  to perform?
• What data will the actor create, store, change,
  remove or add in the system?
• What external changes does the system need to
  know about?
• What changes or events will the actor of the
  system need to be informed about?
• Insist on task observation if the system already
  exists (interface engineering or reengineering)
• Ask to speak to the end user, not just to the
  software contractor
• Expect resistance and try to overcome it

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Example Accident Management System

• What needs to be done to report a Cat in a Tree
  incident?
• What do you need to do if a person reports
  Warehouse on Fire?
• Who is involved in reporting an incident?
• What does the system do if no police cars are
  available? If the police car has an accident on
  the way to the cat in a tree incident?
• What do you need to do if the Cat in the Tree
  turns into a Grandma has fallen from the
  Ladder?
• Can the system cope with a simultaneous incident
  report Warehouse on Fire?

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Scenario Example Warehouse on Fire

• Bob, driving down main street in his patrol car
  notices smoke coming out of a warehouse. His
  partner, Alice, activates the Report Emergency
  function from her laptop.
• Alice enters the address of the building, a
  brief description of its location (i.e., north
  west corner), and an emergency level. In addition
  to a fire unit, she requests several paramedic
  units on the scene given that area appear to be
  relatively busy. She confirms her input and waits
  for an acknowledgment.
• John, the Dispatcher, is alerted to the emergency
  by a beep of his workstation. He reviews the
  information submitted by Alice and acknowledges
  the report. He creates allocates a fire unit and
  two paramedic units to the Incident site and
  sends their estimated arrival time (ETA) to
  Alice.
• Alice received the acknowledgment and the ETA.

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Observations

• Concrete scenario
• Describes a single instance of reporting a fire
  incident.
• Does not describe all possible situations in
  which a fire can be reported.
• Participating actors
• Bob, Alice and John

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Next Goal

• Find all the use cases in the scenario that
  specifies all possible instances of how to report
  a fire
• Example Report Emergency in the first
  paragraph of the scenario is a candidate for a
  use case
• Describe each of these use cases in more detail
• Describe the Entry Condition
• Describe the Flow of Events
• Describe the Exit Condition
• Describe Exceptions
• Describe Special Requirements (Constraints,
  Nonfunctional Requirements)

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Heuristics

• First name the use case
• Use case name ReportEmergency
• Then find the actors
• Generalize the concrete names (Bob) to
  participating actors (Field officer)
• Participating Actors
• Field Officer (Bob and Alice in the Scenario)
• Dispatcher (John in the Scenario)
• Then concentrate on the flow of events
• Use informal natural language

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ReportEmergency Flow Events

• The FieldOfficer activates the Report Emergency
  function of her terminal. FRIEND responds by
  presenting a form to the officer.
• The FieldOfficer fills the form, by selecting the
  emergency level, type, location, and brief
  description of the situation. The FieldOfficer
  also describes possible responses to the
  emergency situation. Once the form is completed,
  the FieldOfficer submits the form, at which
  point, the Dispatcher is notified.
• The Dispatcher reviews the submitted information
  and creates an Incident in the database by
  invoking the OpenIncident use case. The
  Dispatcher selects a response and acknowledges
  the emergency report.
• The FieldOfficer receives the acknowledgment and
  the selected response.

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Use Case Example

• Use case name ReportEmergency
• Participating Actors
• Field Officer (Bob and Alice in the Scenario)
• Dispatcher (John in the Scenario)
• Exceptions
• The FieldOfficer is notified immediately if the
  connection between her terminal and the central
  is lost.
• The Dispatcher is notified immediately if the
  connection between any logged in FieldOfficer and
  the central is lost.
• Special Requirements
• The FieldOfficers report is acknowledged within
  30 seconds. The selected response arrives no
  later than 30 seconds after it is sent by the
  Dispatcher.

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How to Specify

• Name of Use Case
• Actors (Description of Actors involved in use
  case)
• Entry condition (This use case starts when)
• Flow of Events (Free form, informal natural
  language)
• Exit condition (This use cases terminates
  when)
• Exceptions (Describe what happens if things go
  wrong)
• Special Requirements (Nonfunctional Requirements,
  Constraints)

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Use Case for Incident Management
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A new example

• Consider the case of a person joining and
  belonging to a health club

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Step 1 identify Business Actors

• When looking for actors, ask the following
  questions
• Who or what provides inputs to the system?
• Who or what receives outputs from the system?
• Are interfaces required to other systems?
• Are there events that are automatically triggered
  at a predetermined time?
• Who will maintain information in the system?

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The Process of Requirements Use-Case Modeling

• Objective is to elicit and analyze enough
  requirements information to prepare a model that
• Communicates what is required from a user
  perspective.
• Is free of specific details about how the system
  will be built or implemented.
• To effectively estimate and schedule project, may
  need to include preliminary system
  implementation assumptions.
• Steps
• Identify business actors.
• Identify business use cases.
• Construct use-case model diagram.
• Documents business requirements use-case
  narratives.

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Sample List of Actors
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Step 2 Identify Business Requirements Use Cases

• During requirements analysis, strive to identify
  and document only the most critical, complex, and
  important use cases, often called essential use
  cases.
• When looking for use cases, ask the following
  questions
• What are the main tasks of the actor?
• What information does the actor need form the
  system?
• What information does the actor provide to the
  system?
• Does the system need to inform the actor of any
  changes or events that have occurred?
• Does the actor need to inform the system of any
  changes or events that have occurred?

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Use Case Association Relationship

• Association a relationship between an actor
  and a use case in which an interaction occurs
  between them.
• Association modeled as a solid line connecting
  the actor and the use case.
• Association with an arrowhead touching the use
  case indicates that the use case was initiated by
  the actor.
• Association lacking arrowhead indicates a
  receiver actor.
• Associations may be bidirectional or
  unidirectional.

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Use Case Extends Relationship

• Extension use case a use case consisting of
  steps extracted from a more complex use case in
  order to simplify the original case and thus
  extend its functionality.
• Relationship between the extension use case and
  the use case it is extending is called an extends
  relationship.
• Represented as an arrowheaded line beginning at
  the extension use case and point to the use case
  it is extending.
• Each extends relationship line is labeled
  ltltextendsgtgt.

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Use Case Uses Relationship

• Abstract use case a use case that reduces
  redundancy among two or more other use cases by
  combining the common steps found in those cases.
• An abstract case is available for use by any
  other use case that requires its functionality.
• Relationship between the abstract use case and
  the use case that uses it is called a uses (or
  includes) relationship.
• Depicted as an arrowheaded line beginning at
  the original use case and pointing to the use
  case it is using.
• Each uses relationship line is labeled
  ltltusesgtgt.

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Use Case Depends On Relationship

• Depends On a use case relationship that
  specifies which other use cases must be performed
  before the current use case.
• Can help determine sequence in which use cases
  need to be developed.
• Depicted as an arrowheaded line beginning at
  one use case and pointing to a use case it is
  dependent on.
• Each depends on relationship line is labeled
  ltltdepends ongtgt.

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Use Case Inheritance Relationship

• Inheritance a use case relationship in which
  the common behavior of two actors initiating the
  same use case is extrapolated and assigned to a
  new abstract actor to reduce redundancy.
• Other actors can inherit the interactions of the
  abstract actor.
• Depicted as an arrowheaded line beginning at
  one actor and pointing to the abstract actor
  whose interactions the first actor inherits.

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Sample Context Diagram
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Sample Use-Case Glossary
continued
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Sample Use-Case Glossary (continued)
continued
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Sample Use-Case Glossary (concluded)
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Step 3 Construct Use-Case Model Diagram
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Step 4 Document Business Requirements Use-Case
Narratives

• Document first at high level to quickly obtain an
  understanding of the events and magnitude of the
  system.
• Then expand to a fully-documented business
  requirement narrative.
• Include the use cases typical course of events
  and its alternate courses.

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Sample High-Level Version of a Use-Case Narrative
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Sample Expanded Version of a Use-Case Narrative
continued
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Sample Expanded Version of a Use-Case Narrative
(cont)
continued
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Sample Expanded Version of a Use-Case Narrative
(cont)
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Use Cases and Project Management

• Use-case model can drive the entire development
  effort.
• Project manager or systems analyst uses business
  requirements use cases to plan (estimate and
  schedule) the build cycles of the project.
• Build cycles are scoped on the basis of the
  importance of the use case and the time it takes
  to implement the use case.
• To determine importance of the use cases, will
  create
• Use-case ranking and evaluation matrix
• Use-case dependency diagram

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Use-Case Ranking and Priority Matrix

• In most projects, the most important use cases
  are developed first.
• Use-case ranking and priority matrix a tool
  used to evaluate use cases and determine their
  priority.
• Evaluates use cases on a scale of 1 to 5 against
  six criteria.
• Significant impact on the architectural design.
• Easy to implement but contains significant
  functionality.
• Includes risky, time-critical, or complex
  functions.
• Involves significant research or new or risky
  technology.
• Includes primary business functions.
• Will increase revenue or decrease costs.

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Sample Use-Case Ranking and Priority Matrix
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Use-Case Dependency Diagram

• Use-case dependency diagram a graphical
  depiction of the dependencies among use cases.
• Provides the following benefits
• Graphical depiction of the systems events and
  their states enhances understanding of system
  functionality.
• Helps identify missing use cases.
• Helps facilitate project management by depicting
  which use cases are more critical.

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Sample Use-Case Dependency Diagram