Structuring Systems Requirements

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Structuring Systems Requirements

• Use Case Description and Diagrams

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What Is Requirements Structuring?

• The process of analyzing, organizing, and
  modeling the requirements obtained via
  interviews, questionnaires, observation, and
  document analysis
• Relevant UML models (i.e. ways to model the
  requirements) include use cases, class diagrams,
  interaction diagrams, and activity or statechart
  diagrams

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What Is a Use Case?

• A depiction of a systems behavior, under various
  conditions, as the system responds to requests
  from users
• Interaction of an external entity (or actor) with
  the system
• The user ? uses the system
• The actor ? represents a role the user can play

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Use case modeling is utilized to demonstrate
the functionality of a system Interactions
between system and user
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UML Use Case Diagram Symbols
Use Case
Actor
Boundary
Connection
Include relationship
Extend relationship
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What Is an Actor?

• An external entity that interacts with the system
• Most actors represent user roles, but actors can
  also be external systems.
• An actor is a role, not a specific user one user
  may play many roles, and an actor may represent
  many users.

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What Is a Boundary?

• The dividing line between the system and its
  environment
• Use cases are within the boundary
• Actors are outside of the boundary(i.e. external
  entities)

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What Is a Connection?

• An association between an actor and a use case
• Depicts a usage relationship
• Connection does not indicate data flow

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What Is an ltltincludegtgt Relationship?

• A connection between two use cases
• Indicates a use case that is used (invoked) by
  another use case
• Links to general purpose functions, used by many
  other use cases
• Also called an abstract use case since it is
  never performed by itself

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Functionality is split out into a separate use
case that can be ltltincludedgtgt by other use cases
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What Is an ltltextendgtgt Relationship?

• A connection between two use cases
• Extends a use case by adding new behavior or
  actions
• Specialized use case extends the general use case

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Include vs. Extend

• Use ltltextendgtgt if you want to model an extension
  to, or a variation of, a complete use case that
  exists in its own right
• Use ltltincludegtgt if you want to factor the common
  behavior among two or more use cases into a
  single generalized use case

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Actors can be grouped in generalization
categories.
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Written Use Cases

• Document containing detailed specifications for a
  use case
• Contents can be written as simple text or in a
  specified format

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Sample Format for Written Use Case

• Title descriptive name, matches name in use
  case diagram
• Primary actor usually a user role
• Stakeholders any group or individual with an
  interest in the function of the use case
• Precondition conditions that must be satisfied
  in order to execute the use case

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Sample Format for Written Use Case (Continued)

• Minimal guarantee outputs that can be expected
  if the service attempt failed
• Success guarantee outputs that can be expected
  if the service succeeds
• Trigger an event or action that initiates the
  use case
• Main success scenario description of sequence
  of interactions between actor and use case during
  the use case execution
• Extensions detailed description of how errors
  are dealt with (i.e. else statement that follows
  an if-then)

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A sample written use case
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• Any Questions?