Use Case Diagrams, Exams of Architecture

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Use Case Diagrams

Tutorial

What is a use case?

• A requirements analysis concept
• A case of a use of the system/product
• Describes the system's actions from a the point of

view of a user

• Tells a story
  • A sequence of events involving
  • Interactions of a user with the system
• Specifies one aspect of the behavior of a system,

without specifying the structure of the system

• Is oriented toward satisfying a user's goal

Use Case Descriptions

• actors - something with a behavior or role, e.g., a

person, another system, organization.

• scenario - a specific sequence of actions and

interactions between actors and the system, a.k.a. a

use case instance

• use case - a collection of related success and failure

scenarios, describing actors using the system to

support a goal.

What is an Actor?

• Include all user roles that interact with the system
• Include system components only if they responsible for initiating/triggering a use case. - For example, a timer that triggers sending of an e-mail reminder
• primary - a user whose goals are fulfilled by the system
  • importance: define user goals
• supporting - provides a service (e.g., info) to the system
  • importance: clarify external interfaces and protocols
• offstage - has an interest in the behavior but is not primary or supporting, e.g., government - importance: ensure all interests (even subtle) are identified and satisfied

Finding Actors [2]

Ask the following questions:

• Who are the system’s primary users?
• Who requires system support for daily tasks?
• Who are the system’s secondary users?
• What hardware does the system handle?
• Which other (if any) systems interact with the system in question?
• Do any entities interacting with the system perform multiple roles as actors?
• Which other entities (human or otherwise) might have an interest in the system's output?

What is a user story?

1.Who? 2.Does what? 3.And why?

As a , I want so that

• An abbreviated description of a use case
• Used in agile development

Answers 3 questions:

Use Case Diagram Objective

• Built in early stages of development
• Purpose
  • Specify the context of a system
  • Capture the requirements of a system
  • Validate a systems architecture
  • Drive implementation and generate test cases
  • Developed by analysts and domain experts

How do use case diagrams fit in?

This applies also to use case descriptions. Diagram reproduced from www.edrawsoft.com.

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Elements of use case diagram: Actor

• Actor is someone interacting with use case (system function). Named by noun.
• Similar to the concept of user, but a user can play different roles; (example: a prof. can be instructor and researcher – plays 2 roles with two systems).
• Actor triggers use case.
• Actor has responsibility toward the system (inputs), and Actor have expectations from the system (outputs).

Name

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Elements of use case diagram: Use Case

• System function (process – automated or manual).
• Named by verb.

Do something

• Each Actor must be linked to a use case, while some use cases may not be linked to actors. = Use Case

Linking Use Cases

• Association relationships
• Generalization relationships
  • One element (child) "is based on" another element (parent)
• Include relationships
  • One use case (base) includes the functionality of another (inclusion case)
  • Supports re-use of functionality
• Extend relationships
  • One use case (extension) extends the behavior of another (base)

Use Case Levels

19

registration

graduate registration

non-graduate registration

More about Generalization

Generalization Example

The actor Order Registry

Clerk can instantiate the

general use case Place

Order.

Place Order can also be

specialized by the use

cases Phone Order or

Internet Order.