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Lecture 4 :Use cases III (UC description)

Outline

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Concept of Use Case Description

Levels of Use Case Description

Reading and writing Use Case Descriptions

Use Case Diagram of the Customer Support System

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Use Cases grouped into system modules

Note: Same actor interacts with different modules

Use Case Description

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Complements Use Case Diagram

A breakdown of a single use case (e.g., sequence of steps included in the function “Look up item availability”); process logic included

In contrast to Use Case Diagram, Use Case Description captures variations of a Use Case

Example: “Create new order” can be done via phone+clerk and via Internet ordering – 2 scenarios

Level of Use Case Description

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Three levels of detail: UC* Brief description

Summary of what system does in response to actor’s actions

UC Intermediate description Shows steps in use case, if-then

UC Full description Includes Brief description, expands intermediate

description, shows scenarios

* UC=Use Case

Brief Description of Create New Order Use Case

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• Same description that is usually captured in initial Use Case Diagrams

Intermediate Use Case Description

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Telephone Order Scenario for Create New Order Use Case

Full Use Case Description

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Shows steps (“Flow of Events”) broken down to the actor and the system side – useful!

Full Use Case Description

Telephone Order Scenario for Create New Order Use Case

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Use-Cases – Common Mistakes Complex diagram No system 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 …

Writing Use Case Descriptions

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1. Select a use case

2. Write abbreviated full description (Use case name, Scenario (if any), Business Event, Actors, Flow of steps, Exception conditions)

3. For figuring Flow of steps,

- Keep in mind general system model: Input-Processing-Output

- Steps should be at nearly the same level of abstraction (each makes nearly same progress toward use case completion)

4. For figuring exception conditions, focus on if-then logic.

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Combining Processes Number Limit:

The diagram should have between 3 to 10 base use-case. No more than 15 use cases (base + included + extending).

Abstraction: All use-cases should be in similar abstraction levels.

Size: Use cases should be described in half a page or

more. Interaction:

Use-cases which are carried out as part of the same interaction.

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Dividing Processes Size:

If a use-cases takes more than a page, consider include/extend

Weak dependency: If the dependency between two parts of a use-

case is weak, they should be divided.

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More Guidelines Factor out common usages that are required

by multiple use cases If the usage is required use <<include>> If the base use case is complete and the usage

may be optional, consider use <<extend>> A use case diagram should:

contain only use cases at the same level of abstraction

include only actors who are required