Use Case Descriptions

1
Use Case Descriptions

• The text to describe a particular use case
  interaction - in the form of a 2-way dialogue
  between the actor and the system
• provides the supporting detail for the use case
  diagram - not to be started until the diagram is
  complete/nearly complete
• also known as Use Case Scripts or Use Case
  Scenarios (beware - the latter has another
  meaning)

2
What to describe

• Describe the most common/normal form of
  interaction first - the basic course
• Describe possible variations separately - the
  alternative courses
• The script should be in a conversational style
• actor requests.
• System responds by.
• Actor does..
• Etc..

3
Example of a Use Case Description

• In the video rental shop, the interaction between
  Counter Assistant and Rent Video use case may be
• Actor Actions System Response
• 1. Customer tenders video(s) to be
• rented and membership card
• 2. Counter assistant enters member 3. System
  provides member details and
• no.into system status of loans and fines
• 4. Assistant enters identification of each
• video to be rented 5. System accepts ids and
  gives fee payable
• 6. Assistant requests payment, takes
• money and enters payment made 7. System logs
  payment details

4
Guidelines

• Include a series of numbered sections or steps
  which describe noteworthy events and possibly
  related context, constraints and business rules
• steps may alternate between actor and system, or
  may be a series of consecutive steps taken by
  either of them
• written from the users point of view
• consist of users vocabulary

5
Conversational Style

• This conversational style script (as if for a
  theatre play) is a good compromise between the
  advantages and disadvantages of other methods
• it is quick and easy to write (important for
  capturing early, outline information)
• it is quick and easy to read and to understand
• it encourages concise-ness
• it identifies the required sequence of actions
• it highlights causes and effects

6
Styles of Description

• In addition to the conversational style script,
  there are other ways of describing the
  interactions e.g.
• unstructured narrative
• structured English
• decision tree
• decision table

7
Unstructured Narrative

• A text description of what happens in standard
  English sentences.
• Advantages
• easy to write
• Disadvantages
• easy to include ambiguity,
• lengthy both to read and to check,
• does not highlight cause and effect
• does not highlight sequence of actions

8
Example of Narrative Description

• How long does it take to understand this
  narrative?
• A video shop primarily rents videos to customers.
  Customers can only borrow videos if they are
  registered members, and need to produce their
  membership card each time they borrow a video.
•  Customers come into the shop and once they have
  chosen the videos that they wish to borrow they
  take them to the checkout and hand them to the
  shop assistant. The shop assistant then enters
  the membership number into the system. This will
  produce on screen the members personal details,
  and whether any other videos are currently on
  loan. Customers cannot borrow more videos if
  there is an overdue balance owing. Providing the
  customer does not owe money, the shop assistant
  enters the video code which shows the rental
  period and the rental amount. This is repeated
  for each video. The shop assistant then asks the
  customer for the amount of money. Customers can
  pay by cash or credit card.

9
Structured English

• A text description of what happens but using a
  limited range of phrases.
• Advantages
• concise
• eliminates ambiguity
• highlights sequence of actions
• Disadvantages
• does not highlight cause and effect
• may use phrases which are unfamiliar to some users

10
Examples of other Methods of Description

• You may also like to look at
• Decision Trees
• Decision Tables

11
Essential Use Cases

• These are used during the feasibility and
  analysis stages of the project.
• The aim is to be free of implementation detail to
  show the essence of the business requirements
  (the conceptual model).
• Enables analysts, developers, users and clients
  to understand the scope of the problem and the
  processes required

12
Real Use Cases

• Now the Essential Use Cases will be used as the
  basis for lateral, creative thinking with the
  opportunity for new ideas on how create the
  system.
• Real Use Cases are used to document the design of
  the project i.e. how it will work in reality.
• For a user interface it may include prototype
  screen shots, print layouts, form layouts, menus.
• For a system interface it may include file
  layouts.

13
Templates

• Here is a templatefor documenting the scripts
  (uctempbl.rtf)
• Here is an example use case script on the
  template for just the basic course of processing
  (uctempex.rtf)

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Template Sections

• Use Case - its identifier/name
• Actors - list of actors involved. Show which one
  initiates the use case
• Overview - short outline description summarising
  the use case
• Type - category of the use case
• Cross References - use case relationships
  (covered later)

15
Categories of Use Cases

• 1. Primary - major common process e.g. Rent Video
• 2. Secondary - minor or rare processes e.g.
  Request to supply unstocked New Video
• 3. Optional - processes that may or may not be
  used

16
Alternative Courses

• Alternative courses can describe alternative
  events to the typical story. These are the less
  common, the exceptional or error cases.
• Place all the alternatives after all the typical
  course of events e.g.
• 7. Customer pays clerk by cash or credit
• Alternative Courses
• 7. Customer has unpaid late charges and will not
  pay them. Collect payment or cancel rental
  transaction

17
Summary

• Use Case descriptions supply the detail of system
  requirements
• Conversational scripts are used to describe the
  interactions between actors and use cases
• The basic course may be followed by 1 or more
  alternative courses
• Essential Use Cases are used during Analysis,
  Real Use Cases are used during Design