Modelling Requirements - Ch. 11

Question 1

What is a Use Case?

Answer 1

In UML a use case is something that an actor wants the IT system to do; a ‘case of use’.

Each use case has a stated goal and contains a description of the actions that the system must perform in order to achieve it.

A Use Case has:

• Actors (matchstick if human or rectangle with an <<actor>> before system name if actor is asystem)</actor>
• Each use case is shown as an oval and respresents a function the system will perform in response to the actor (verb-noun, e.g. ‘Book Room’)
• A defined boundary indicated by drawing a large box around all of the use cases with the actors outside the box.
• Associations indicate which actors will need to interact with which use case (lines to link actors with use cases).
• <<include>> use cases included in a use case shown on diagram</include>
• <<extend>> use cases linked, but with the originating ‘extend’ use case having its own use case diagram (extension of diagram)</extend>

Pros:

Very useful during workshops as they are so easily understood.

Question 2

What is the purpose of a Data Model?

Answer 2

• allows stakeholders who will need to use the system or obtain information from it to agreed on data to be recorded and retrieved.
• provides basis for database design or evaluation of off-the-shelf applications.
• helps the analyst understand data required to support process improvements.
• provides mechanism for communicating data requirements forward into the design and build of an IT system.

Question 3

What is an Entity Relationship Diagram?

Answer 3

Identfies and provides understanding of:

• Data attributes
• Entities (grouping of attributes)
  
  • Entity type and Entity occurent (e.g. Book vs book title such as Alice In Wonderland)
  • Entity type = table
  • Entity occurence = record with unique identifier (e.g. account_number)
  • Represented by a box
  • Singular Noun name
  • physical (e.g. order, customer, supplier)
  • conceptual (e.g. booking, appointment)
  • active (e.g. meeting, course)
  • parent entity (‘one’ end) may exist without child entries
  • child entity (‘many’ end) must have a parent
  • many to many relationships need to be resolved by adding an intermediate entity (e.g. ‘employee’ to ‘project’ = add ‘contract’ entity)
• Relationship between entities (business connection shown as a line linking entities)
• Optionality
  
  • if relationship is optional (e.g. ‘customer call’ to ‘order’), the relationship will be shown by a dotted line
  • fully mandatory relationships are shown by solid lines
• Relationship names (e.g. ‘sales region’ responsible_for AND ‘customer’ allocated_to)
• Exclusivity arc (e.g. ‘employee’ allocated EITHER TO ‘sales region’ OR ‘office’)

Question 4

What is a Class Model?

Answer 4

Shows graphically the classes in a system and their associations with each other (similar to entity relationship diagrams).

Contains:

• objects (e.g. account_number) - objects are sent messages that invoke a response
• classes (object class, similar to entity type - represented by rectangular boxes with three sections containing:
  
  • name of class ‘Account’ ‘Payment’ ‘Transaction’ ‘OrderLine’
  • attributes (items of data about the class ‘name’ ‘dateLastPayment’ ‘creditLimit’)
  • Operations (invoked by messages beign sent toi t) named the same as the message that invokes it ‘Replace creditLimit with newCreditLimit’
• associations