Identifying Functionality

Our functional use case for Video Checkout, above, is still focused on the business process; but its level of detail is sufficient for us to be able to define specific functionality - and get right to the business objects that we'll need to create. First, let's go through and list the functionality that's described:

• The customer has an ID number
  
  
• The video rental transaction is handled via a specific screen
  
  
• The customer's ID number is sufficient to retrieve the customer's information
  
  
• The customer's name and address are in the database
  
  
• Videos each have an ID number
  
  
• Once the customer data is shown, the cursor moves to the video entry field
  
  
• The video's ID number is sufficient to retrieve the video's information
  
  
• The video's title and rental price are in the database
  
  
• The system must keep a running subtotal for all videos entered
  
  
• The system must calculate the tax
  
  
• The system must calculate a total rental fee

Notice that not all the functionality that's been described fits into business objects. For instance, the requirement that 'once the customer data is shown, the cursor moves to the video entry field' is not a business rule, but a presentation rule. This functionality is no less important to the end user of the system, but we do need to make sure that presentation rules are handled by the user-interface, not the business objects.