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.