The business tier (also called the middle tier) receives requests from the presentation tier
and returns a result to the presentation tier depending on the business logic it contains. Almost
any event that happens in the presentation tier usually results in the business tier being called
(utilized), except events that can be handled locally by the presentation tier, such as simple
input data validation, and so on. For example, if the visitor is doing a product search, the presentation
tier calls the business tier and says, ???Please send me back the products that match
this search criterion.??? Most of the time, the business tier needs to call the data tier for information
to be able to respond to the presentation tier??™s request.
The data tier (sometimes referred to as the database tier) is responsible for managing the
application??™s data and sending it to the business tier when requested. For the TShirtShop
e-commerce site, you??™ll need to store data about products (including their categories and their
departments), users, shopping carts, and so on. Almost every client request finally results in
the data tier being interrogated for information (except when previously retrieved data has
been cached at the business tier or presentation tier levels), so it??™s important to have a fast
CHAPTER 2 ?– LAYING OUT THE FOUNDATIONS 15
database system.
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