Multitier Application Architecture
Web-based applications are multitier applications (sometimes referred to as n-tier appli-cations) that divide functionality into separate tiers (i.e., logical groupings of functionality). Although tiers can be located on the same computer, the tiers of web-based applications often reside on separate computers. Figure 21.3 presents the basic structure of a three-tier web-based application.
The bottom tier (also called the data tier or the information tier) maintains the appli-cation’s data. This tier typically stores data in a relational database management system (RDBMS). We discuss RDBMSs in Chapter 22. For example, a retail store might have an inventory information database containing product descriptions, prices and quantities in stock. Another database might contain customer information, such as user names, billing addresses and credit card numbers. These may reside on one or more computers, which together comprise the application’s data.
The middle tier implements business logic, controller logic and presentation logic to control interactions between the application’s clients and its data. The middle tier acts as an intermediary between data in the information tier and the application’s clients. The middle-tier controller logic processes client requests (such as requests to view a product
catalog) and retrieves data from the database. The middle-tier presentation logic then pro-cesses data from the information tier and presents the content to the client. Web applica-tions typically present data to clients as XHTML documents.
Business logic in the middle tier enforces business rules and ensures that data is reli-able before the application updates a database or presents data to users. Business rules dic-tate how clients access data, and how applications process data. For example, a business rule in the middle tier of a retail store’s web-based application might ensure that all product quantities remain positive. A client request to set a negative quantity in the bottom tier’s product information database would be rejected by the middle tier’s business logic.
The top tier, or client tier, is the application’s user interface, which gathers input and displays output. Users interact directly with the application through the user interface, which is typically a web browser, keyboard and mouse, or a mobile device. In response to user actions (e.g., clicking a hyperlink), the client tier interacts with the middle tier to make requests and to retrieve data from the information tier. The client tier then displays the data retrieved for the user. The client tier never directly interacts with the information tier.
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