A three-tier model means you don't have to build, install, or configure the client tier. Any user who has a web browser can use the web database application, usually without needing to install additional software, be using a specific operating system, or own a particular hardware platform. This means an application can be delivered to any number of diverse, geographically dispersed users. The advantage is so significant that our focus in this tutorial is entirely on three-tier solutions with this thin-client web browser architecture.
But what are the alternatives to a thin client? A custom-built Java applet is an example of a thicker client that can still fit the three-tier model: the user downloads an applet and runs more of the overall application logic on her platform. The applet still interacts with a middle tier that, in turn, provides an interface to the database tier. The advantage is customization: rather than using the generic browser solution, a custom solution can eliminate many problems inherent in the statelessness, security, and inflexibility of the Web. The applet might not even use HTTP to communicate with the middle-tier application logic.
A thick client is also part of a traditional two-tier solution, also known as a client/server architecture. Most traditional database applications-such as those in the bank-have only two tiers. The client tier has most of the overall application logic, and the server tier is the DBMS itself. The advantage is that a customized solution can be designed to meet the exact application requirements without any compromises. Disadvantages are the lack of hardware and operating system flexibility and the requirement to provide software to each user.
