Three-Tier Architectures
This book shows you how to develop web database applications that are built around the three-tier architecture model shown in Figure 1-3. At the base of an application is the database tier, consisting of the database management system that manages the data users create, delete, modify, and query. Built on top of the database tier is the middle tier , which contains most of the application logic that you develop. It also communicates data between the other tiers. On top is the client tier , usually web browser software that interacts with the application.

Figure 1-3. The three-tier architecture model of a web database application
The three-tier architecture is conceptual. In practice, there are different implementations of web database applications that fit this architecture. The most common implementation has the web server (which includes the scripting engine that processes the scripts and carries out the actions they specify) and the database management system installed on one machine: it's the simplest to manage and secure, and it's our focus in this book. With this implementation on modern hardware, your applications can probably handle tens of thousands of requests every hour.
For popular web sites, a common implementation is to install the web server and the database server on different machines, so that resources are dedicated to permit a more scalable ...