The Middle Tier

In most three-tier web database systems, the majority of the application logic is in the middle tier. The client tier presents data to and collects data from the user; the database tier stores and retrieves the data. The middle tier serves most of the remaining roles that bring together the other tiers: it drives the structure and content of the data displayed to the user, and it processes input from the user as it is formed into queries on the database to read or write data. It also adds state management to the HTTP protocol. The middle-tier application logic integrates the Web with the database management system.

In the application framework used in this tutorial, the components of the middle tier are a web server, a web scripting language, and the scripting language engine. A web server processes HTTP requests and formulates responses. In the case of web database applications, these requests are often for programs that interact with an underlying database management system. The web server we use throughout this tutorial is the Apache Software Foundation's Apache HTTP server, the open source web server used by more than 60% of Internet connected computers.[2]

From The Netcraft Web Server Survey, (April 2001).

We use the PHP scripting language as our middle-tier scripting language. PHP is an open source project of the Apache Software Foundation and, not surprisingly, it is the most popular Apache HTTP server add-on module, with around 40% of the Apache HTTP servers having PHP capabilities.[3] PHP is particularly suited to web database applications because of its integration tools for the Web and database environments. In particular, the flexibility of embedding scripts in HTML pages permits easy integration with the client tier. The database-tier integration support is also excellent, with more than 15 libraries available to interact with almost all popular database management systems.

From the Security Space web server survey, Apache module report, (April 2001).