At first, the Web was used to access static documents. Now companies want to use web browsers as a way to provide access to corporate databases. This requires linking the web server to the database system. The methods used to link web servers and DBMS systems have evolved into the three-tier network architecture shown below. In the "front" tier, is the user interface and a web browser running on a PC or "thin client". The front tier communicates with a web server in the "middle tier." When the user request is for something more complex than a simple web page, the web server passes the request to an application server. The application server role is to handle the business logic required to process the request. Often, the request will involve access to a corporate database. The database systems run in the "back" tier of the architecture. SQL is the standard database language for communication between the application server and back-end database. All of the packaged application server products provide a SQL-based API for database access.
