(1) Before the Web, the term referred to a computer in a client/server environment that performed the business logic (the data processing). In a two-tier client/server environment, which is most common, the user's machine performs the business logic as well as the user interface, and the server provides the database processing. In a three-tier environment, a separate computer (application server) performs the business logic, although some part may still be handled by the user's machine. After the Web exploded in the mid-1990s, application servers became Web based (see definition #2 below). See file server.
An application server in a three-tier client/server environment provides middle tier processing between the user's machine and the database management system (DBMS).
(2) Since the advent of the Web, the term most often refers to software in an intranet/Internet environment that hosts a variety of language systems used to program database queries and/or general business processing. These scripts and services, such as JavaScript and Java server pages (JSPs), typically access a database to retrieve up-to-date data that is presented to users via their browsers or client applications.
The application server may reside in the same computer as the Web server (HTTP server) or be in a separate computer. In large sites, multiple computers are used for both application servers and Web servers (HTTP servers). Examples of Web application servers include BEA Weblogic Server and IBM's WebSphere Application Server. See Web server.
There is overlap between an application server and a Web server, as both can perform similar tasks. The Web server (HTTP server) can invoke a variety of scripts and services to query databases and perform business processing, and application servers often come with their own HTTP server which delivers Web pages to the browser.
Application servers have become the middleware for the enterprise as they increasingly provide more hooks into legacy applications. This is a J2EE-compliant application server running only Java and using Enterprise JavaBeans (EJBs) for the business logic. See
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