An application server is an application program that accepts connections in order to service requests, by sending back responses. An application server can run remotely (connected to client through a computer network) or can exist on the same computer where the client application is running. Examples include file server, database server, backup server, print server, mail server, web server, FTP server, application server, VPN server, DHCP server, DNS server, WINS server, logon server, security server, domain controller, backup domain controller, proxy server, firewall, etc.
Application server are developed to support the quick development of the enterprise applications. They provide security and state maintenance with the data base and persistence.
An application server may be a part of a three tier architecture model. A three tier architecture includes the Client Tier, Middle Tier and the EIS (Enterprise Information System) Tier. It may consists of Presentation Tier (as the GUI interface), Middle Tier as the collection of business logic application and the EIS (Enterprise Information System). The view tier is nothing but the web based graphical user interface to interact with the clients, Middle tier is the combination of web containers and the EJB containers. EIS contains persistence and the database management systems to support the applications. JEE Platform requires database to store the business data. This database is accessible by means of JDBC, JDO or the SQLJ APIs. We can also access the database through enterprise beans, web components and the application client components.