An Application server is a server side program that is used to provide service to the client simply by recieving request and sending back response to the client. It is also known as appserver. Generally complex transaction based applications uses application servers.
Application server are developed to support to built-in
redundancy, monitors for high-availability, high-performance distributed
application services, support for complex database access and quick development
of the enterprise applications. Application servers are also used for security
and state maintenance with the data base and persistence. They are sometimes
referred to as middleware because they work as a mediator between the database
servers and the end user, and they often connect the two. Application server can
exist on the on a remote computer provided computer must be connected to the
client through the computer network or they can exist on the same computer on
which the client's application is running. There are several types of
application servers are available such as 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.
An application server can also works as a part of a three-tier architecture model. Client Tier, Middle Tier and the EIS (Enterprise Information System) Tier are three tier of a three-tier architecture. Client tier also known as Presentation Tier (as the GUI interface), Middle Tier as the collection of business logic application and the EIS (Enterprise Information System).