Explore the World of Application Servers

This article explores the world of the Application Server and answers some of the most frequently asked questions like:

What is an Application server?
What is a 3-Tier architecture?
Where exactly the application server fits in?
For what exactly the application server is meant?
What are the different types of application servers?
What all are the application servers avalaible on the market?
What should one look for when opting for an application server?

An Application server is a server program that resides in the server (computer) and provides the buiness logic for the application programme. The server can be a part of the network, more precisly the part of the distributed network. The server program is a program that provides its services to the client program that resides either in the same computer or on another computer connected through the network.

Application servers are mainly used in web based applications that have 3-Tier arhitecture.

1st Tier: Front end - Browser (thin client), a GUI interface lying at the client/workstation.

2nd Tier: Middle tier - Application Server - set of application program.

3rd Tier: Back end - Database Server.

The application server is a second / middle Tier of the 3- tier Architecture. In other words now application servers are integeral part of the 3-tier Acrtitecture.

The Application server syncs / combines with the web server for processing the request made by the client.

If we look at the request-response flow between client, web server and application server, we come to know that the client's request first goes to the web server which sends the required information to the application server that sends back the response to the webserver after taking an appropriate action. The webserver futher sends the processed information back to the client. Web server use different approaches or technology for forwarding or receiving back processed information. Some of the most common approaches are given below.

CGI (Common Gateway Interface)- can be written either in JAVA, C, C++, and Perl.
FastCGI - Improved CGI programming in which each request is not handled as a different process like CGI.
ASP (Active Server Pages) - A Microsoft Technology
JSP (Java Server Pages).
Java Servlets - Sun's Technology
Java Script (Server Side) - NetScape Technology requires livewire for database connectivity.
etc.

First let's have a look at the web application architecture without considering the application server as part of the web application architecture.

All three can either be on one machine or spread over the network on different machines as shown above.

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