Notes

Application Server is a term that sometimes is mixed with a web server. While a web server handles mainly HTTP protocols, the application server deals with several different protocols, including, but not limited, to HTTP.

The Web server's main job is to display the site content and the application server is in charge of the logic, the interaction between the user and the displayed content. The application server is working in conjunction with the web server, where one displays and the other one interacts.

The information traveling back and forth between the server and its client is not restricted to simple display markup, but to interaction between the two.

In most cases, the server creates this interaction through a component API, such as J2EE (Java 2 Platform), EJB (Enterprise JavaBean) and other different application software models.

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An example:

The best way to understand the difference between the scenarios where an application server works with the web server versus a scenario where there isn't an application server is through an online store.

Scenario 1: Web server without an application server

you have an online store with only a web server and no application server. The site will provide a display where you can choose a product from. When you submit a query, the site performs a lookup and returns an HTML result back to its client. The web server sends your query directly to the database server (be patient, I will explain this one in our next nugget) and waits for a response. Once received, the web server formulates the response into an HTML file and sends it to your web browser. This back and forth communication between the server and database server happens every time a query is run.

Scenario 2: Web server with an application server

if the query you want to run has already been done previously and no data has changed since then, the server will generate the results without having to send the request to the database server. This allows a real-time query where a second client can access the same info and receive real time, reliable information without sending another duplicate query to the database server. The server basically acts as an intermediate between the database server and the web server. This allows the information pulled to be reusable while in the first scenario, since this info is embedded in a particular and "customized" HTML page, this is not a reusable process. A second client will have to request the info again and receive another HTML embedded page with the info requested -highly inefficient. Not to mention that this type of server is very flexible due to its ability to manage its own resources, including security, transaction processing, messaging and resource pooling.