Three‐Tier Applications

This is extended in the three‐tier model by the addition of a middle layer that sits between the client and the data sources. The three physical tiers break down as follows:

• The client tier is the front‐end tool with which the end user interacts. It may take the form of  any type of user interface, such as a Visual Basic application, or (increasingly commonly) a web browser. In ASP applications, the client tier usually takes the form of a web browser.
• The middle tier (also referred to as the business tier) represents most of the logic that makes the application functional. In the case of ASP this is where our ASP pages reside, upon the web server.
• The data tier represents the storage mechanism used to hold persistent data. This could be a relational database, text‐based file, directory, mail server etc.

The Client Tier

The client browser requests an ASP page from the server, and is presented with an HTML form for

entering data about the size of wall and brick. The user then clicks on the Calculate button, which sends the form details to the web server. Later, the client receives the response from the web server and displays the results of the calculation.

The Middle Tier

This is where our web server resides, holding (among att other things) the ASP pages and our components. When the client requests the form, the web server creates it as HTML using WroxBloxForm.asp. When the user clicks Calculate on the client end, the values are sent to WroxBloxResult.asp. WroxBloxResult.asp creates an instance of the Brick Calculator component, also on the middle tier, which uses the passed parameter values to calculate the number of bricks required, and returns the result to the page. The page then writes this back to the client.

The Data Tier

OK, so our example doesn't use the data tier. However, if we needed to collect or store any persistent

data in a database, such data would reside in this tier. Busy sites will gain great performance advantages from actually having the database residing on a separate physical server, because databases typically require a lot of system resources. During the design phase, this is not always possible but the notion of a logical data tier still exists, whether or not the database resides on a separate server.

Image Credit

https://www.softwaretestingclass.com/what-is-difference-between-two-tier-and-three-tier-architecture/