New Architecture – Two-tier Architecture vs. Three-tier Architecture

The typical client/server architecture is best described as two-tier, simply because it only consists of two parts working together.  The client talks directly to the server.  It is typically used in small environments (less than 50 users).  Two-tiered architecture works effectively for simpler applications using only one relational database and a small number of users.  The high amount of traffic in a two-tier architecture can overload the network.  More users increase the number of data transfer trips across the physical boundaries of the network.  The two-tiered architecture retains the client and server exchange even when no work is being done allowing fewer individuals to access the application. 

As applications become more complex, when a number of databases can be accessed, or the number of users becomes larger, the two-tiered approach starts to fall short.  To properly scale an application to accommodate more users, requires moving to a three-tier architecture.

Three-tier software architecture overcomes limitations and disadvantages inherent in two-tier architecture.  A middle tier (Business Layer) is placed between the client and the server. The Business Layer middle provides process management where business logic and rules are executed and can accommodate hundreds of users (as compared to only 50-100 users with the two tier architecture) by providing functions such as queuing, application execution, and database staging.