Tiered Software Architectures

http://www.sei.cmu.edu/str/descriptions/clientserver_body.html

Client/server architecture (2 tier architectures).
This approach split the processing management between the user system interface environment and the database management server environment. The client initiate the request (Remote procedure call or SQL ) sent to the server, and the servers respond to client with the answer.
This is good at providing distributed computer within a small size network, may be upto 100 simultaneous connections. If more than that, performance will deteriorate due to process of sending the "keep-alive" message. Furthermore, using proprietary databse restricts flexibility and choice of DBMS for application, it is not portable between different paltforms.

Three tier architectures is also referred to as the multi-tier architecture, a middle tier was added between the user system interface client environment and the database management server environment to perform queuing, application execution, and database staging. In addition the middle layer adds scheduling and prioritization for work in progress. There are a variety of ways of implementing this middle tier, such as transaction processing monitors, message servers, or application servers. The three tier client/server architecture has been shown to improve performance for groups with a large number of users (in the thousands) and improves flexibility when compared to the two tier approach. Flexibility in partitioning can be a simple as "dragging and dropping" application code modules onto different computers in some three tier architectures. A limitation with three tier architectures is that the development environment is reportedly more difficult to use than the visually-oriented development of two tier applications.

(see more on Three Tier Software Architectures)

Author: Darleen Sadoski, GTE
Copyright 2007 by Carnegie Mellon University