2-Tier client/Server Models
With two-tier client/server, the user system interface is usually located in the user’s desktop environment and the database management services are usually in a server.
Processing management is spilt between the user system interface environment and the database management server environment. The database management server provides stored procedures and triggers.
In 2-tier client/server applications, the business logic is buried inside the user interface on the client or within the database on the server in the form of stored procedures. Alternatively, the business logic can be divided between client and server.
The three-tier architecture emerged to overcome the limitations of the two-tier architecture. In the three-tier architecture, a middle tier was added between the user system interface client environment and the database management server environment.
There are a variety of ways of implementing this middle tier, such as transaction processing monitors, message servers, or application servers.
In 3-tier client/server application, the business logic resides in the middle tier, separate from the data and user interface. In this way, processes can be managed and deployed separately from the user interface and the database.