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A three-tier application is a specific type of n-tier architecture. In the case of three-tier architecture, the tiers are as follows:
N-tier denotes a software engineering concept used for the design and implementation of software systems using client/server architecture divided into multiple tiers. This decouples design and implementation complexity, thus allowing for the scalability of the deployed system.
In a three-tier application, the user interaction is managed by the presentation tier, which provides an easy-to-operate front end. The business rules are managed by the business tier, which controls and operates the entire application framework. The underlying data is stored and served by the data storage tier, also known as data persistence.
The three tiers are loosely coupled to each other, with predetermined and stable interfaces. This decoupling allows for significant changes to occur within the design, implementation and scale of each tier, without impacting the other tiers.
The business rules are removed from the client and are executed in the application server, also known as the middle tier. The application server ensures that the business rules are processed correctly. It also serves as an intermediary between the client application and database server.
The advantage of a three-tier application over a two-tier application is the added modularity. This allows for the replacement of any tier without affecting the other tiers and the separation of business-related functions from database-related functions. Finally, a three-tier application significantly increases a system's load balancing, scalability for performance and maintainability.