Three-tier is a client–server architecture in which the user interface, functional process logic (“business rules”), computer data storage and data access are developed and maintained as independent modules, most often on separate platforms.
The three-tier model is a software architecture pattern.
Apart from the usual advantages of modular software with well-defined interfaces, the three-tier architecture is intended to allow any of the three tiers to be upgraded or replaced independently in response to changes in requirements or technology. For example, a change of operating system in the presentation tier would only affect the user interface code.
Three-tier architecture has the following three tiers:
Presentation tier
This is the topmost level of the application. The presentation tier displays information related to such services as browsing merchandise, purchasing and shopping cart contents. It communicates with other tiers by outputting results to the browser/client tier and all other tiers in the network. (In simple terms it’s a layer which users can access directly such as a web page, or an operating systems GUI)
Application tier (business logic, logic tier, data access tier, or middle tier)
The logical tier is pulled out from the presentation tier and, as its own layer, it controls an application’s functionality by performing detailed processing.
Data tier
This tier consists of database servers. Here information is stored and retrieved. This tier keeps data neutral and independent from application servers or business logic. Giving data its own tier also improves scalability and performance.