Three-tier (computing)

In computing, Three-tier is a client-server architecture[?] in which the user interface, functional process logic ("business rules") and data storage[?] and data access[?] are developed and maintained as independent modules[?], most often on separate platforms[?]. The term "three-tier" or "three-layer" seems to have originated witin Rational Software[?] or Microsoft.

The three-tier model is considered to be a software architecture and a software design 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 as requirements or technology change. For example, an upgrade of desktop operating system[?] from Microsoft Windows to Unix would only affect the user interface code.

Typically, the user interface runs on a desktop PC or workstation and uses a standard graphical user interface, functional process logic may consist of one or more separate modules running on a workstation server[?] or application server, and an RDBMS on a database server[?] or mainframe contains the data storage logic[?]. The middle tier may be multi-tiered itself (in which case the overall architecture is called an "n-tier architecture").

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