In Computing, Three-tier is a client-server architecture in which the user interface, Business logic ( business rules ), data storage and data access are developed and maintained as independent module (computing)s, most often on separate platform (computing)s. The term three-tier or three-layer , as well as the concept of multitier architectures, seems to have originated within Rational Software.

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, a change of desktop operating system from Microsoft Windows to Unix would only affect the user interface code.

Typically, the user interface runs on a desktop personal computer 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 ).

It seems similar, although defined in slightly different terms, to the Model-view-controller concept and the pipes and filters concept.

==Web services usage==

In the Web service field, three-tier is used to refer to Websites, often Electronic commerce websites, which are built using three tiers: # A front end static content serving Web server # A middle dynamic content processing and generation level Application server, for example J2EE platform based in modern usage # A back end Database, comprising both data sets and the Database management system or RDBMS software that manages and provides access to the data.

Three-tier websites evolved from earlier two-tier websites which initially used webservers with both static and dynamic content, often Common Gateway Interface or CGI scripts, and a database back end. Separating out the static and dynamic page generation, and using software platforms with more programming features and structure, made website creation and management easier.

=See also=