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.

Application tier (Business Logic/Logic Tier/Data Access Tier/Middle Tier)

The logic 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.

At first glance, the three tiers may seem similar to the MVC (Model View Controller)concept; however, topologically they are different. A fundamental rule in a three-tier architecture is the client tier never communicates directly with the data tier; in a three-tier model all communication must pass through the middleware tier. Conceptually the three-tier architecture is linear. However, the MVC architecture is triangular:the View sends updates to the Controller, the Controller updates the Model, and the View gets updated directly from the Model.

From a historical perspective the three-tier architecture concept emerged in the 1990s from observations of distributed systems (e.g.,web applications) where the client, middleware and data tiers ran on physically separate platforms. Whereas MVC comes from the previous decade (by work at Xerox PARCin the late 1970s and early 1980s) and is based on observations of applications that ran on a single graphical workstation; MVC was applied to distributed applications much later in its history (see Model 2).

1. A front-end web server serving static content, and potentially some are cached dynamic content.
2. A middle dynamic content processing and generation level application server, for example Java EE, ASP.NET, PHP platform.
3. A back-end database, comprising both data sets and the database management system or RDBMS software that manages and provides access to the data.

Data transfer between tiers is part of the architecture. Protocols involved may include one or more of SNMP, CORBA, Java RMI, .NET Remoting, Windows Communication Foundation, sockets, UDP, web services or other standard or proprietary protocols. Often Middlewareis used to connect the separate tiers. Separate tiers often (but notnecessarily) run on separate physical servers, and each tier may itselfrun on a cluster.

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