This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2009)

Three-tier education refers to those structures of schooling, which exist in some parts of England, where pupils are taught in three distinct school types. A similar experiment was also trialled in Scotland. (See separate article)

In three-tier authorities children begin their compulsory education in a First school. Sometimes also called a Lower school (or more simply, primary school), these schools cater for children aged up to an age between 8 and 10, and cover all of Key Stage 1 and the first years of Key Stage 2. Children then transfer to a Middle school. These schools cater for children during a period of 3 to 4 years between the ages of 8 and 14, depending on the local authority. These years cover parts of both Key Stage 2 and Key Stage 3. Following this, children transfer to a High school, sometimes known as an Upper school for the remainder of their compulsory education, and sometimes on into sixth form.

Many local authorities trialled systems as this for a period from as early as 1968, with many more following suit during the early 1970s, particularly in 1972 when the Raising of school leaving age in England and Wales from 15 to 16 saw many secondary schools lacking the adequate class space for 11-16 year olds.^[1]

However, most have since reverted to align their schools to the National Curriculum.

Some areas reverted to the traditional age ranges as long ago as the late 1970s. One of the first areas to revert to the traditional age ranges was Halesowen in the West Midlands, which abandoned 5-9 first, 9-13 middle and 13-16/18 secondary schools in 1982 after just 10 years in use.

Central Bedfordshire is now the only authority with an exclusively three-tier structure. A further 18 authorities have a mix of standard two-tier and three-tier provision with middle schools. Leicestershire operates a three-tier system in some area, with children moving from primary school into secondary school after year six, and then to upper school after year nine.

A three-tier system also exists in Gibraltar.

The main routes through education are shown in the diagram below, with three-tier routes being shown in blue and mauve:

See also[link]

• Education in England
• List of middle schools in England

References[link]

External links[link]

• Northumberland Education Action Group - campaign to retain three-tier education in Northumberland
• Parents Against Change, Suffolk UK Action Group - campaign to retain three-tier education in Suffolk

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2008)

In software engineering, multi-tier architecture (often referred to as n-tier architecture) is a client–server architecture in which the presentation, the application processing, and the data management are logically separate processes. For example, an application that uses middleware to service data requests between a user and a database employs multi-tier architecture. The most widespread use of multi-tier architecture is the three-tier architecture.

N-tier application architecture provides a model for developers to create a flexible and reusable application. By breaking up an application into tiers, developers only have to modify or add a specific layer, rather than have to rewrite the entire application over. There should be a presentation tier, a business or data access tier, and a data tier.

The concepts of layer and tier are often used interchangeably. However, one fairly common point of view is that there is indeed a difference, and that a layer is a logical structuring mechanism for the elements that make up the software solution, while a tier is a physical structuring mechanism for the system infrastructure.^{[1] [2]}

There have been growing concerns in recent years that many organizations are facing an excessive number of layers in their multi-layered architecture. These concerns stem from sprawling application architecture's that are not well managed or designed, in which development teams create an ever-growing number of "wrapper" layers that add zero or negative value to future maintainability. The resulting architecture resembles a Rube Goldberg Machine and scares organizations from solving the root cause of the sprawling layers, thus creating more layers.

Three-tier architecture[link]

Visual overview of a Three-tiered application

Three-tier^[3] 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. It was developed by John J. Donovan in Open Environment Corporation (OEC), a tools company he founded in Cambridge, Massachusetts.

The three-tier model is 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 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.

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 or application server, and an RDBMS on a database server or mainframe contains the computer data storage logic. The middle tier may be multi-tiered itself (in which case the overall architecture is called an "n-tier architecture").

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, or 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 on its own tier also improves scalability and performance.

Comparison with the MVC architecture[link]

At first glance, the three tiers may seem similar to the model-view-controller (MVC) 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 middle 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, middle ware and data tiers ran on physically separate platforms. Whereas MVC comes from the previous decade (by work at Xerox PARC in 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 later in its history (see Model 2).

Today, MVC and similar model-view-presenter (MVP) are Separation of Concerns design patterns that apply exclusively to the presentation layer of a larger system. In simple scenarios MVC may represent the primary design of a system, reaching directly into the database; however, in most scenarios the Controller and Model in MVC have a loose dependency on either a Service or Data layer/tier.

Web development usage[link]

In the web development field, three-tier is often used to refer to websites, commonly electronic commerce websites, which are built using three tiers:

1. A front-end web server serving static content, and potentially some cached dynamic content. In web based application, Front End is the content rendered by the browser. The content may be static or generated dynamically.
2. A middle dynamic content processing and generation level application server, for example Ruby on Rails, Java EE, ASP.NET, PHP, ColdFusion, Perl platform.
3. A back-end database or data store, comprising both data sets and the database management system or RDBMS software that manages and provides access to the data.

Other considerations[link]

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 middleware is used to connect the separate tiers. Separate tiers often (but not necessarily) run on separate physical servers, and each tier may itself run on a cluster.

Traceability[link]

The end-to-end traceability of data flows through n-tier systems is a challenging task which becomes more important when systems increase in complexity. The Application Response Measurement defines concepts and APIs for measuring performance and correlating transactions between tiers.

Comments[link]

Generally, the term tiers is used to describe physical distribution of components of a system on separate servers, computers, or networks (processing nodes). A three-tier architecture then will have three processing nodes. Layers refer to a logical grouping of components which may or may not be physically located on one processing node.

See also[link]

• Client–server model
• Database-centric architecture
• Front-end and back-end
• Hierarchical internetworking model
• Open Services Architecture
• Rich Internet application
• Service layer
• Web application
• Load balancing (computing)
• Multilayered architecture

External links[link]

• Linux journal, Three Tier Architecture
• Microsoft Application Architecture Guide
• Example of free 3-tier system

References[link]

1. ^ Deployment Patterns (Microsoft Enterprise Architecture, Patterns, and Practices)
2. ^ Fowler, Martin "Patterns of Enterprise Application Architecture" (2002). Addison Wesley.
3. ^ Eckerson, Wayne W. "Three Tier Client/Server Architecture: Achieving Scalability, Performance, and Efficiency in Client Server Applications." Open Information Systems 10, 1 (January 1995): 3(20)

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.