Introduction: Three-Tiered Web Architecture

Source: Think Quest New York City (

In a three-tier architecture data access is mediated by a middle layer, called the application layer. In large-scale applications, the three tiers may be on separate machines. However, in smaller applications, all three may run on the same machine.

Data Tier (Back end)

  • Holds all the data for the web application.
  • Database Server -- manages the storage and retrieval of the data.
  • MySQL -- Open Source Database Management System (DBMS) Server

Application Tier (Middleware)

  • Also called the logic tier or business tier.
  • Controls the functionality of the application: logic, decision-making, processing, calculations.
  • Web Server (Apache HTTP)
  • Sahana (Server-side PHP Scripts)
  • OpenMRS (Server-side Java Applications)

Client Tier (Front end)

  • Also called the presentation layer.
  • Usually a web-based GUI (graph user interface)
  • Browser Software (Firefox)
  • Javascript (client-side) script

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