Introduction: Three-Tiered Web Architecture

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)
 * HTML, CSS
 * Javascript (client-side) script