3-Tier Client-Server Architecture

As the need for enterprise scalability is increasing, it challenged the traditional 2 tier architecture. In 1995, a new variant of 2 tier architecture came into existence where there was reduction in client-side headache. Such systems comprised of following three levels/tiers:

1. User Interface Layer: This layer/tier comprised of functionality for simple GUI required for communication with database server and it runs on end-users computer. It sometimes performs a role of validating the input at users' side.
2. Business Logic & Data Processing Layer: It acts as a middleware which runs on separate machine and whose responsibility is to handle business and application logic. Sometimes it is also known as "Application Server".
3. Database Server: It stores the data required by middle layer. This layer may run on separate server known as database server whose primary job is to handle all database related requests from client side or through middle layer.

As there is clear separation between business logic and user interface, client will require very few resources. So it is known as "Thin Client". Fig.9.6 shows the architecture of 3-tier systems.

Advantages of 3-Tier Architecture:

• It is less expensive as client requires very limited amount of resources like hardware,memory etc.
• Application maintenance is centralized with transfer of business logic for many clients into single application server.
• The added modularity makes it convenient for modification or replacement of any one of the tier without affecting others.
• Load balancing is easier with separation of core business logic from database functions.

This architecture easily maps to web environment with a web browser acting as "Thin Client' and web server acting as application server. This architecture can be easily extended to n.. tiers which will provide more flexibility and modularity.