3-tier Architecture

In the beginning, there were mainframes. Every program and piece of data was stored in a single almighty machine. Users could access this centralized computer only by means of dumb terminals. (See Figure 1.)

In the 1980s, the arrival of inexpensive network-connected PCs produced the popular two-tier client-server architecture. In this architecture, there is an application running in the client machine which interacts with the server—most commonly, a database management system (see Figure 2). Typically, the client application, also known as a fat client, contained some or all of the presentation logic (user interface), the application navigation, the business rules and the database access. Every time the business rules were modified, the client application had to be changed, tested and redistributed, even when the user interface remained intact. In order to minimize the impact of business logic alteration within client applications, the presentation logic must be separated from the business rules. This separation becomes the fundamental principle in the three-tier architecture.

Figure 2. Two-Tier Client-Server Architecture

In a three-tier architecture (also known as a multi-tier architecture), there are three or more interacting tiers, each with its own specific responsibilities (see Figure 3):

Figure 3. Three-Tier Architecture

• Tier 1: the client contains the presentation logic, including simple control and user input validation. This application is also known as a thin client.

• Tier 2: the middle tier is also known as the application server, which provides the business processes logic and the data access.

·Tier 3: the data server provides the business data.

These are some of the advantages of three-tier architecture:

• It is easier to modify or replace any tier without affecting the other tiers.

• Separating the application and database functionality means better load balancing.

• Adequate security policies can be enforced within the server tiers without hindering the clients.