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.
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):
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 a 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.
In order to demonstrate these design concepts, the general outline of a simple three-tier “Hangman” game will be presented (check the source code in the archive file). The purpose of this game, just in case the reader isn't familiar with it, is to try to guess a mystery word, one letter at a time, before making a certain number of mistakes.
The data server is a Linux box running the MiniSQL database management system. The database is used to store the mystery words. At the beginning of each game, one of these words is randomly selected.
At the client side, a Java applet contained in a web page (originally obtained from a web server) is responsible for the application's graphical user interface (see Figure 4). The client platform may be any computer with a web browser that supports applets. The game's logic is not controlled by the applet; that's the middle tier's job. The client only takes care of the presentation logic: getting the user's input, performing some simple checking and drawing the resulting output.
The server in the middle tier is a Java application, also running within a Linux box. The rules of the “Hangman” game (the business rules) are coded in this tier. Sockets and JDBC, respectively, are used to communicate with the client and the data server through TCP/IP.
Figure 5 presents a UML (Unified Modeling Language) deployment diagram that shows the physical relationship among the hardware nodes of the system.
Even though the design described gives the impression of requiring a different machine for each tier, all tiers (each one running on a different process) can be run in the same computer. This means the complete application is able to run in a single Linux system with a graphical desktop, and it doesn't even have to be connected to the Net!
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