Two-Tier Architectures in Brief Some two-tier architectures combine most of the application logic layer tasks with the presentation layer, while others combine most of the application logic layer with the data layer. A two-tier architecture could have one tier consisting of client PCs containing application logic code and database access routines and the other tier consisting of one or more databases. This arrangement is often referred to as a fat client. Another way to implement the two-tier architecture is placing the application logic layer with the data layer to form a tier and having the presentation layer in the other tier. Here, the database would rely on stored procedures and triggers to implement most of the application logic. This arrangement is often referred to as a thin client. Figure 1-1 shows the differences between fat and thin client arrangements. Figure 1-1: Fat and thin clients compared and contrasted As you can see in Figure 1-1, the fat client architecture containing application logic code is a combination of the functionality of the presentation and application logic layers. The thin client architecture has the application logic code buried within the DBMS in the form of stored procedures (code stored within the database that performs some application-specific task) and triggers (a feature of a DBMS that executes stored procedures based on one or more conditions). Most two-tier architectures fall somewhere in between these extremes. The dashed line represents the tier boundary. Conventional wisdom these days is that two-tier architectures can satisfactorily handle a hundred or so users. For larger numbers of users, performance may start to degrade because of the client s need to maintain a connection to the server. These constant connections drain network bandwidth and use scarce database connections. This problem is more severe in the fat client than in the thin client scenario. For the fat client implementation, every request for data requires reaching across the network, dipping into the database, and returning data to the client. For the thin client implementation, one request for data can trigger a DBMS stored procedure that executes on the server. This stored procedure could return the same data that a fat client would need multiple requests for. Although using stored procedures helps alleviate the bandwidth problem, the thin client still requires the database connection.
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