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Chapter 3 • Enterprise Systems Architecture 71

resources. If this is the case, IT must now go back to management and explain the problem and
the solution, which is often either a replacement of network switches and routers or more band-
width for WAN and Internet-facing connections. Proper design, planning, and fund allocation are
key to a successful ERP deployment that will be scalable for the future.
The three-tier client–server architecture, seen in Figure 3-4, has been shown to improve
performance for groups with a large number of users (in the thousands) and improves flexibility
when compared with the older and somewhat obsolete two-tier approach. One way to help
ensure scalability is to reduce some of the burden of processing and database access from the
users’ client computers. This is at the heart of the three-tier approach. With this approach, which
is sometimes also referred to as application partitioning, the bulk of the complex business
processing is performed on separate computers called application servers. Because the applica-
tion servers do the complex processing and report generation, the amount of data that must be
passed from the database server to the (many) client computers is greatly reduced, as is the
amount of computing work each client computer must perform. This processing conservation
reduces the load on the network, which is a key consideration for applications with large
numbers of users. It also reduces the hardware requirements for the client computers. For this
reason, mainframes have found their new role as servers in three-tier architectures.

Benefits and Limitations
Three-tier applications provide several benefits over traditional client–server applications including
the following:
• Scalability. Three-tier architecture allows easier architecture to add, change, and remove
applications because the user interface and database are not affected by upgrades to
applications.
• Reliability. Three-tier architecture makes it easier to increase reliability of a system by
implementing multiple levels of redundancy. In addition, scheduling and prioritization of
jobs can be managed better from a central location.
• Flexibility. By separating the business logic of an application from its presentation logic,
three-tier architecture makes the application much more flexible to changes. Flexibility in
partitioning can be as simple as “dragging and dropping” application code modules onto
different computers.

GUI
Applications
Data

Application
Presentation Layer Layer Data Layer
FIGURE 3-4 A Three-Tier ERP Architecture