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72 Chapter 3 • Enterprise Systems Architecture

• Maintainability. Support and maintenance costs are less on a single server than it would be
to maintain each installation or upgrade on a desktop client because the middle layer adds
scheduling and prioritization for work in progress.
• Reusability. Separating the application into multiple layers makes it easier to implement
reusable components.
• Security. Three-tier architecture provides higher security because there is less software on
the client machines, which means the IT staff has more control over the ERP system.
Three-tier applications also have some limitations, including the following:

• Economics. Three-tier applications require additional hardware and software infrastructure
to support the middle layer, which can increase the overall platform costs.
• Complexity. A key limitation with three-tier architectures is that the development environ-
ment is reportedly more difficult to use than the visually oriented development of two-tier
applications.

The benefits of three-tier architectures outweigh the limitations in the long run and are more
commonly used in many large-scale distributed systems and enterprise applications including a
large number of e-commerce solutions. Component technologies such as Enterprise Java Beans
(EJB) and CORBA Component Model (CCM) support the middle tier of three-tier architectures.
They provide frameworks for component development and deployment. Many Web services
based on hypertext transfer protocol (HTTP) and extensible markup language (XML) similarly
make use of three-tier architectures. Even though no two three-tier systems may be alike, they
share similar requirements and consequently similar system designs.

Web Services Architectures
In the last decade, many ERP vendors have introduced Web (or Internet)-based architecture for
their systems. This is often described as a fourth tier where the Web tier is split into services tier
6
and browser tier. The ERP systems focus on the Internet to provide a powerful new functionality
for Internet-based access and integration. This architecture leverages a number of Internet
technologies and concepts to deliver simple, ubiquitous access to ERP modules and enable the
open flow of information between systems. This functionality is primarily supported through the
following Internet access technologies:
•Web server
•ERP portal
•Back-end server integration
•Browser plug-ins or applets
This next-generation architecture leverages a number of Internet technologies and con-
cepts to deliver simple, ubiquitous access to ERP application modules and to enable the open
flow of information between systems. Using the Internet architecture as the foundation, end
users can access ERP applications over the Web browser, as well as more easily integrate their
PeopleSoft applications with existing internal systems and external trading partner systems. The
Internet architecture can be server-centric or client-centric.

6 Sandoe, K., Corbitt, G., and Boykin, R. (2001). Enterprise Integration. New York: John Wiley & Sons, 79–81.