In Chapters 4 and 5, you??™ll learn how to design the database for optimum
performance.
These tiers are purely logical??”there is no constraint on the physical location of each tier.
In theory, you are free to place all of the application, and implicitly all of its tiers, on a single
server machine, or you can place each tier on a separate machine if the application permits
this. Chapter 22 explains how to integrate functionality from other web sites using XML Web
Services. XML Web Services permit easy integration of functionality across multiple servers
without the hassle of customized code.
An important constraint in the three-tier architecture model is that information must flow
in sequential order among tiers. The presentation tier is only allowed to access the business
tier, and it can never directly access the data tier. The business tier is the brain in the middle
that communicates with the other tiers and processes and coordinates all the information flow.
If the presentation tier directly accessed the data tier, the rules of three-tier architecture programming
would be broken.
These rules may look like limitations at first, but when utilizing an architecture, you need
to be consistent and obey its rules to reap the benefits. Sticking to the three-tier architecture
ensures that your site remains easily updated or changed and adds a level of control over who
or what can access your data.
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