When most people talk about n-tier applications, they’re talking about physical models in which
the application is spread across multiple machines with different functions: a client, a web server, an
application server, a database server, and so on. And this isn’t a misconception—these are indeed
n-tier systems. The problem is that many people tend to assume there’s a one-to-one relationship
between the layers (tiers) in a logical model and the tiers in a physical model, when in fact that’s not
always true.
A physical n-tier architecture is quite different from a logical n-layer architecture. An n-layer
architecture has nothing to do with the number of machines or network hops involved in running
the application. Rather, a logical architecture is all about separating different types of functionality.
The most common logical separation is into an Interface layer, a Business layer, and a Data layer.
These may exist on a single machine or on three separate machines—the logical architecture doesn’t
define those details.
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