A three-tier architecture is any system which enforces a general separation between the following three parts:
Applied to web applications and distributed programming, the three logical tiers usually correspond to the physical separation between three types of devices or hosts:
However, inside of the application server, there is a further division of program code into three logical tiers. This is kind of fractal: the part (app server object design) resembles the whole (physical system architecture). In a classic JSP/Servlet system, these objects are usually implemented as:
In an EJB system, the three logical tiers are usually implemented somewhat differently:
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