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technology (IT) executives had hoped for in terms of reuse are seldom real-
ized, multi-tiered architecture is here to stay. Three-tiered architecture is, in
fact, the industry standard and a wide variety of application development
environments from Smalltalk to Visual Basic support and encourage the use
of this standard architecture.
In general, a three-tiered architecture has the following layers:
• Interface layer
• Business layer
• Data layer
The driving force behind three-tiered architecture is the need to support
both flexibility and robustness in applications. De-coupling the interface
layer from the database offers the advantage of changes in the database
that need not affect the interface layer directly, thereby isolating the effects
of a change in either layer. The interface layer describes how the applica-
tion interacts with the outside world. If the outside world is comprised of
end users, then the interface layer refers to a user interface. Alternatively,
if it is comprised of client applications, it refers to an applications interface.
Arguably, the main payoff involved in object-oriented architectures is
not reuse but rather change management. Effective change management is
also the goal of three-tiered architectures. Since three-tiered architectures
are easier to implement with object-based (if not object-oriented) systems,
new life has been extended to object-based systems. In this article, a dis-
tinction is being made between object-oriented and object-based systems.
Object-based systems implement classes and objects, but do not permit
other aspects of object-oriented programming like inheritance and poly-
morphism. So whereas the three pillars of object-oriented programming
can be said to be encapsulation, inheritance, and polymorphism, object-
based programming concerns itself with mainly encapsulation.
A leading example of an object-based application development is Visual
Basic. Visual Basic is to the client-server world what Cobol is to the main-
frame world. Since classes in Visual Basic are implemented using DCOM
(distributed component object model), it is extremely easy to develop and
deploy components using Visual Basic.
An object-based component can be described as a set of objects collab-
orating to provide a common functionality and implementing a common
interface. Thus, an object-based component improves the encapsulation
aspect of object-based applications. By virtue of this it also increases the
flexibility as well as robustness of an object-based application, since
changes to the component are isolated.
It has already been argued that the main thrust towards three-tiered archi-
tecture is coming from a need for effective change management. Change
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