Understanding the 3 Layers of an Application
May 1, 2000
All applications can be broken into three layers
- Presentation Layer - What the user sees. (aka GUI,
Client-side, or "view")
- Business Rules Layer - The underlying processing engines
and their rules (aka middle-tier, backend, or "control")
- Data Layer - The physical data storage layer (aka: "model")
Most applications have requirements that span all three of the layers.
That is, most applications need a user interface (1) and process (2)
user requests based upon some underlying data (3).
However, as a developer, it is usually better to make sure that
within the design of your application, that you separate the three
layers.
Why?
Well, mainly because things change. Especially in the world of
computers. In fact, things don't just change, they change
extremely quickly and often dramatically.
As a result, developers must often modify applications to meet
new requirements in short order. And how flexible your application
will be, will depend on how well the three layers have been
separated.
Let's understand this by example. Suppose you wrote an application
that drew its data from a simple delimited data file.
If the data manipulation code were embedded into the
presentation and Rules layer, what do you suppose would
happen if you decided at a later date that you preferred
to store your data in a relational database like Oracle?
(Or what would happen if the technical environment suddenly
changed and B2B XML streams were the norm?)
Well, in this case, you would most certainly have to
rewrite your application because your code would be
hand-carved for flat files.
As you might imagine, this is incredibly inefficient.
Rewriting is never a very good situation to be in.
Since the environment in which your application must
survive will change so fast, you need a design that
will allow you to switch in new technologies without
having to rewrite.
In order to do this however, the layers must be very
distinct...they must be "pluggable". In the example below,
we show that an "interface" can sit between the application
(the presentation and rules layers) and the data layer.
The interface separates them safely into distinct layers.
As you might imagine, a good program will also separate the presentation
layer from the rules layer as well. With good separation,
one can switch presentation drivers as desired....whether the client
uses a browser, a handphone, or a PDA should not matter!
Monolithic, single-scale applications
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