three_layers.html

this gives an introduction to microsoft dna.

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Java (Parts 1 and 2) in Slovak</A><BR>
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Introduction to Microsoft DNA
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Understanding the 3 Layers of an Applicaiton
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<P>

All applicaitons can be broken into three layers

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<OL>
<LI><B>Presentation Layer</B> - What the user sees. (aka GUI, 
    Client-side, or "view")
<LI><B>Business Rules Layer</B> - The underlying processing engines 
    and their rules (aka middle-tier, backend, or "control")
<LI><B>Data Layer</B> - The physical data storage layer (aka: "model")
</OL>

<P>

Most applications have requitements 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).

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However, as a developer, it is usually better to make sure that 
within the design of your applicaiton, that you separate the three 
layers.

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Why?

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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.

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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.

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Let's understand this by exaple. Suppose you wrote an applicaiton 
that drew its data from a simple delimited data file.

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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 relationsal database like Oracle?  
(Or what would happen if the technical environment suddenly 
changed and B2B XML streams were the norm?)

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Well, in this case, you would most certainly have to 
<B>rewrite</B> your application because your code would be
hand-carved for flat files.

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As you might imagine, this is incredibly inefficient.  
Rewriting is never a very good situation to be in.

<P>

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.

<P>

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 presentatin and rules layers) and the data layer.

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The interface separates them safely into distinct layers.

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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!

<P>

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