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At the most generic level, a business's information systems are

collections of data, otherwise known as databases. A database might

reside on one computer (with another computer able to transparently

take its place in the event of a catastrophe). Sometimes, you'll hear

this part referred to as the back-end or the Enterprise Information

System (EIS). This is where database management systems -- such as

Oracle 8, Microsoft SQL Server 2000, and other commercial, broadly

adaptable solutions -- provide persistent data storage. They also

provide access to that data via some variation of Structured Query

Language (SQL).

Having established its base of stored data, an enterprise information

system's next goal is to extract meaningful data (i.e., information)

from the databases. This represents the business-logic layer, or

business tier. To provide an over-simple example, Java classes at the

business level might be tasked with figuring out daily profit or loss.

These classes would reach into the database, extract the day's sales

and expenditure figures, and subtract. They could then present the

result -- the profit or loss -- to any authorized asker.

In the simplest case, the next level is the client application. The

client tier represents the piece of processing power that someone or

something is actually using to do useful work. It might be some sort of

reporting program that presents profit and loss figures to a manager.

This piece of the puzzle refers to processing components on the

business level for most of the values it needs (i.e., it does

relatively little processing on its own), and so can be pretty simple.

Because the client doesn't have to do much processing on its own, it

can be "thin," meaning it requires relatively few computing resources.

In the extreme case, a client can be something like a dumb terminal or

a network-connected personal digital assistant (PDA).

What's the advantage of architecture like this? Well, the use of thin

clients is a big advantage, because it frees computing resources at the

client end for other uses (or obviates the need to buy and maintain

those extra resources in the first place). This architecture also is

scalable, meaning that you can just write, in the example above, your

profit calculation class once and use it in a variety of client

applications (the reporting tool, the accounting suite, the Web-

accessible monitoring tool for store managers, and so on).

All of these pieces talking to one another in a coordinated way is the

J2EE architecture's biggest challenge, and it's a big part of what J2EE

is about. We'll discuss that in coming columns.

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12y ago

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