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    <p>Rule Engines can offer a lot of value, in certain instances.</p> <p>First, many rule engines work in a more declarative way. A very crude example would be AWK, where you can assign regexes to blocks of code. When the regex is seen by the file scanner, the block of code is executed. </p> <p>You can see that in this case, if you had, say, a large AWK file and you wanted to add Yet Another "rule", you can readily go to the bottom of the file, add you regex and logic, and be done with it. Specifically, for many applications, you're not particularly concerned with what the other rules are doing, and the rules don't really interoperate with each other.</p> <p>Thus the AWK file becomes more like a "rule soup". This "rule soup" nature lets folks focus very tightly on their domain with little concern for all of the other rules that may be in the system.</p> <p>For example, Frank is interested in orders with a total of more than $1000, so he puts in to the rule system that he's interested. "IF order.total > 1000 THEN email Frank".</p> <p>Meanwhile, Sally wants all orders from the west coast: "IF order.source == 'WEST_COAST' THEN email Sally".</p> <p>So, you can see in this trivial, contrived case, that an order can satisfy both rules, yet both rules are independent of each other. A $1200 order from the West Coast notifies both Frank and Sally. When Frank is no longer concerned, he'll simply yank his rule out from the soup.</p> <p>For many situations, this flexibility can be very powerful. It can also, like this case, be exposed to end users for simple rules. Using high level expressions and perhaps lightweight scripting.</p> <p>Now, clearly, in a complicated system there are all sorts of interrelationships that can happen, an this is why the entire system is not "Done with rules". Someone, somewhere is going to be in charge of the rules not getting out of hand. But that doesn't necessarily lessen the value such a system can provide.</p> <p>Mind this doesn't even go in to things like expert systems, where rules fire on data that rules can create, but a simpler rules system.</p> <p>Anyway, I hope this example shows how a rules system can help augment a larger application.</p>
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