raise error saying : *shooh shooh shooh, constraint breaking not allowed*. a value that already exists in some other row, then take action. The value that was to be set to the column *sno* is :. Observe that if the modification is being done to the *sno*. Whenever any data modification is being done in that table,. What is the mechanism for implementing this in the database level? Event : To showcase, suppose here is a db table schema with some constraint of non null and unique : Implementation of those are, simply put, nothing but type 2 rules. Recall from databases that there is this thing called a trigger, which is used to implement this other thing called the constraint. What Martin Fowler is not telling is that type 2 rule engines are used everywhere when one uses a database supporting constraints! Anytime you are using a database trigger, you are using a type 2 rule engine. Instead, I am going to showcase how one can be easily build, based on a rule engine that everyone used, without knowing much about rule engines or Type-2 systems. Now, I am not going to bore you with the first one, because no matter how fancy it sounds like, it never is good enough for serious inferencing. In fact whether or not one should use a rule engine is highly debatable topic. 
You can read more about them in some detail here. Type - 2 : event driven, and acronym as ECA : Event-Condition-Action.It may also have a hidden model under the hood, for probabilistic sophistication. Type -1 : the direct result of A.I., and generally uses inferencing using some modified versions of rete algorithm.There are two general classes of rules engines. One can put rules declaratively, and it's appropriate execution would be taken care by the engine.
Hence, once the maturity and the complexity levels reaches a certain threshold, an organisation try switching to the closest declarative style of storing business logic a.k.a business rule engines. They are indeed the poor mans (imperative languages) declarative implementation (more like a proxy). For expositions on how this realisation happens please read :
It is also a matter of fact that sooner or later, developers using such languages and paradigm figures out that it is a sin to write business logic inside compiled code, or rather any imperative code. As it is unfolding now, despite many tries from the other sides of the force, most of the languages used by industry are imperative in nature.
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