Using a rule engine provides a framework that allows to externalize business logic in a common place which in turn empower subject matter experts of the business to easily change and manage the rules. Coding such rules directly into the application makes maintenance difficult and expensive because the rules change so often. This article describes how to architect and build a service that uses Drools to provide business decisions.

James C. Owen of KSBC has kindly provided PDF'ized versions of Dr Charles Forgy's thesis, where he defines the RETE algorithm (Charles Forgy is the inventor of the RETE algorithm, which is kind of important to rule engines ;).

For those who are interest in this, you can find them here. They are broken down by chapter as they are pretty large. There is a whole lotta history here (which I thoroughly enjoy). The 70's and 80's were a golden age for software (and music !).

Drools are using RETE as an algorithm for pattern matching. It uses a network of nodes. This network/tree has one entry point and many exit points (each one represents a rule consequence). To be more precise, the network is a directed acyclic graph of nodes. The network is created when we compile rules. It is then used at runtime. We insert objects into the network. Objects propagate through the network and if they reach exit-points of the network, activations are being created. They are later on executed after we call workingMemory.fireAllRules().

Using a rules engine can lower an application's maintenance and extensibility costs by reducing the complexity of components that implement complex business logic. This updated article shows you how to use the open source Drools rules engine to make a Java™ application more adaptive to changes. The Drools project has introduced a new native rule expression language and an Eclipse plug-in, making Drools easier to use than ever before.

String comparison algorithms are plentiful in computer science. Expert systems also require extensive pattern matching during their execution, but it is a special case of pattern matching. The Rete matching algorithm was invented to speed up that pattern-matching process.