How Drools Rule Engine Works

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Drools Rule Engine Works

How Does Drools Rule Engine Works?

The rule engine is a complex event processing system based on rule chains. Here we are going to discuss how Drools Rule Engine works.

Introduction to Drools Rule Engine

Enterprise systems usually have multiple layers. From top to bottom, they are Presentation Layer, Business Logic Layer. The middle layer is a business logic layer representing the core of the application where all of the business processes and decisions occur.

We had a lot of frameworks that covered the User Interface and the Service layer aspects but no proven framework/tool to handle the business logic layer.

Also, the need to build a more complex system is increasing. We are trying to automate all kinds of business processes and implement complex business decisions. However, these processes and decisions are not very well represented using traditional programming languages such as Java; hence needed a framework/tool for the business layer as well and that gave rise to Drools (Rule Engine). Drools is a powerful decision management system with complex event processing features.

Drools is a part of the JBoss Enterprise BRMS product since federating in 2005, is a Business Rule Management System (BRMS) and rules engine written in Java which implements and extends the Rate pattern-matching algorithm.

Rule Engine – Drools

Rule Engine: The rule engine is the computer program that delivers KRR functionality to the developer.

Rule: A rule is a two-part structure.


When all the conditions are met, a rule will fire, i.e., actions will execute.


rule “Hello Jhon”


User (name == “Jhon”)


System.out.println(“Hello Jhon”);


Difference Between a Java Method and Rule

Java Method

public void (User user) {

if (user.getName().equalsIgnoreCase(“Jhon”)){

System.out.println(“Hello Jhon”);




rule “Hello Jhon”


User (name == “Federe”);


System.out.println(“Hello Jhon”);


  • The method must be invoked directly, whereas Rules can never be invoked directly.
  • Specific instances of objects (data) will be passed to Methods, whereas specific object instances cannot be passed to the Rules.

Traditional Programming Vs. Declarative Programming

Traditional Approach

  • There is no standard template to define and code business logic.
  • Mapping a requirement to code is costly.
  • Mapping a requirement to code is difficult.
  • Business logic and infrastructure code are tightly coupled.
  • Business analysts have no way of verifying the implemented logic.

Declarative Approach

  • The rule can be declared, and it resembles very close to the requirements.
  • Rule Engine can apply these rules to the data very efficiently.
  • Easy to maintain.
  • Business analysis can easily verify the declared rules.
  • Requirements and Rules will be more likely mapped one-to-one.

Advantages and Disadvantages of using Rule Engine


  • Rules are easier to understand for both developers and business analysts.
  • Rules are easily maintainable.
  • Improved performance: Rule Engine uses the PHREAK algorithm, which states that the performance of the engine does not depend on the number of rules.
  • Rules can be reused.
  • It can be easily integrated with business processes.
  • Rules can be modified and deployed without having to bring down the application.


  • Developers need to learn this new way of programming.
  • Difficult to troubleshoot issues. We need to understand the working of the rule engine.
  • The Rule engines consume more memory.

When not to use Rule Engine

  • When the Rules are very minimal in an application.
  • If the business logic is more stable and does not change often, the use of Rule Engine is not recommended.
  • If there needs to be control over the code execution, for instance, a specific algorithm, the rule engines are not a good choice.
  • Do not have sufficient training on Rules Engine for the developers.


Author: SVCIT Editorial

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