A Rule Engine is a Logic, or a Condition, like “When some conditions are evaluated to true, then do some task.”A rule engine may be viewed as a sophisticated if/then statement interpreter. The if/then statements that are interpreted are called rules. Imagine a rule engine as a system that takes data and rules as input. It will apply those rules to the data and will give us an output based on the rule definition. Let’s take the example of online shopping where we like to give customers certain promotions or discounts. The if portions of rules contain conditions such as cart.getTotal() > 1000. The then portions of rules contain actions such as addDiscount(amount).
if (cart.getTotal() > 1000)
addDiscount(amount);
Above define shopping cart and customer as the data where rules will be executed if they meet the conditions defined in the ruleset.
Greater Flexibility
Keeping your rules into a Knowledge Base lets you adapt easily to your decisions when they are changing.
Easy to Understand
Rules are easy to understand as compared to other business logic. And this also creates a bridge between Business Analyst and Developers to understand and implement the business logic.
Reduced Complexity
It reduced the complexity as we no longer need to build the rule engine logic in the source.
Reusability
By keeping rules in one place leads to greater reusability of your business rules.
Drools
Drools is a Business Rules Management System (BRMS) solution. It provides a core Business Rules Engine (BRE), a web authoring and rules management application (Drools Workbench), full runtime support for Decision Model and Notation (DMN) models at Conformance level 3, and an Eclipse IDE plugin for core development.
OpenL Tablets
EasyRule is a lightweight rule engine API. It provides the Rule abstraction to create rules with conditions and actions, and the RulesEngine API that runs through a set of rules to test conditions and execute actions. Here is some core feature of the EasyRule:
Easy Rules
Easy Rules is a simple Java rules engine providing a lightweight and POJO based framework to define the business. It can create complex rules from primitive ones by using the composite pattern. This framework, in contrast to the most traditional rules engines, doesn’t make use of XML files or any Domain Specific Language files to segregate rules from the application. It uses annotation-based classes and methods for injecting business logic into the application. Easy Rules can be handy for developers to create and maintain applications with business logic that’s entirely separated from the application itself. On the other hand, as this framework doesn’t implement the JSR94 standard and the business logic has to be coded straight to Java code.
RuleBook
RuleBook provides a simple and intuitive DSL that is flexible. If you have a larger collection of Rules, we can build them as annotated POJOs and RuleBook can transform the whole package into a RuleBook instantly. It provides an easy-to-use Lambda enabled Domain Specific Language or using POJO.
In this blog, we are going to cover the Drools Rule Engine. Let’s take a real-time example so that we will get a better idea. Consider Amazon live sale is going on, in that Amazon is offering multiple discounts on different bank cards.
In the above diagram, the basic flow is if the customer is having an HDFC card then the customer will get a 10% discount on the purchase of > 10000. Similarly, DBS cardholders will get a 10% discount on the purchase of > 5000, and ICICI cardholders will get a 10% discount on the purchase of > 15000.
Steps–
So far so good. Now let’s start doing some code,
1. drools-decision tables
2. drools-core
3. drools-compiler
3. We will be going to keep the example simple so we will just create one package in that there will be basically 4 files.
4. In resources
Now we are done with the project setup, let’s add some code in class files.
DroolsConfig.java
package com.drools.example; import org.kie.api.KieServices; import org.kie.api.builder.KieBuilder; import org.kie.api.builder.KieFileSystem; import org.kie.api.builder.KieModule; import org.kie.api.builder.KieRepository; import org.kie.api.builder.ReleaseId; import org.kie.api.runtime.KieContainer; import org.kie.api.runtime.KieSession; import org.kie.internal.io.ResourceFactory; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; @Configuration public class DroolConfig { private KieServices kieServices = KieServices.Factory.get(); private KieFileSystem getKieFileSystem() { KieFileSystem kieFileSystem = kieServices.newKieFileSystem(); kieFileSystem.write(ResourceFactory.newClassPathResource("offer.xls")); return kieFileSystem; } @Bean public KieContainer getKieContainer() { // Container created getKieRepository(); KieBuilder kb = kieServices.newKieBuilder(getKieFileSystem()); kb.buildAll(); KieModule kieModule = kb.getKieModule(); return kieServices.newKieContainer(kieModule.getReleaseId()); } private void getKieRepository() { final KieRepository kieRepository = kieServices.getRepository(); kieRepository.addKieModule(new KieModule() { public ReleaseId getReleaseId() { return kieRepository.getDefaultReleaseId(); } }); } @Bean public KieSession getKieSession(){ //session created return getKieContainer().newKieSession(); } }
OfferController.java
package com.drools.example; import org.kie.api.runtime.KieSession; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.web.bind.annotation.PostMapping; import org.springframework.web.bind.annotation.RequestBody; import org.springframework.web.bind.annotation.RestController; @RestController public class OfferController { @Autowired private KieSession session; @PostMapping("/order") public OrderEntity orderNow(@RequestBody OrderEntity order) { session.insert(order); session.fireAllRules(); return order; } }
OrderEntity.java
package com.drools.example;
public class OrderEntity {
private String name;
private String cardType;
private int discount;
private int price;
//getters and setters
}
SpringDroolsApplication.java
package com.drools.example;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class SpringDroolsApplication {
public static void main(String[] args) {
SpringApplication.run(SpringDroolsApplication.class, args);
}
}
We are done with the coding part, now let’s check the API.
Case 1
Request Body
{
"name":"Mobile",
"cardType":"HDFC",
"price":9000
}
Response Body
{
"name": "Mobile",
"cardType": "HDFC",
"discount": 0,
"price": 9000
}
Case 2
The customer has an HDFC card and the order amount is greater than 10000
API URL: http://localhost:9091/order
Request Body
{
"name":"Mobile",
"cardType":"HDFC",
"price":15000
}
Response Body
{
"name": "Mobile",
"cardType": "HDFC",
"discount": 10,
"price": 15000
}
That’s it. We have successfully implemented the Rule Engine using the Drools decision table. The complete project you can find here.
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