BAEL-803: Backward Chaining with Drools (eugenp#2741)

* spring beans DI examples

* fix-1: shortening examples

* List of Rules Engines in Java

* BAEL-812: Openl-Tablets example added

* BAEL-812: artifacts names changed

* BAEL-812: moving rule-engines examples to rule-engines folder

* BAEL-812: removing evaluation article files

* BAEL-812: folder renamed

* BAEL-812: folder renamed

* BAEL-812: pom.xml - parent added

* BAEL-1027: Introduction to GraphQL - initial commit

* BAEL-781: Explore the new Spring Cloud Gateway

* BAEL-781: Fix UserService.java

* BAEL-781: Fix user-service pom.xml

* BAEL-781: remove eureka-server from the example

* BAEL-781: modifying example

* BAEL-803: Backward Chaining wih Drools

* BAEL-803: pom.xml

Author: felipeazv

drools/backward-chaining/pom.xml

@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+
+	<modelVersion>4.0.0</modelVersion>
+
+	<artifactId>drools-backward-chaining</artifactId>
+	<version>1.0</version>
+	<name>drools-backward-chaining</name>
+
+	<parent>
+		<groupId>com.baeldung</groupId>
+		<artifactId>parent-modules</artifactId>
+		<version>1.0.0-SNAPSHOT</version>
+	</parent>
+
+	<properties>
+		<runtime.version>6.4.0.Final</runtime.version>
+	</properties>
+
+	<dependencies>
+		<dependency>
+			<groupId>org.kie</groupId>
+			<artifactId>kie-api</artifactId>
+			<version>${runtime.version}</version>
+		</dependency>
+		<dependency>
+			<groupId>org.drools</groupId>
+			<artifactId>drools-core</artifactId>
+			<version>${runtime.version}</version>
+		</dependency>
+		<dependency>
+			<groupId>org.drools</groupId>
+			<artifactId>drools-decisiontables</artifactId>
+			<version>${runtime.version}</version>
+		</dependency>
+	</dependencies>
+</project>

drools/backward-chaining/src/main/java/com/baeldung/BackwardChainingBeatles.java

@@ -0,0 +1,28 @@
+package com.baeldung;
+
+import org.kie.api.KieServices;
+import org.kie.api.runtime.KieContainer;
+import org.kie.api.runtime.KieSession;
+
+import com.baeldung.model.Beatle;
+
+public class BackwardChainingBeatles {
+    public static void main(String[] args) {
+        
+            KieServices ks = KieServices.Factory.get();
+            KieContainer kContainer = ks.getKieClasspathContainer();
+            KieSession kSession = kContainer.newKieSession("ksession-backward-chaining");
+            // drools session base on the xml configuration (<strong>kmodule.xml</strong>)
+
+            // graph population
+            kSession.insert(new Beatle("Starr", "drums"));
+            kSession.insert(new Beatle("McCartney", "bass"));
+            kSession.insert(new Beatle("Lennon", "guitar"));
+            kSession.insert(new Beatle("Harrison", "guitar"));
+
+            kSession.insert("Ringo"); // invoke the rule that calls the query implentation of backward chaining
+            kSession.fireAllRules(); // invoke all the rules
+        
+    }
+
+}

drools/backward-chaining/src/main/java/com/baeldung/model/Beatle.java

@@ -0,0 +1,33 @@
+package com.baeldung.model;
+
+import org.kie.api.definition.type.Position;
+
+public class Beatle {
+
+    @Position(0)
+    private String lastName;
+    @Position(1)
+    private String instrument;
+
+    public Beatle(String lastName, String instrument) {
+        this.lastName = lastName;
+        this.instrument = instrument;
+    }
+
+    public String getInstrument() {
+        return instrument;
+    }
+
+    public void setInstrument(String instrument) {
+        this.instrument = instrument;
+    }
+
+    public String getLastName() {
+        return lastName;
+    }
+
+    public void setLastName(String lastName) {
+        this.lastName = lastName;
+    }
+
+}

drools/backward-chaining/src/main/resources/META-INF/kmodule.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<kmodule xmlns="http://jboss.org/kie/6.0.0/kmodule">
+    <kbase name="backward_chaining" packages="backward_chaining">
+        <ksession name="ksession-backward-chaining"/>
+    </kbase>
+</kmodule>

drools/backward-chaining/src/main/resources/backward_chaining/Beatles.drl

@@ -0,0 +1,44 @@
+package com.baeldung
+
+import com.baeldung.model.Beatle;
+
+
+query whichBeatle(String lastName, String instrument) 
+  Beatle(lastName, instrument;)
+  or 
+  (Beatle(lastName, null;) 
+    and 
+   whichBeatle(null, instrument;)) //recursive call to the function that allows to search in a derivation tree structure
+end
+
+rule "Ringo"
+  when
+    String(this == "Ringo")
+    whichBeatle("Starr", "drums";)
+  then
+    System.out.println("The beatle is Ringo Starr");
+end
+
+rule "Paul"
+  when
+    String(this == "Paul")
+    whichBeatle("McCartney", "bass";)
+  then
+    System.out.println("The beatle is Paul McCartney");
+end
+
+rule "John"
+  when
+    String(this == "John")
+    whichBeatle("Lennon", "guitar";)
+  then
+    System.out.println("The beatle is John Lennon");
+end
+
+rule "George"
+  when
+    String(this == "George")
+    whichBeatle("Harrison", "guitar";)
+  then
+    System.out.println("The beatle is George Harrison");
+end