Practical Guide to Using Drools DMN Engine without Kie Server

1 Background Introduction

1.1 What is DMN

DMN (Decision Model and Notation) is a standard for representing business decisions and rules, enabling stakeholders to understand decision processes quickly through visual diagrams and tables.

1.2 Why Use DMN

In the ZhaiZhai Book project, pricing logic became too complex to maintain in pure Java code; DMN provides a visual, editable representation that reduces dependence on developers and improves communication.

1.3 What is Drools

Drools is an open‑source Java rule engine that implements the DMN specification, acting as a concrete implementation of the DMN interface.

1.4 Why Choose Drools

Drools runs on the existing Java environment (zero extra deployment cost), fully supports the highest DMN conformance level, and benefits from an active community and robust tooling.

2 Drools Engine Application

Content based on Drools version 7.61.0; later versions may differ.

2.1 Official Recommended Direct Approach

Deploy a Kie Server to execute DMN rules and use the Drools web workbench for editing, testing, and publishing rules via the Kie Server REST API.

2.2 Limitations in the ZhaiZhai Book Project

The project cannot adopt the official approach because its internal framework does not include JBoss‑based Kie Server, and external dependencies increase operational risk.

3 Practicing Drools Engine without Kie Server

3.1 Online Editing of DMN Rules

Kogito provides an all‑in‑one JavaScript file that enables online BPMN/DMN editing; the project wrapped this script to integrate editing into its backend UI, adding record lists for version control.

3.2 Executing DMN Rules with Drools Engine

Without Kie Server, the engine is invoked directly in code. First, add Drools dependencies, then create and use the engine as shown:

KieServices kieServices = KieServices.Factory.get();
ReleaseId releaseId = kieServices.newReleaseId("com", "my-kjar", "1.0.0");
KieContainer kieContainer = kieServices.newKieContainer(releaseId);
DMNRuntime dmnRuntime = KieRuntimeFactory.of(kieContainer.getKieBase()).get(DMNRuntime.class);
String namespace = "my-namespace";
String modelName = "dmn-model-name";
DMNModel dmnModel = dmnRuntime.getModel(namespace, modelName);
DMNContext dmnContext = dmnRuntime.newContext();
 dmnContext.set("inputData", "123");
DMNResult dmnResult = dmnRuntime.evaluateAll(dmnModel, dmnContext);
for (DMNDecisionResult dr : dmnResult.getDecisionResults()) {
    log.info("Decision: '" + dr.getDecisionName() + "', Result: " + dr.getResult());
}

The DMN model must be packaged in a JAR that the Kie container can resolve via Maven.

3.3 Completing the Process Flow

Since the all‑in‑one JS lacks built‑in validation, the project uses KieContainer.verify() to check rules before saving:

Results verify = kieContainer.verify();
for (Message message : verify.getMessages()) {
    log.info(message.getLevel().name() + ": " + message.getText());
}

3.4 DLC – Running Drools without Maven

When Maven is unavailable (e.g., on a Spark cluster), the engine can be fed a manually constructed JAR containing the DMN file and required metadata:

// Generate a JAR with kmodule.xml, pom files, and the DMN rule
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
JarOutputStream jos = new JarOutputStream(outputStream);
// kmodule.xml
jos.putNextEntry(new JarEntry("/META-INF/kmodule.xml"));
jos.write("
".getBytes(StandardCharsets.UTF_8));
// pom.properties
jos.putNextEntry(new JarEntry("/META-INF/maven/com.myspace/test/pom.properties"));
jos.write("groupId=com.myspace\nartifactId=test\nversion=1.0.0".getBytes(StandardCharsets.UTF_8));
// pom.xml
jos.putNextEntry(new JarEntry("/META-INF/maven/com.myspace/test/pom.xml"));
jos.write("
...
".getBytes(StandardCharsets.UTF_8));
// DMN file
jos.putNextEntry(new JarEntry("/config/rules/test.dmn"));
jos.write(dmn.getBytes(StandardCharsets.UTF_8));
jos.finish();
ByteArrayInputStream inputStream = new ByteArrayInputStream(outputStream.toByteArray());
KieServices kieServices = KieServices.Factory.get();
KieRepository repository = kieServices.getRepository();
KieModule kieModule = repository.addKieModule(kieServices.getResources().newInputStreamResource(inputStream));
// Use a custom ClassLoader instead of MavenClassLoaderResolver
KieContainer kieContainer = kieServices.newKieContainer(kieModule.getReleaseId(), ProjectClassLoader.findParentClassLoader());

This approach eliminates the need for a Maven environment while still allowing the Drools engine to locate and execute the DMN rules.

4 Summary

DMN offers clear advantages: rule updates without code changes, a single codebase for multiple scenarios, visual workflow, and participation from non‑developers. Drawbacks include visual clutter for complex graphs, occasional need for custom code, and slower execution compared to pure Java.

5 References

Drools official site, DMN tutorials, Kogito sandbox, KIE portal, and OMG DMN specifications.