Drools Interview Questions

Last updated on Jan 09, 2024

Drool is a business rule management system consists of backward and forward interfaces. If you are looking for the top 15 Drool interview questions and answers, then you are at the right place to learn and explore the important concepts of the Drool tool. As per the latest research, more than 1.78% of the market depends on the drool tool. Are you ready to upgrade drool skills? Then here you go, HKR’s recently designed drool interview questions and answers will help you to crack any complex interview. What you are waiting for, let’s begin the journey;

 Most Frequently Asked Drools Interview Questions

What is a Drool tool?

Ans: Drool is nothing but a business rule management system (BRMS) tool, which is provided by the JBoss community. This type of tool is designed to implement complex applications based on automatic behavior and rules. Drool is also known as the framework, this framework is developed on the base of the algorithm called rete pattern-matching algorithms. Drool is written using java and it’s an open-source project tool used by JBoss and Red Hat. The two main keywords here we use are Logic and data.

What are the main parts of Drool tools?

Ans: The following are the two important parts of the Drool tool such as;

1. Authoring -> this process is used to generate Rules files or DRL files.

2. Runtime -> this process is used to create working memory and handling all types of activations.

What do you mean by Rule in Drool?

Ans: A rule is a piece of knowledge that is used to perform specific tasks based on the conditions.

The syntax is as follows;


 < Condition is true>


 < Take desired action>









What are the important components of the enterprise version?

Ans: Below are the important components of the enterprise version;

1. JBoss enterprise web platform-> this contains software infrastructures and BRMS components.

2. JBoss enterprise application platform or JBoss enterprise SOA platform-> this consists of software infrastructures, and run only BRMS components.

3. Business rules manager-> this consists of drools guvnor and GUI editors.

4. Business Rules repository -> this consists of Drools Guvnor.

Below are the components of the JBoss community version:

1. Drools Guvnor or Business rule manager

2. Drools expert or rule engine.

3. Drools flow or process/workflow

4. Drools fusion or event processing /temporal reasoning.

5. Drools planner or Optaplanner.

What do you mean by pattern matching?

Ans: Pattern matching is a process of matching any new or existing facts which can be done against production rules. The complete pattern matching is performed by the Inference Engine.

There are many algorithms used to perform pattern matching;

1. Linear

2. Treat

3. Rete

4. Leaps.

What are Drool workbench, Expert, and Fusion?

Ans: Drool workbench uses the web user interface to perform authoring and management tasks.

Drool expert is also known as a business rule engine.

Drool fusion is the feature used to process complex events.

What is a Knowledge session in Drool?

Ans: The knowledge session in Drool nothing but a session retrieved from the knowledge base. This is also known as the main interface for the drool engine.

There are two types of knowledge session available such as;

1. Stateless knowledge session

2. Stateful knowledge session.

What is the global variable and nomenclature in Drool?

Ans: The global variables are the keywords used in Drool. This global variable is a variable and which are visible to all the rules available in the DRL file.

Nomenclature is also known as a production rule system that uses the enhanced Rete algorithm implementations.

What are the key features of the JBoss Enterprise Application Platform?

Ans: Some of the important key features of the Jboss enterprise application platform;

1. Eclipse based IDEs are available using the Jboss developer studio.

2. Java persistence uses hibernate.

3. Supports the Java EE and web service standard tools.

4. Object request broker in Drool uses the JACOBS which is interoperability with any CORBA objects.

. What do you understand by Drools stateful vs stateless knowledge session?

Ans: Let me explain one by one;

1. Stateless session:

a. Any changes which are made while executing the rules will not affect the rule engine.

b. dispose() method in drool is called automatically mainly used to release the session.

c. Any changes made in the executing rules will not affect the rule engine, this means that any rule is modified no other re-activation rule engines will take place.

2. Stateful session:

a. Any changes which are made while executing the rules will affect the rule engine.

b. Dispose  () method in drool is called to release the session to avoid any memory leaks.

c. Any changes made in the executing rule will affect the rule engine for a particular fact; you can also reactivate all the drools rules and also modify them.

. Which drools attribute have you used?

Ans: Below are the important drools attributes which we are using;

1. Salience -> salience in drool is used to figure out which drool tool we have to fire first.

2. No loop -> with the help of this attribute, you can avoid an indefinite looping system.

3. Update statement -> this attribute updates the statement and also modifies the rule facts.

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. List the different execution control statements used in the drool tool?

Ans: Some of the different execution control statements used in drool tool such as;

1. Agenda group: this is an optional keyword in Drool. If any type of rule is not assigned to any group, it will belong to the main agenda by default.

2. Activation group: this is a reserved keyword available in drl file. Here either singular or multiple rules will be belonging to a particular activation group.

3. Rule flow-group: this is similar to the agenda group. This is primarily used in the JBPM process to process rule flows.

. What are the benefits of using rule engines in business project management?

Ans: Below are the important benefits of using rule engines in business project management such as;

1. Offers high flexibility

2. it’s very easy to learn

3. Helps to reduce the intricacy

4. Reusability

. How to remove objects from the Knowledge session?

Ans: By using the retract method to remove the object from the knowledge session.

. What is the restriction in java code?

Ans: In drools, the user cannot use the java code inside the “when” part of the rules, and it can also be used in the “then” part.


In this blog, I have tried my best to explain the top 15 important drools interview questions and answers asked by any top recruiter. Learning HKR’s drools interview questions and answers will help you to gain knowledge and crack any interview. These interview questions are designed for both basic to experienced professionals.

. What do you mean by Dialect?

MVEL refers to an expression language used for Java  based applications. Field and getter/method access is supported here.

. Why is Drool used rather than other programming languages?

Rule logic can be performed by programming languages such as Java. For example, in jewelry shops, we need to maintain the business logic of discount calculation according to the type of Jewellery. The rules may change on a daily basis. So the code needs to be updated on a regular basis. For this we need a developer for doing the code changes which will not be a good idea. Defining the logic as the rules in the business rule management system is a good practice. Even though the discount is  changed it can be handled by even the non-technical person.

. What is accumulating in the DRL Drools?

Accumulate is defined as a conditional element that can be found in version 4.0 of Drool. It can be used  for iterating over the objects list and assisting validation in the Java Operation.

Ex. accumulate (Counter() ; $cnt : count())

. What is KIE?

KIE refers to Knowledge is everything. This is a new approach for creating a knowledge base and Knowledge session. It is available in versions 6.0 onwards. Here Knowledge base refers to an interface that deals with a set of processes and rules. The Primary task of the Knowledge base is storing and reusing the rules as the rules creation is quite expensive. Rules are available in Package org.drools.KnowledgeBase. They are usually called Knowledge or Knowledge definitions. Knowledge base offers methods to create a session.

. Name the productized version of the Drools.

The Productized version of Drools is the JBoss Enterprise BRMS. It comes with enterprise-level support. 

. What standard is supported by Drool?

Enterprise framework for construction, enforcement and maintenance of the business policies as well as the JSR-94 standards for the business rule engine are available in Drools.

. What do you mean by Drool flow?

Drool flow offers workflow features to the drool platform. Workflow narrates the order of the steps that are to be carried out. Drool flow enables users to monitor and execute their business logic. 

. Define decision tables.

In drools, the decision tables are similar to excel-based tables. Even a nontechnical person will be able to understand or change the logic in the tables. Decision table is the best choice as it can be managed by the non-programmers while the .drl file is complex to manage.

. In Drools, how will the backward chaining be implemented?

The user first makes the decision and will check if the decision is true or false by monitoring the sequence of events in the background chaining. This  is also called the derivation queries and the drools implement it through the query constructs.

. What framework did you integrate Drools with?

I have integrated Drools with the Spring Framework.

. Differentiate Drool from the other programming languages.

In contrast to other programming languages, Drool provides various modifications that can be applied to coding and will not work on a specific set of patterns. For Businesses Conventional programming languages will not offer any management solution that is offered by Drool.

. Drools process refers to a framework that offers a workflow as well as several business processes.

In Drool, “$” symbol is used before every variable to make sure that POJO class and the variables of rules are not different.

. What is a Drool planner?

Drool Planner is also known as automated planning. During NP-hard planning issues, Drool planner optimizes the process.

. What is the use of JBPM?

JBPM indicates a lightweight workflow engine that is written in Java and enables you to execute the business processes along with the present-day BPMN 2.0 specification. It can be executed on any Java environment we use as an application/service.

. Differentiate StringBuffer class with StringBuilder class.

  • StringBuffer is Synchronized. Here we cannot call the method simultaneously by threads.
  • StringBuilder is unsynchronized. The method can be called by the thread simultaneously.
  • StringBuilder is very efficient when compared to StringBuffer.

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. In Drool, which tool can be used for managing the complex event processing?

For Event processing and temporal reasoning, Drool fusion will be used.

. Why is Dialect used in Drools?

Dialect is used for writing Java Code in the rules and points to the setter and getter of variables of POJO.

. Is it possible to use the round trips in between the rules that are modified in Guvnor, load them in UI, update as well as upload to Guvnor?

We cannot ensure this. It is better not to try it to avoid complications.

. Is it possible to work with a Custom UI rather than the Guvnor Rule?

Yes! We can store the rules in the custom domain model in the database and produce DRL from the same model. It assists in rendering Guvnor redundant in a custom environment.

. Why is the symbol “$” used in front of every variable?

In Drool, “$” symbol is used before every variable to make sure that POJO class and the variables of rules are not different.

. Define the terms Drools Workbench, Fusion and Expert.

Drools Workbench is indeed a web user interface to authorize and manage. Drools Fusion refers to a complicated event processing capability while Drools Experts refers to a business rule engine.

. How can we halt processes in the DRL file?

The user has to use the drools. halt()/stop in the result part of the application.

. How can we fetch and play with rules in the java code?

Rules are not available as the Java codes. Guvnor REST API can be used for downloading the DRL source code for the rules and then again upload them all.

. Who uses the Business Rule engine?

Business Rules Engine is used by Drools experts through the Rete algorithm as well as the Drools Rule Language.

. What is not recommended while using the global variables?

Modification of global value in the rule is not recommended. Rather, the value must be set from the application with the help of the Working memory interface.

. What do you mean by the Drools Expert and Drools Guvnor?

Drools Experts refers to the Business Rules Engine while the Drools Guvnor refers to the Business Rules Manager.

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. What do you mean by the Business Rules Manager?

Business Rules Manager is indeed a centralized repository for Drools knowledge bases. It has a rich web based GUI (graphical user interface) and tools as well as editor for managing a huge number of rules.

. What are Drools' Recursive Rules?

The best example for Recursion in Drools is the fibonacci series. It is implemented using the keywords insert and modifies.

. List the important parts of the Recursion in Drools.

Most important parts in the process of implementation of the recursive rules in Drools are the insertion of keywords in the ‘then’ part, ‘breaking condition’ and not keyword in the ‘when’ part.

. In fibonacci series, what condition is used for breaking the series?

In the Fibonacci series, the breaking condition is sequence==1.

About Author

As a Senior Writer for HKR Trainings, Sai Manikanth has a great understanding of today’s data-driven environment, which includes key aspects such as Business Intelligence and data management. He manages the task of creating great content in the areas of Digital Marketing, Content Management, Project Management & Methodologies, Product Lifecycle Management Tools. Connect with him on LinkedIn and Twitter.

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