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Drools is the Business Rule Management System (BRMS) provided by the JBoss Community for implementing complex applications with automatic behaviors based on rules. The framework is essentially based on an extension of the Rete pattern-matching algorithm.
The JBoss Enterprise Web Platform is the software infrastructure that supports the running of BRMS components.
MVEL is an expression language for Java-based applications. It supports field and method/getter access. It is based on Java syntax.
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The rules logic can be done in a programming language like Java. Consider a Jewellery shop which needs to maintain business logic of calculating discount based on the type of Jewellery. But such rules may change daily and need to be updated regularly in our code which is not good. Also, we will always require a Developer for making such changes. So it's a good practice to define this logic as rules in a Business Rule Management System. If tomorrow the discount is to be changed this can be done even by a non-technical person.
JBoss Enterprise BRMS is a business rule management system and reasoning engine for business policy and rules development, access, and change management. JBoss Enterprise BRMS is a productized version of Drools with enterprise-level support available. JBoss Rules is also a productized version of Drools, but JBoss Enterprise BRMS is the flagship product.
Components of the enterprise version:
1. JBoss Enterprise Web Platform – the software infrastructure, supported to run the BRMS components only
2. JBoss Enterprise Application Platform or JBoss Enterprise SOA Platform – the software infrastructure, supported to run the BRMS components only
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Business Rules Engine – Drools Expert using the Rete algorithm and the Drools Rule Language (DRL)
3. Business Rules Manager – Drools Guvnor – Guvnor is a centralized repository for Drools Knowledge Bases, with rich web-based GUIs, editors, and tools to aid in the management of large numbers of rules.
4. Business Rules Repository – Drools Guvnor
Drools and Guvnor are JBoss Community open source projects. As they are mature, they are brought into the enterprise-ready product JBoss Enterprise BRMS.
Components of the JBoss Community version:
1. Drools Guvnor (Business Rules Manager) – a centralized repository for Drools Knowledge Bases
2. Drools Expert (rule engine) – uses the rules to perform reasoning
3. Drools Flow (process/workflow), or jBPM 5 – provides for workflow and business processes
4. Drools Fusion (event processing/temporal reasoning) – provides for complex event processing
5. Drools Planner/OptaPlanner (automated planning) – optimizes automated planning, including NP-hard planning problems
Example
This example illustrates a simple rule to print out information about a holiday in July. It checks a condition on an instance of the Holiday class and executes Java code if that condition is true.
rule "validate holiday"
dialect "mvel"
dialect "java"
when
$h1 : Holiday( month == "july" )
then
System.out.println($h1.name + ":" + $h1.month);
end
The purpose of dialect “mvel” is to point the Getter and Setters of the variables of your Plain Old Java Object (POJO) classes. Consider the above example, in which a Holiday class is used and inside the circular brackets (parentheses) “month” is used. So with the help of dialect “mvel” the getter and setters of the variable “month” can be accessed.
Dialect “java” is used to help us write our Java code in our rules. There is one restriction or characteristic on this. We cannot use Java code inside the “when” part of the rule but we can use Java code in the “then” part.
We can also declare a Reference variable $h1 without the $ symbol. There is no restriction on this. The main purpose of putting the $ symbol before the variable is to mark the difference between variables of POJO classes and Rules.
Users cannot use Java code inside the “when” part of the rules. However, it can be used in the “then” part.
Rules are pieces of knowledge often expressed as, “When some conditions occur, then do some tasks.”
When
Then
The most important part of a Rule is its when part. If the when part is satisfied, the then part is triggered.
rule
when
then
end
Drools Workbench is the web user interface for authoring and management. Drools Expert is the business rule engine and Drools Fusion is the complex event processing feature.
Accumulate is a conditional element that was introduced in Drools version 4.0. It is used to iterate over the list of objects and help validation of data in Java operation.
Ex. accumulate (Counter() ; $cnt : count())
KIE stands for Knowledge is everything. It stands for Knowledge is everything. From Drools 6.0 onwards a new approach is used to create a Knowledge Base and a Knowledge Session. The knowledge base is an interface that manages a set of rules and processes. The main task of the knowledge base is to store and re-use rules because the creation of rules is very expensive. Rules are contained inside the package org. drools.KnowledgeBase. These are commonly referred to as knowledge definitions or knowledge. The knowledge base provides methods for creating a Session.
JBoss Enterprise BRMS is the productized version of Drools and it has enterprise-level support.
The knowledge session is retrieved from the knowledge base. It is the main interface for interacting with the Drools Engine. The knowledge session can be of two types:
1. Stateless Knowledge Session
2. Stateful Knowledge Session
Using retract method to remove the object from the knowledge session can help.
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The global variable in Drools is the keyword. Global is the variable used in Drool and is visible in all rules inside the DRL file.
Drool is also called a production rule system that uses an enhanced implementation of the Rete algorithm.
Drool supports enterprise framework for enforcement, construction, and maintenance for business policies along with JSR-94 standards for the business rule engine.
JBoss enterprise version has the following components - JBoss Enterprise application of SOA platform, Business Rules Engine, JBoss Enterprise web platform, and Business Rules Manager.
Some important features of the JBoss Enterprise Application Platform are as follows -
Drool flow provides workflow capabilities to the drool platform. Workflow describes the order of steps that need to be executed. Drool flow allows users to execute and monitor their business logic.
In drools, decision tables are excel-based tables. The logic in these tables can be understood and changed by the non-technical person. As the .drl file is difficult to manage, therefore, decision table is a good option that can be easily managed by non-programmers.
Stateless Session
Any changes in the facts while executing rules are not made aware to the rule engine.
dispose() method is called automatically to release the session.
Any changes in the facts while executing rules are not made aware to the rule engine so if any rule is modified no other re-activation of rules will take place.
Stateful Session
Any changes in the facts while executing rules are made aware of the rule engine.
dispose() method should be called to release the session to avoid memory leaks.
As any changes in facts are available to the rule engine so if a rule is modified for a particular fact, this change will re-activate all the rules and fire the rules that are built on modified fact.
In background chaining, the first user takes a decision and then checks whether the decision is true or not by tracking the sequence of events. It is also referred to as derivation queries and drools implements that through query construct.
There are the following attributes used in Drools -
Yes, I have used Drools with Spring Framework.
Unlike other programming languages, Drool offers several modifications to be applied to coding and does not work on a certain set of patterns. Conventional programming languages do not provide any management solution for businesses which is the main feature of Drool.
Following are the advantages of using rule engines in business project management -
Drool process is a framework that provides a flow of work along with various business processes.
In Drools, we use "$" before every variable to ensure that there is no difference between the POJO class and variables of rules.
Drool planner is also called automated planning that optimizes the process during NP-hard planning issues.
JBPM is a lightweight workflow engine written in Java and allows you to execute business processes with the latest BPMN 2.0 specification. It runs on any Java environment you have as an application or service.
Global is a keyword used while defining any variable which would be visible to all the rules in a DRL file. A global variable can be used with any type of object. Global must be used very carefully with the DRL file because any changes in the global variable are not notified to the working memory.
StringBuffer | StringBuilder |
StringBuffer is synchronized ie. two threads can’t call the method at the same time | StringBuilder is unsynchronized ie. two threads can call the method at the same time |
Less efficient as compared to StringBuilder | Better than StringBuffer |
Drool fusion is used for event processing and temporal reasoning.
Dialect is used to write Java code in the rules and points to the Getter and Setters of the variables of POJO (plain old Java objects)
There is no such way to ensure this. Newcomers should not at all try this to avoid complications.
It is possible to store the rules in the custom domain model in the database and generate the DRL from the same model. This helps render the Guvnor redundant in the custom environment.
A user cannot use Java code inside the “when” part of the rules. However, it can be used in the “then” part.
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Drools Workbench is the web user interface for authoring and management. Drools Expert is the business rule engine and Drools Fusion is the complex event processing feature.
The user has to resort to drools. halt() in the consequence (then) part of the application.
By using the retract method, we can remove the object from the knowledge session.
Rules do not exist as the Java codes. Use the Guvnor REST API to download the DRL source code for the rules and thereafter upload them all over again.
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