Drools教學

任何Java企業級應用可以分成三個部分 −

• UI – 用戶界麵（前端）
• 服務層連接到數據庫
• 業務層

我們有一些框架，用來處理UI和業務層在一起，例如，Spring和Struts。然而，我們冇有一個標準的方式來處理業務邏輯，直到Drools的出現。

Drools是什麼?

Drools是一個業務邏輯集成平台（BLip）。它是用Java編寫。它是由JBoss和紅帽公司擴展支持，並實現Rete模式匹配算法的一個開源項目。

通俗地說，Drools是一種工具，使我們能夠分離內部業務流程，找到邏輯和數據的集合。我們需要注意的兩個重要關鍵詞是邏輯和數據。

Drools的被分成兩個主要部分：編寫和運行係統。

• 製作: 製作過程涉及創建規則文件（.DRL文件）。
• 運行時: 它涉及到創建工作存儲器和處理活化。

什麼是規則引擎？

Drools is Rule Engine or a Production Rule System that uses the rule-based approach to implement and Expert System. Expert Systems are knowledge-based systems that use knowledge representation to process acquired knowledge into a knowledge base that can be used for reasoning.

A Production Rule System is Turing complete with a focus on knowledge representation to express propositional and first-order logic in a concise, non-ambiguous and declarative manner.

The brain of a Production Rules System is an Inference Engine that can scale to a large number of rules and facts. The Inference Engine matches facts and data against Production Rules – also called Productions or just Rules – to infer conclusions which result in actions.

A Production Rule is a two-part structure that uses first-order logic for reasoning over knowledge representation. A business rule engine is a software system that executes one or more business rules in a runtime production environment.

A Rule Engine allows you to define “What to Do” and not “How to do it.”

What is a Rule?

Rules are pieces of knowledge often expressed as, "When some conditions occur, then do some tasks."

When
    <Condition is true>
Then
    <Take desired Action>

The most important part of a Rule is its when part. If the when part is satisfied, the thenpart is triggered.

rule  <rule_name>
      <attribute> <value>
      
      when
         <conditions>
      
      then
         <actions>
end

Pattern Matching

The process of matching the new or existing facts against Production Rules is called Pattern Matching, which is performed by the Inference Engine. There are a number of algorithms used for Pattern Matching including:

• Linear
• Rete
• Treat
• Leaps

Drools Implements and extends the Rete Algorithm. The Drools Rete implementation is called ReteOO, signifying that Drools has an enhanced and optimized implementation of the Rete algorithm for object-oriented systems.

Advantages of a Rule Engine

Declarative Programming

Rules make it easy to express solutions to difficult problems and get the solutions verified as well. Unlike codes, Rules are written in less complex language; Business Analysts can easily read and verify a set of rules.

Logic and Data Separation

The data resides in the Domain Objects and the business logic resides in the Rules. Depending upon the kind of project, this kind of separation can be very advantageous.

Speed and Scalability

The Rete OO algorithm on which Drools is written is already a proven algorithm. With the help of Drools, your application becomes very scalable. If there are frequent change requests, one can add new rules without having to modify the existing rules.

Centralization of Knowledge

By using Rules, you create a repository of knowledge (a knowledge base) which is executable. It is a single point of truth for business policy. Ideally, Rules are so readable that they can also serve as documentation.

Tool Integration

Tools such as Eclipse provide ways to edit and manage rules and get immediate feedback, validation, and content assistance. Auditing and debugging tools are also available.