RickJWagner writes "Drools (sometimes called 'JBoss Rules') is a Business Rules Engine and supporting ecosystem. Drools, like other BREs, promises to lower the barriers to entry for application programming. Armed with this book, can a Business Analyst be used to write application logic? I don't believe so, and I'll tell you why." Keep reading for the rest of RickJWagner's review.Business Rules Engines, especially those based on the Rete algorithm, strongly favor rules written in 'if/then' format. (Sometimes the marketers will call this 'when/then' logic.) The basic premise is that you write your rules as a series of rules that individually specify matching logic for some objects you make available to the engine, then you specify what to do if any of the objects match your conditions. Example "IF there is a customer with age > 60, THEN allow senior discount". That's the marketing promise, anyway.
|Drools JBoss Rules 5.0 Developer's Guide|
|summary||Guides you through all of the features of Drools, such as dynamic rules, the event model, and Rete implementation with high performance indexing.|
This book does a great job of showing you how to build a banking application, complete with validation, data transformation, and reporting functions. Each of these are implemented using Drools, of course, and workable code is provided at every step. The author takes care to explain nearly every line of code provided and highlights important classes and features as they occur. I think the author did well here.
Writing business rules is quite a bit different than writing logic in a language like Java or C++. I'd compare it more directly to writing SQL-- you're declaratively specifying which objects (out of a group) you want something to happen to, so you're thinking in terms of matching logic rather than ordered steps in an algorithm. You also don't always have complete control over the order in which your rules are fired, so it's not like garden-variety coding where it can be treated like a 5-step recipe. It just takes a different mindset. Once you're used to it, things are easier to understand, and this book can help. (By the way, I've fooled around with BREs for about a decade now, and support a production application that uses Drools, so I'd consider myself moderately skilled in BRE usage.)
In the course of writing the banking application the book is anchored upon, the author occasionally makes design decisions that are specific to doing things "The Rule Engine Way". One example is the use of 'global' facilities for validation reporting. The author might have chosen to implement this another way, but chose what he considered the best path and briefly explained his reasoning in making the choice. That's exactly the kind of thing that I think a BRE-literate reader would find of value-- expert insights into how to use this tool, not mere explanations of syntax, etc. Unfortunately, these insights were relatively few in nature and not highlighted where they were presented, so they might not be apparent to readers that aren't already thinking in the BRE way.
One thing the book glossed over that I wish was given more coverage is Guvnor, the Drools Business Rule Management System. Basically, a BRMS is a web application used to change existing rules, write new rules (provided they have been pre-templated by a rule author, usually), and version the rules. I'm told this is one of the key differentiators between Drools and commercial offerings like IBM's JRules, so it's a little disappointing that it was given virtually no coverage in this book.
As the author fleshes out the banking application, we encounter a little scope bleed as the reader is introduced to iBatis, Spring and Tomcat. While I see how these are necessary for the provided application, I viewed them as distractions and potentially barriers to successful implementation for some readers. To counter that, I offer the author kudos for covering a multitude of Drools facets like Domain Specific Language inclusion, Complex Event Processing, and rule ordering through "Drools Flow". All these are valuable tools in the Drools user's toolbox and they are given adequate coverage.
As I hinted at in the opening paragraph, marketers of BREs love to show demonstrations where rules are written in shocking clear 'if/then' syntax. These rules are purported to control powerful application logic and can be maintained by low-cost business analysts. Is this reality with Drools? No, I'm afraid not. It's also not true with JRules, Blaze, or any other Rete-powered BRE. What marketers will show you is how easy rule maintenance can be-- but they're not showing you how difficult things can be when your logic doesn't neatly fit the 'if/then' paradigm. For example, commercial vendors love to show insurance logic where they offer rules like 'IF the driver's age is over 25, THEN give them a discount'. Next time you see one of those, ask the marketer to show you something along the lines of 'Calculate the average age of the drivers in the household'. Notice how that doesn't say 'IF'? Requests of this type will typically require a skilled rule author, not a business analyst copying from a rule template. This type of logic does not play to the strengths of the engine. Actually, implementing this type of logic can be fiendishly difficult-- that's the reason BRE developers are among the best paid of application developers (Check Dice or Monster.com). I say all this to let you know BRE usage sometimes is easy, sometimes is really hard. In a workspace like that, I like to have advice handy from a multitude of providers, and I'll be happy to add this book to my reference collection. I just wish there were more highlighted best practices in this book to help the user leverage the author's expert experience. (By the way, there are a few more books on rules engines available, but most all of what I've seen is truly awful. I do believe they were written by business analysts, and probably ones who have never actually written an application powered by a BRE. I do not find that fault with this book.)
So, what's the verdict? I'm glad I read this book (twice, to make sure I got everything) and would recommend it to anyone using Drools. If you're not yet a Drools user, I don't think this book offers enough remedial material to effectively help you get on board-- for that I recommend the excellent documentation offered online with the product. (By the way, I hope you like cheese. The Drools doc authors seem to have some sort of cheese fixation, so references to cheese are plentiful.) For a Drools user like me, this book offers a view at parts of the toolkit I hadn't yet used and a view of how an expert user might go about designing an application. I'll call it a keeper.
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