Data Driven Rules Engine - Drools

Question

I have been evaluating Drools as a Rules Engine for use in our Business Web Application.

My use case is a Order Management Application.
And the rules are of following kind:
- If User Type is "SPECIAL" give an extra 5% discount.
- If User has made 10+ Purchases already, give an extra 3% discount.
- If Product Category is "OLD", give a Gift Hamper to the user worth $5.
- If Product Category is "NEW", give a Gift Hamper to the user worth $1
- If User has made purchases of over $1000 in the past, Shipping is Free

The immediate challenge i see is that:
- There is no meaningful UI that i can offer to the end users to modify the rules.
- Guvnor UI or any Editor to modify drl files is just not acceptable from end user point of view - Most of these Rules will operate on often huge data available in db

So,
- I want a way for Admin users to specify these Rule from within my Web App UI.
- Could i store these "Rules" in database, and then operate on them via Drools - at least that allows me to "modify" these Rules via my "own" UI. So this is something like a Decision Table in DB.
- What is the best way to go about this?

Answer 1

You asked me to give an answer to your question, given my answer to Data driven business rules. My answer to that question was that SQL is a bad solution to execute business rules stored in the database. The person who asked that question wanted to generate SQL expressions from their stored business rules, and I cautioned against doing that, because it would lead to problems in security, testability, performance, and maintenance.

I have not used Drools, but I gather from documentation that it includes Guvnor, a business rules manager that supports using an RDBMS as a repository for user-defined rules.

[Drools] Guvnor uses the JCR standard for storing assets such as rules. The default implementation is Apache Jackrabbit, http://jackrabbit.apache.org. This includes an out of the box storage engine/database, which you can use as is, or configure to use an existing RDBMS if needed. (http://docs.jboss.org/drools/release/5.2.0.Final/drools-guvnor-docs/html/chap-database_configuration.html)

Apache Jackrabbit is not an RDBMS, it is "a content repository is a hierarchical content store with support for structured and unstructured content, full text search, versioning, transactions, observation, and more." This seems like a more appropriate repository for Drools.

But Drools doesn't say it tries to use SQL to execute those business rules. It has a separate component, Drools Expert (Rules Engine) to do that.

Drools Expert is a declarative, rule based, coding environment. This allows you to focus on "what it is you want to do", and not the "how to do this". (http://www.jboss.org/drools/drools-expert.html)

SQL is also a declarative programming language, but it's designed to perform relational operations on table-structured data. A language to implement a rules engine has different goals, and can probably do things that SQL can't (and vice-versa).

So I would suggest if you use Drools, feel free to use an RDBMS as a repository as they document (use their JCR-compliant implementation of content repository, do not try to design your own). Then use their Drools Expert as a specialized language designed for executing rules.

Answer 2

• There is no meaningful UI that i can offer to the end users to modify the rules.

Out of the box, Guvnor provides web based decision tables (and Excel if you prefer), as you say you would like to provide. It provides guided editors for more complex rules, but your rules would appear to be very simple.

• Guvnor UI or any Editor to modify drl files is just not acceptable from end user point of view

As mentioned, Guvnor supports decision tables. If you don't like the layout of the Guvnor web application, then you can just embed the Guvnor editors into your own web application.

• Most of these Rules will operate on often huge data available in db

The size of your database is irrelevant to the use of Guvnor. Guvnor is for editing rules, not runtime evaluation. Drools Expert is the runtime rules engine. It's fast. It can deal with very large volumes of data and very large volumes of rules. All you need to do is write database queries to get relevant chunks of that data into the rules engine at runtime. You need to do that, whatever solution you try to implement.

On a side-note, if what you're really after is an explanation of when rules engines are good (and bad) solutions to a problem, then I would recommend reading the Why use a Rule engine? section of the Drools Expert manual.

Answer 3

Generally, I've found it is easier to work at a more abstract level, such as a Domain Model, and have some sort of programmatic conversion from that to Drools rules, instead of dealing with Drools rules directly. That way, you can store your Domain Model however you like, and you can build UIs around it, etc, and still have the option to generate Drools rules on demand. Then challenge with this is creating a programmatic transformation from your model to Drools rules, but templating tools will help here. I've used Groovy templating for this, and it has worked well.