Add rules engine listener

[cemo]

I would like to fetch some data from database and pass into per rule. I noticed that there is RuleListener but it seems it is triggered per rule. I think that an API for per execution.

[fmbenhassine]

Hi,

Yes the rule listener is executed per rule.

What do you mean an API per execution? Could you please give an example?

Kr
Mahmoud

[cemo]

I am in the process of implementing coupon campaigns. I would like to query database coupons before evaluations starting. I will pass them to rules as a fact. Specifically I need some hooks in com.clovify.mm.core.jeasy.LifeCycleAwareRulesEngine#apply before LOGGER.info("Rules evaluation started");.

BTW I had implemented a custom RulesEngine but It might be something wrong. I wanted to discuss as well. What is the best place to fetch some data for rules?

[fmbenhassine]

Thank you for the example, I see now. Basically you are looking for something like a RulesEngineListener which is executed per rule set, not per rule. I'm thinking of:

public interface RulesEngineListener {
    void beforeFiringRules(Rules rules, Facts facts);
    void afterFiringRules(Rules rules, Facts facts);
}

With this listener, you can add custom behaviour before/after firing the whole rule set.

Is this correct? I want to make sure I correctly understand your request.

What is the best place to fetch some data for rules?

The best place would be in the RulesEngineListener#beforeFiringRules method 😄

[cemo]

Yeah exactly :) By the way I had added almost same class :)

public interface LifeCycleListener {
  void beforeEvaluates(Rules rules, Facts facts);
  void afterEvaluates(Rules rules, Facts facts);
}

But RulesEngineListener reflects better need. :)

[fmbenhassine]

ok cool. But in hindsight, do we really need such a listener? What would be the added value compared to something like:

Data data = .. // fetch data before rule set execution
facts.put("data", data);

rulesEngine.fire(rules, facts);

// check facts after rule set execution

The RuleListener makes sense since we don't have access to facts before/after the execution of each rule (unless there is only one registered rule of course). But in the case of RulesEngineListener, we have access to the facts before/after the whole execution. Why would I need to implement an listener, register it and make the engine call it for me instead of writing the code before/after calling the engine as in the example. Do you see the point?

[cemo]

I will have some modules which will encapsulated all logic. Such an interface will be a contract between modules and core logic.

[fmbenhassine]

That makes perfect sense. The interface actually has some benefits:

• may serve as contract between different modules (as you said)
• may encapsulate some logic that can be reused
• is more elegant than putting logic before/after running the engine (as in my previous comment)
• is consistent with the existing API: having a RuleListener per rule and a RulesEngineListener per rule set makes sense

So let me plan this for next release!
I'll let you know when a snapshot version with the addition is published to give it a try before the release.

Thank you for this suggestion!

Kr,
Mahmoud

[cemo]

@benas

This commit is nice. The code reside in

easy-rules/easy-rules-core/src/main/java/org/jeasy/rules/core/DefaultRulesEngine.java

Lines 101 to 109 in 42251de

	public void fire(Rules rules, Facts facts) {
	if (rules.isEmpty()) {
	LOGGER.warn("No rules registered! Nothing to apply");
	return;
	}
	logEngineParameters();
	log(rules);
	log(facts);
	apply(rules, facts);

can be refactored into a listener. Instead of apply method, you can just use fire method. This will be nice since we can delegate in this case to default implementation.

[fmbenhassine]

yes that was coming 😄 I was planning to add a DefaultRulesEngineListener (like the current DefaultRuleListener) to encapsulate the logging. This will remove the need for the apply method.

I will deploy v3.1 snapshot and keep you posted.
Thanks for the review!

[fmbenhassine]

This issue should be fixed now: methods were renamed to be able to use them in both fire and check methods. A default rules engine listener was added to encapsulate the logging logic.

Version 3.1.0-SNAPSHOT is deployed to maven central, @cemo can you give a try to the listener you requested?

[cemo]

tested.

[fmbenhassine]

Great! thank you for your time!