air conditioning

This tutorial shows an example of how to use the InferenceRulesEngine. We would like to implement a simple air conditioning system with the following requirement: Given the temperature as a fact, when it is hot, then the system should cool the air until a certain degree. We consider 25 degrees as the threshold for "hot".

First, let's create a condition to define when it is "hot":

public class HighTemperatureCondition implements Condition {

    @Override
    public boolean evaluate(Facts facts) {
        Integer temperature = facts.get("temperature");
        return temperature > 25;
    }

    static HighTemperatureCondition itIsHot() {
        return new HighTemperatureCondition();
    }

}

Then an action to decrease the temperature:

public class DecreaseTemperatureAction implements Action {

    @Override
    public void execute(Facts facts) throws Exception {
        System.out.println("It is hot! cooling air..");
        Integer temperature = facts.get("temperature");
        facts.put("temperature", temperature - 1);
    }

    static DecreaseTemperatureAction decreaseTemperature() {
        return new DecreaseTemperatureAction();
    }
}

Now, let's create a rule using the previous condition and action:

Rule airConditioningRule = new RuleBuilder()
        .name("air conditioning rule")
        .when(itIsHot())
        .then(decreaseTemperature())
        .build();

Here we use the static methods itIsHot() and decreaseTemperature() defined respectively in HighTemperatureCondition and DecreaseTemperatureAction.

Finally, let's use a InferenceRulesEngine to fire this rule:

public class Launcher {

    public static void main(String[] args) {
        // define facts
        Facts facts = new Facts();
        facts.put("temperature", 30);

        // define rules
        Rule airConditioningRule = new RuleBuilder()
                .name("air conditioning rule")
                .when(itIsHot())
                .then(decreaseTemperature())
                .build();
        Rules rules = new Rules();
        rules.register(airConditioningRule);

        // fire rules on known facts
        RulesEngine rulesEngine = new InferenceRulesEngine();
        rulesEngine.fire(rules, facts);
    }

}

To run this tutorial, you can follow these instructions:

$ git clone https://github.com/j-easy/easy-rules.git
$ cd easy-rules
$ mvn install
$ cd easy-rules-tutorials
$ mvn exec:java -P runAircoTutorial

You should see the following output:

INFO: Selecting candidate rules based on the following facts: [ { temperature : 30 } ]
INFO: Rule 'air conditioning rule' triggered
It is hot! cooling air..
INFO: Rule 'air conditioning rule' performed successfully
INFO: Selecting candidate rules based on the following facts: [ { temperature : 29 } ]
INFO: Rule 'air conditioning rule' triggered
It is hot! cooling air..
INFO: Rule 'air conditioning rule' performed successfully
INFO: Selecting candidate rules based on the following facts: [ { temperature : 28 } ]
INFO: Rule 'air conditioning rule' triggered
It is hot! cooling air..
INFO: Rule 'air conditioning rule' performed successfully
INFO: Selecting candidate rules based on the following facts: [ { temperature : 27 } ]
INFO: Rule 'air conditioning rule' triggered
It is hot! cooling air..
INFO: Rule 'air conditioning rule' performed successfully
INFO: Selecting candidate rules based on the following facts: [ { temperature : 26 } ]
INFO: Rule 'air conditioning rule' triggered
It is hot! cooling air..
INFO: Rule 'air conditioning rule' performed successfully
INFO: Selecting candidate rules based on the following facts: [ { temperature : 25 } ]
INFO: No candidate rules found for facts: [ { temperature : 25 } ]

As you can see, the InferenceRulesEngine will continuously select and fire candidate rules until no more rules are applicable. If we used a DefaultRulesEngine instead, only the first run would have been executed and the temperature would have remained at 29.