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A few months ago we started redesigning the Drools lowest level executable model and making it accessible to end user with a Java 8 API. To demonstrate the flexibility of this approach I tried to integrate it with a reactive stream and in particular to use this stream as a data source for Drools.

To show how this works I created a simple temperature server that provides a RxJava Observable emitting every second the temperature for a given town and terminates after 5 seconds. There is also a second factory method that allows to merge more of these Observables in order to have a single Observable that emits the temperature for more than one town at the same time.

public class TempServer {
    public static Observable<TempInfo> getFeed(String town) {
        return Observable.create(subscriber ->
                                         Observable.interval(1, TimeUnit.SECONDS)
                                                   .subscribe(i -> {
                                                       if (i > 5) subscriber.onCompleted();
                                                       try {
                                                           subscriber.onNext(TempInfo.fetch(town));
                                                       } catch (Exception e) {
                                                           subscriber.onError(e);
                                                       }
                                                   }));
    }

    public static Observable<TempInfo> getFeeds(String... towns) {
        return Observable.merge(Arrays.stream(towns)
                                      .map(TempServer::getFeed)
                                      .collect(toList()));
    }
}

where the TempInfo.fetch method just returns a random temperature between -20 and 50 degrees

public TempInfo(String town, int temp) {
    this.town = town;
    this.temp = temp;
}

public static TempInfo fetch(String town) {
    return new TempInfo(town, random.nextInt(70) - 20);
}

Using an improved version of the Java 8 DSL presented in the former article I defined the following 2 rules:

Variable<TempInfo> temp = any( TempInfo.class );
Variable<Person> person = any( Person.class );

Rule r1 = rule("low temp")
        .view(
                subscribe(temp, "tempFeed"),
                expr(temp, t -> t.getTemp() < 0),
                input(person, "persons"),
                expr(person, temp, (p, t) -> p.getTown().equals(t.getTown()))
             )
        .then(on(person, temp)
                      .execute((p, t) -> System.out.println(p.getName() + " is freezing in " + p.getTown() + " - temp is " + t.getTemp())));

Rule r2 = rule("high temp")
        .view(
                subscribe(temp, "tempFeed"),
                expr(temp, t -> t.getTemp() > 30),
                input(person, "persons"),
                expr(person, temp, (p, t) -> p.getTown().equals(t.getTown()))
             )
        .then(on(person, temp)
                      .execute((p, t) -> System.out.println(p.getName() + " is sweating in " + p.getTown() + " - temp is " + t.getTemp())));

Here I’m using 2 different kinds of data sources: a passive one that can be considered a mere store of facts:

DataStore persons = storeOf(new Person("Mark", 37, "London"),
                            new Person("Edson", 35, "Toronto"),
                            new Person("Mario", 40, "Milano"));

that can be bound to a specific Drools KieSession with

bindDataSource(ksession, "persons", persons);

and a reactive one taken from the TempServer implemented above

Observable<TempInfo> tempFeed = TempServer.getFeeds( "Milano", "London", "Toronto" );

that can also be bound to the same KieSession in a similar way

bindRxObservable( ksession, "tempFeed", tempFeed );

Having done this you can fire those 2 rules and obtain an output like the following:

Mark is freezing in London - temp is -9
Edson is sweating in Toronto - temp is 42
Mario is sweating in Milano - temp is 42
Mario is sweating in Milano - temp is 49
Mark is freezing in London - temp is -17
Edson is sweating in Toronto - temp is 40
Edson is sweating in Toronto - temp is 47
Mario is freezing in Milano - temp is -14
Mark is freezing in London - temp is -8
Mark is freezing in London - temp is -17

The complete test case to run this example is available here.