How to make some asynch calls in a loop

Question

In a loop i need to make some checks, performed actually in some another verticle. In each iteration of my loop i need to check the response code, returned from those verticle and make some decision accordingly. In some other words i need to stop the execution of my loop and somehow wait till asynch. call returns. But such execution stop violates the vert.x philosophy, which states that main thread execution should be never stopped. How can i do it in the scope of Vert.x? So far i don't know how to do this. Any suggestions/code samples/urls to smth. like a solution would b highly appreciated.

Thanks!

score 3 · Accepted Answer · answered Jul 25 '16 at 18:23

When working with Vert.x you need to think less in terms of loops, and more in terms of callbacks. You should use eventBus to communicate between vertices. Let's say that what you want is something similar to this pseudocode:

for (int i = 0; i < 4; i++) {
   int result = getVerticleResult();
   System.out.println(result);
}

So, just a very basic example

class LooperVerticle extends AbstractVerticle {

    private int i = 4;

    @Override
    public void start() throws Exception {
        doWork();
    }

    private void doWork() {
        vertx.eventBus().send("channel", "", (o) -> {
            if (o.succeeded()) {
                System.out.println(o.result().body());
                i--;
                if (i > 0) {
                    doWork();
                }
            }
        });
    }
}

class WorkerVerticle extends AbstractVerticle {

    @Override
    public void start() throws Exception {

        vertx.eventBus().consumer("channel", (o) -> {
            // Generate some random number
            int num = ThreadLocalRandom.current().nextInt(0, 9);

            // Simulate slowness
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            o.reply(num);
        });
    }
}

To test:

public class EventBusExample {

    public static void main(String[] args) {
        Vertx vertx = Vertx.vertx();
        vertx.deployVerticle(new LooperVerticle());
        vertx.deployVerticle(new WorkerVerticle());
    }
}

score 1 · Answer 2 · answered Jul 25 '16 at 11:50

1

I think you need to use FutureTask and store them in a Collection and use FutureTask.get() to retrieve the result when needed which is a blocking call.

answered Jul 25 '16 at 11:50

TMtech

1,076
10
14

score 1 · Answer 3 · answered Jul 25 '16 at 13:46

It sounds like a use case for reactive steam processing. In general such problem could be solved using 2 parties:

a producer that executes tasks and returns asynchronous results
a handler that subscribes to results and performs another tasks

There is a way to configure producer to perform tasks only when there is a subscriber. And on other side subscriber can decide to unsubscribe from producer on some condition.

I'm not familiar with vertx capabilities for reactive streams. But I would start from RxJava integration http://vertx.io/docs/vertx-rx/java/

How to make some asynch calls in a loop

3 Answers3