Using Project Reactor in a Vert.x application

Question

I'm using a library in a Vert.x application which returns Project Reactor type Mono.

I have a verticle which receives this reactive type and is intended to send the content through the event bus to another verticle:

import io.vertx.core.AbstractVerticle;
import io.vertx.core.eventbus.Message;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

import java.time.Duration;

public class HelperVerticle extends AbstractVerticle
{
    public static final String ADDRESS = "address_1";

    @Override
    public void start() throws Exception
    {
        vertx.eventBus().consumer(ADDRESS, this::consume);
    }

    private void consume(Message<Object> message)
    {
        Mono.delay(Duration.ofMillis(3000)) 
            .thenReturn("Content of Mono.") // this would come from external library
            .publishOn(Schedulers.fromExecutor(vertx.nettyEventLoopGroup())) // is this needed?
            .subscribe(output ->
            {
                System.out.println("My verticle: " + Thread.currentThread().getName());
                message.reply(output + " " + message.body());
            }, error -> message.fail(1, error.getMessage()));
    }
}

Is this right approach? Should I switch to Vert.x event loop thread pool before sending the message to the event bus? Is there anything I should be aware of when using these libraries together?

Answer 1

The code looks good to me, except you shouldn't use the Netty event loop group as executor but rather the verticle context:

public class HelperVerticle extends AbstractVerticle
{
    public static final String ADDRESS = "address_1";

    private Scheduler scheduler;

    @Override
    public void start() throws Exception
    {
        scheduler = Schedulers.fromExecutor(command -> context.runOnContext(v -> command.run()));
        vertx.eventBus().consumer(ADDRESS, this::consume);
    }

    private void consume(Message<Object> message)
    {
        Mono.delay(Duration.ofMillis(3000)) 
            .thenReturn("Content of Mono.") // this would come from external library
            .publishOn(scheduler)
            .subscribe(output ->
            {
                System.out.println("My verticle: " + Thread.currentThread().getName());
                message.reply(output + " " + message.body());
            }, error -> message.fail(1, error.getMessage()));
    }
}

With such a scheduler, you get the insurance that the verticle state will not be modified by a thread other than the event loop it's been assigned.