Execution threads when facing streams

[ichrist-gr]

In the below service i want to manipulate stored documents from mongodb. Method mongoService.retrieveExecutableTasks() returns Multi and sendHttpRequest() returns Uni so we expect them to be executed on the I/O threads by default. In this case we have to do with streams and handle the backpressure issue. Let's take a look at my solution:

On top of class i have instantiated:
EventLoopGroup vertxEventLoop = Vertx.vertx().nettyEventLoopGroup();

The below method is triggered from Quartz scheduler every 1s
`public void executablesPollingMechanism() {
Date lastExecutionDate = new Date(Instant.now().toEpochMilli());

Uni.createFrom()
    .nullItem()
    .emitOn(vertxEventLoop)
    .onItem()
    .transformToMulti(o -> mongoService.retrieveExecutableTasks(lastExecutionDate))
    .emitOn(vertxEventLoop)
    .onItem()
    .transformToUniAndConcatenate(executable -> sendHttpRequest(executable, lastExecutionDate))
    .onSubscription()
    .invoke(subscription -> subscription.request(schedulerConfigs.requestAmount()))
    .onRequest()
    .invoke(Utils::handleItems)
    .subscribe()
    .with(response -> {});

}`

public static void handleItems(long value) { Subscription subscription = new AtomicReference<Subscription>().get(); if (Objects.nonNull(subscription)) { subscription.request(value); } }

I realize that adding the vertx-event-loop threads programmatically is not the best solution.
My problem though is, that if i do not add the emitOn() in both called methods (retrieveExecutableTasks() and sendHttpRequest()),
the execution is happening on the NIO IO Thread (created by Quarkus).

Eventually, is there a reason why the executions do not happen in vertx-event-loop threads and how can i overcome the emitOn(), so that my executions run on the proper vertx-event-loop threads

[ichrist-gr]

Problem fixed if we use vertx verticle