Vert.x Kafka not adhering to RxJava thread assignment? - rx-java2

Question

Given the following code:
kafkaConsumer
.rxSubscription()
.subscribeOn(Schedulers.io())
.map(s -> {
logger.info("Mapping on Thread: " + Thread.currentThread().getName());
return s;
})
.observeOn(Schedulers.computation())
.subscribe(
set -> {
logger.info("Subscribing on Thread: " +Thread.currentThread().getName());
});
where kafkaConsumer is a Vert.x KafkaConsumer, I expect that the
.map(s -> {
logger.info("Mapping on Thread: " + Thread.currentThread().getName());
return s;
})
would happen on the Reactive IO Thread. However, it executes on the Vert.x event-loop Thread. When I run the following test class, the same scenario runs the map method on the IO thread as expected.
public class ThreadTesting {
public static void main(String args[]) {
Vertx vertx = Vertx.vertx();
Observable.fromArray(new String[] {"start"})
.flatMapSingle(s -> method1())
.subscribeOn(Schedulers.io())
.map(
s -> {
System.out.println("mapping 2 on Thread: " + Thread.currentThread().getName());
return s.concat(method2());
})
.observeOn(Schedulers.computation())
.subscribe(
str -> {
System.out.println("Subscribing on Thread: " + Thread.currentThread().getName());
},
onError -> {
onError.printStackTrace();
});
}
public static Single<String> method1() {
System.out.println("Executing method 1 on Thread: " + Thread.currentThread().getName());
AsyncResultSingle<String> vertxSingle = new AsyncResultSingle<>(
h -> {
h.handle(Future.succeededFuture("method 1 string"));
});
return vertxSingle;
}
public static String method2() {
System.out.println("Executing method 2 on Thread: " + Thread.currentThread().getName());
return "method 2 String";
}
}
What causes this discrepancy in Thread execution to happen?

Answer 1

The Vert.x KafkaConsumer emits items asynchronously on an event-loop thread, even if you subscribed to it on the io scheduler.
In your snippet, you try to force items to be emitted on the computation scheduler. It works, but not on the observable you expect: it applies to the observable returned by the map operation.
If you want map to operate on the computation scheduler, you need to apply the observeOn operator before:
kafkaConsumer
.rxSubscription()
.subscribeOn(Schedulers.io())
.observeOn(Schedulers.computation())
.map(s -> {
logger.info("Mapping on Thread: " + Thread.currentThread().getName());
return s;
})
.subscribe(
set -> {
logger.info("Subscribing on Thread: " +Thread.currentThread().getName());
});