Project Reactor: About the side effect operator

About side effect

Side effect is the code that to monitor particular signals which flow in the operator chain. Following is available operator and please check the javadoc on when these functions will be called.

• doOnSubscribe()
• doOnNext()
• doOnError()
• doOnComplete()
• doOnTerminate()
• doAfterTerminate()
• doOnRequest()
• doOnCancel()
• doOnEach()
• doFirst()
• doFinally()

Direction of signal flow

In rare situation, 2 or more of the same operator will be chain together. There is a problem that will operator will be called first. This depends on the direction of the signal flow. On example below

package example;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Flux;

public class SignalFlow {

    private static final Logger LOGGER = LoggerFactory.getLogger(SignalFlow.class);

    public static void main(String[] args) {
        Flux<Integer> f1 = Flux.just(1, 2, 3, 4)
                .doOnNext(v -> LOGGER.info("doOnNext (1): {}", v))  // doOnNext (1)
                .doOnNext(v -> LOGGER.info("doOnNext (2): {}", v))  // doOnNext (2)
                .map(v -> v * 2);
        f1.subscribe();

        Flux<Integer> f2 = Flux.just(1, 2, 3, 4)
                .doOnRequest(v -> LOGGER.info("doOnRequest (1): {}", v))    // doOnRequest (1)
                .doOnRequest(v -> LOGGER.info("doOnRequest (2): {}", v))    // doOnRequest (2)
                .map(v -> v * 2);
        f2.subscribe();
    }

}

And the result is

15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (1): 1
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (2): 1
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (1): 2
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (2): 2
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (1): 3
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (2): 3
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (1): 4
15:44:46.451 [main] INFO  example.SignalFlow -- doOnNext (2): 4
15:44:46.451 [main] INFO  example.SignalFlow -- doOnRequest (2): 9223372036854775807
15:44:46.451 [main] INFO  example.SignalFlow -- doOnRequest (1): 9223372036854775807

On operator doOnNext(), the flow is from Publisher to Subscriber, so the result is "doOnNext (1)" and then "doOnNext (2)". But on operator doOnRequest(), the flow is in reverse direction. So "doOnRequest (2)" will be shown first. Following is the summary

Function	Direction of signal flow
doOnSubscribe()	Publisher to Subscriber
doOnNext()	Publisher to Subscriber
doOnError()	Publisher to Subscriber
doOnComplete()	Publisher to Subscriber
doOnTerminate()	Publisher to Subscriber
doAfterTerminate()	Subscriber to Publisher
doOnRequest()	Subscriber to Publisher
doOnCancel()	Subscriber to Publisher
doOnEach()	Publisher to Subscriber
doFirst()	Subscriber to Publisher
doFinally()	Subscriber to Publisher