Flux.generate() is throwing java.lang.IllegalStateException: The generator didn't call any of the SynchronousSink method

Question

I am trying to do my first steps in reactive programming with Spring Boot (2.3.4.RELEASE). So far I am trying to creating an infinte stream of persons in a service method which is called in a REST controller method but it ends with that exception: "java.lang.IllegalStateException: The generator didn't call any of the SynchronousSink method " I tried to google some solutions for a few hours but I haven't found any propper one which fits to what I am trying to do.

This is my service method:

public Flux<PersonEntity> streamPersons() {
        return personRepository.findMinId()
                .zipWith(personRepository.findMaxId())
                .flatMapMany(minMaxTuple ->
                        Flux.<PersonEntity> generate(stream ->
                                personRepository.findById(new Random().longs(minMaxTuple.getT1(), minMaxTuple.getT2()).findFirst().getAsLong()
                                )
                        ).delayElements(Duration.ofMillis(300))
                );
    } 

UPDATE 1

I tried this code snippet from the latest update in the below answer. Method is looking like that now:

    public Flux<PersonEntity> streamPersons() {
        return personRepository.findMinId()
                .zipWith(personRepository.findMaxId())
                .flatMapMany(minMaxTuple ->
                        Flux.<PersonEntity>generate(sink -> {
                            Mono<PersonEntity> foundStock = stockRepository.findById(new Random().longs(minMaxTuple.getT1(), minMaxTuple.getT2()).findFirst().getAsLong());
                            sink.next(foundStock);
                        }
                        ).delayElements(Duration.ofMillis(300))
                );
    }

Unfortunately I am getting a compiler error and I don't know how to get rid of it. The local variable foundStock is a Mono, but the method sink.next(...) requires a regular PersonEntity. How to convert this without blocking?

Update 2

This solution is the one, which works for me. My intention is to get a random person from the database to stream it in an infinte stream to the requesting instance. Special thanks goes to @Toerktumlare !

public Flux<PersonEntity> streamPersons() {
    Mono<Tuple2<Long, Long>> minMaxIdTuple = personRepository.findMinId()
            .zipWith(personRepository.findMaxId());
    Flux<Long> interval = Flux.interval(Duration.ofSeconds(1));
    
    return interval.flatMapSequential(aLong ->
           minMaxIdTuple.map(minMaxTuple ->
                new Random().longs(minMaxTuple.getT1(), minMaxTuple.getT2()))
                    .flatMapMany(longStream -> personRepository.findById(
                         longStream.findFirst().getAsLong()))
                    .flatMap(Flux::just)
        );
    }

My controller method is looking like that:

    @GetMapping(value = "/person/stream", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<PersonEntity> streamPersons() {
        return personService.streamPersons();
    }

The full stacktrace of the above mentioned exception

java.lang.IllegalStateException: The generator didn't call any of the SynchronousSink method
    at reactor.core.publisher.FluxGenerate$GenerateSubscription.slowPath(FluxGenerate.java:276) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
    |_ checkpoint ⇢ Handler first.reactive.steps.controller.provider.PersonController#streamPersons() [DispatcherHandler]
    |_ checkpoint ⇢ HTTP GET "/personapp/api/v1/persons/stream" [ExceptionHandlingWebHandler]
Stack trace:
        at reactor.core.publisher.FluxGenerate$GenerateSubscription.slowPath(FluxGenerate.java:276) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxGenerate$GenerateSubscription.request(FluxGenerate.java:204) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:143) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:237) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxGenerate.subscribe(FluxGenerate.java:83) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Flux.subscribe(Flux.java:8325) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:188) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1782) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:247) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:329) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFlatMap$FlatMapMain.tryEmitScalar(FluxFlatMap.java:480) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:413) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxBuffer$BufferExactSubscriber.onComplete(FluxBuffer.java:179) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onComplete(Operators.java:2016) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxUsingWhen$UsingWhenSubscriber.deferredComplete(FluxUsingWhen.java:402) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxUsingWhen$CommitInner.onComplete(FluxUsingWhen.java:536) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onComplete(Operators.java:2016) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:252) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onComplete(Operators.java:2016) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$MonoSubscriber.onComplete(Operators.java:1824) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreThen$ThenAcceptInner.onComplete(MonoIgnoreThen.java:314) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$MonoSubscriber.onComplete(Operators.java:1824) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreThen$ThenAcceptInner.onComplete(MonoIgnoreThen.java:314) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.pool.SimpleDequePool$QueuePoolRecyclerInner.onComplete(SimpleDequePool.java:555) ~[reactor-pool-0.1.6.RELEASE.jar:0.1.6.RELEASE]
        at reactor.core.publisher.Operators.complete(Operators.java:135) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoEmpty.subscribe(MonoEmpty.java:45) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Mono.subscribe(Mono.java:4213) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.pool.SimpleDequePool$QueuePoolRecyclerMono.subscribe(SimpleDequePool.java:667) ~[reactor-pool-0.1.6.RELEASE.jar:0.1.6.RELEASE]
        at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:153) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:153) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Mono.subscribe(Mono.java:4213) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:97) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreElements$IgnoreElementsSubscriber.onError(MonoIgnoreElements.java:76) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onError(FluxMapFuseable.java:134) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.onError(FluxFilterFuseable.java:156) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onError(FluxFilterFuseable.java:375) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onError(FluxMapFuseable.java:326) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxHandleFuseable$HandleFuseableSubscriber.onNext(FluxHandleFuseable.java:185) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2344) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxHandleFuseable$HandleFuseableSubscriber.request(FluxHandleFuseable.java:243) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:346) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.request(FluxFilterFuseable.java:403) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.request(FluxFilterFuseable.java:184) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:162) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoIgnoreElements$IgnoreElementsSubscriber.onSubscribe(MonoIgnoreElements.java:64) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:90) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.onSubscribe(FluxFilterFuseable.java:81) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onSubscribe(FluxFilterFuseable.java:298) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:255) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxHandleFuseable$HandleFuseableSubscriber.onSubscribe(FluxHandleFuseable.java:148) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoCurrentContext.subscribe(MonoCurrentContext.java:35) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.Mono.subscribe(Mono.java:4213) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxUsingWhen$UsingWhenSubscriber.onComplete(FluxUsingWhen.java:394) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFlatMap$FlatMapMain.checkTerminated(FluxFlatMap.java:838) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFlatMap$FlatMapMain.drainLoop(FluxFlatMap.java:600) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFlatMap$FlatMapMain.drain(FluxFlatMap.java:580) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onComplete(FluxFlatMap.java:457) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:136) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxContextStart$ContextStartSubscriber.onComplete(FluxContextStart.java:115) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at io.r2dbc.postgresql.util.FluxDiscardOnCancel$FluxDiscardOnCancelSubscriber.onComplete(FluxDiscardOnCancel.java:99) ~[r2dbc-postgresql-0.8.5.RELEASE.jar:0.8.5.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onComplete(FluxMapFuseable.java:144) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onComplete(FluxMapFuseable.java:144) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxWindowPredicate$WindowPredicateMain.checkTerminated(FluxWindowPredicate.java:520) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxWindowPredicate$WindowPredicateMain.drainLoop(FluxWindowPredicate.java:468) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxWindowPredicate$WindowPredicateMain.drain(FluxWindowPredicate.java:412) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxWindowPredicate$WindowPredicateMain.onComplete(FluxWindowPredicate.java:293) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxCreate$BaseSink.complete(FluxCreate.java:438) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxCreate$BufferAsyncSink.drain(FluxCreate.java:784) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxCreate$BufferAsyncSink.complete(FluxCreate.java:732) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxCreate$SerializedSink.drainLoop(FluxCreate.java:239) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxCreate$SerializedSink.drain(FluxCreate.java:205) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxCreate$SerializedSink.complete(FluxCreate.java:196) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at io.r2dbc.postgresql.client.ReactorNettyClient$Conversation.complete(ReactorNettyClient.java:709) ~[r2dbc-postgresql-0.8.5.RELEASE.jar:0.8.5.RELEASE]
        at io.r2dbc.postgresql.client.ReactorNettyClient$BackendMessageSubscriber.emit(ReactorNettyClient.java:974) ~[r2dbc-postgresql-0.8.5.RELEASE.jar:0.8.5.RELEASE]
        at io.r2dbc.postgresql.client.ReactorNettyClient$BackendMessageSubscriber.onNext(ReactorNettyClient.java:850) ~[r2dbc-postgresql-0.8.5.RELEASE.jar:0.8.5.RELEASE]
        at io.r2dbc.postgresql.client.ReactorNettyClient$BackendMessageSubscriber.onNext(ReactorNettyClient.java:757) ~[r2dbc-postgresql-0.8.5.RELEASE.jar:0.8.5.RELEASE]
        at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:112) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:845) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:213) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:213) ~[reactor-core-3.3.10.RELEASE.jar:3.3.10.RELEASE]
        at reactor.netty.channel.FluxReceive.drainReceiver(FluxReceive.java:256) ~[reactor-netty-0.9.12.RELEASE.jar:0.9.12.RELEASE]
        at reactor.netty.channel.FluxReceive.onInboundNext(FluxReceive.java:362) ~[reactor-netty-0.9.12.RELEASE.jar:0.9.12.RELEASE]
        at reactor.netty.channel.ChannelOperations.onInboundNext(ChannelOperations.java:358) ~[reactor-netty-0.9.12.RELEASE.jar:0.9.12.RELEASE]
        at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:96) ~[reactor-netty-0.9.12.RELEASE.jar:0.9.12.RELEASE]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:324) ~[netty-codec-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:296) ~[netty-codec-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:714) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:650) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:576) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493) ~[netty-transport-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989) ~[netty-common-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.52.Final.jar:4.1.52.Final]
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.52.Final.jar:4.1.52.Final]
        at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]

Any help or hint is highly appreciated. Thanks in advance

Answer 1

A Mono or a Flux contain something called a sink . The generator function, is the simplest form of generating a steady stream of items. The default sink is synchronous and used for one-by-one emissions, hence synchronousSink .

So by calling the generator method you expose the inner sink, and you need to feed items through the sink api by calling the sinks functions next , complete or error function.

Example:

Flux<String> flux = Flux.generate(
    () -> 0, 
    (state, sink) -> {
      sink.next("3 x " + state + " = " + 3*state); 
      if (state == 10) sink.complete(); 
      return state + 1; 
    });

Here we are using the function signature:

Flux#generate(Callable<S> stateSupplier, BiFunction<S,SynchronousSink<T>,S> generator)

So we are:

• We supply the initial state value of 0.
• We use the state to choose what to emit (a row in the multiplication table of 3).
• We also use it to choose when to stop.
• We return a new state that we use in the next invocation (unless the sequence terminated in this one).

This starts off as soon as we subscribe to the flux.

The above code will generate the following sequence.

3 x 0 = 0
3 x 1 = 3
3 x 2 = 6
3 x 3 = 9
3 x 4 = 12
3 x 5 = 15
3 x 6 = 18
3 x 7 = 21
3 x 8 = 24
3 x 9 = 27
3 x 10 = 30

In your code you are not calling the sinks next method in the generate function.

You can read more about how sinks and the generator function works in the official Reactor Documentation - Producing These docs should be your first source before googling anything else.

UPDATE:

After this answer was posted this comment was posted:

"why does this example work and not my code"

The example he is referring to is:

Flux<Dish> getDishes() {
    return Flux.<Dish> generate(sink -> sink.next(randomDish())) //
                .delayElements(Duration.ofMillis(250));
}

Here we can clearly see that he is using the internal sink and calling sink#next each time to send a random dish out to the subscribing client.

While in the code provided by the thread starter:

Flux.<PersonEntity> generate(stream ->
       personRepository.findById(new Random()
           .longs(minMaxTuple.getT1(), minMaxTuple.getT2())
           .findFirst()
           .getAsLong()))
       .delayElements(Duration.ofMillis(300))

No sink is being used.

Generator methods is for generating items, which means that the sink needs a concrete object. It cannot be a Mono or a Flux . If you need to return such, you convert between them and return them straight to the client.