How can I create reactor Flux from a blocking queue?

Question

I am trying to implement a reactor Flux created from a BlockingQueue but not sure which operator is best for my use case?

I am creating a streaming REST end point, where response is Flux that needs to keep emitting messages from a BlockingQueue as a response to GET REST call.

I have already tried forums and documentation and can only find Flux initiated from iterable collections or reactive data sources, but no examples from any BlockingQueue.

Answer 1

You can try Flux#generate and Queue#peek . Just keep in mind that peek will return null if the queue is empty, and it cannot be used in onNext .

Something like:

Flux.generate(sink -> {
    val element = queue.peek();
    if (element == null) {
        sink.complete();
    } else {
        sink.next(element);
    }
});

There is also Flux#repeatWhen operator, in case you want to re-subscribe to the queue after it was considered empty, eg with:

flux.repeatWhen(it -> it.delayElements(ofSeconds(1)))

Answer 2

An alternative worth considering is to get rid of BlockingQueue and use Sinks instead.

This requires:

1. Creating a sink, eg

   private final Sinks.Many<String> sink = Sinks.many().multicast().onBackpressureBuffer();

2. Exposing a sink as a Flux:

    sink.asFlux()

3. Pushing to a sink:

    sink.tryEmitNext("SOME MESSAGE");