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Hot streams#

In a cold stream, the stream is created when one subscriber subscribes to the stream. So, if no one subscribes, the actual stream is not created, saving resources (that would be wasted because nobody is interested in the items).

In a hot stream, the stream exists before subscribers subscribe. The stream emits items even if no subscribers observe the stream. If there are no subscribers, the items are just dropped. Subscribers only get items emitted after their subscription, meaning that any previous items would not be received.

To create a hot stream, you can use io.smallrye.mutiny.operators.multi.processors.BroadcastProcessor that:

  • drops items if no subscribers are present,
  • forwards items to the set of observing subscribers.
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BroadcastProcessor<String> processor = BroadcastProcessor.create();
Multi<String> multi = processor
        .onItem().transform(String::toUpperCase)
        .onFailure().recoverWithItem("d'oh");

new Thread(() -> {
    for (int i = 0; i < 1000; i++) {
        processor.onNext(Integer.toString(i));
    }
    processor.onComplete();
}).start();

// Subscribers can subscribe at any time.
// They will only receive items emitted after their subscription.
// If the source is already terminated (by a completion or a failure signal)
// the subscriber receives this signal.

Note that the BroadcastProcessor subscribes to the hot source aggressively and without back-pressure. However, the BroadcastProcessor enforces the back-pressure protocol per subscriber. If a subscriber is not ready to handle an item emitted by the hot source, an io.smallrye.mutiny.subscription.BackPressureFailure is forwarded to this subscriber.