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Splitting a Multi into several Multi#

It is possible to split a Multi into several Multi streams.

Using the split operator#

Suppose that we have a stream of strings that represent signals, and that we want a Multi for each kind of signal:

  • ?foo, ?bar are input signals,
  • !foo, !bar are output signals,
  • foo, bar are other signals.

To do that, we need a function that maps each item of the stream to its target stream. The splitter API needs a Java enumeration to define keys, as in:

enum Signals {

Now we can use the split operator that provides a splitter object, and fetch individual Multi for each split stream using the get method:

Multi<String> multi = Multi.createFrom().items(
        "!a", "?b", "!c", "!d", "123", "?e"

var splitter = multi.split(Signals.class, s -> {
    if (s.startsWith("?")) {
        return Signals.INPUT;
    } else if (s.startsWith("!")) {
        return Signals.OUTPUT;
    } else {
        return Signals.OTHER;

        .onItem().transform(s -> s.substring(1))
        .subscribe().with(signal -> System.out.println("input - " + signal));

        .onItem().transform(s -> s.substring(1))
        .subscribe().with(signal -> System.out.println("output - " + signal));

        .subscribe().with(signal -> System.out.println("other - " + signal));

This prints the following console output:

output - a
input - b
output - c
output - d
other - 123
input - e

Notes on using splits#

  • Items flow when all splits have a subscriber.
  • The flow stops when either of the subscribers cancels, or when any subscriber has a no outstanding demand.
  • The flow resumes when all splits have a subscriber again, and when all subscribers have outstanding demand.
  • Only one subscriber can be active for a given split. Other subscription attempts will receive an error.
  • When a subscriber cancels, then a new subscription attempt on its corresponding split can succeed.
  • Subscribing to an already completed or errored split results in receiving the terminal signal (onComplete() or onFailure(err)).
  • The upstream Multi gets subscribed to when the first split subscription happens, no matter which split it is.
  • The first split subscription passes its context, if any, to the upstream Multi. It is expected that all split subscribers share the same context object, or the behavior of your code will most likely be incorrect.