Creates a Source that is materialized as an akka.actor.typed.ActorRef. Messages sent to this actor will be emitted to the stream if there is demand from downstream, otherwise they will be buffered until request for demand is received.

Depending on the defined akka.stream.OverflowStrategy it might drop elements if there is no space available in the buffer.

The strategy akka.stream.OverflowStrategy.backpressure is not supported, and an IllegalArgument("Backpressure overflowStrategy not supported") will be thrown if it is passed as argument.

The buffer can be disabled by using bufferSize of 0 and then received messages are dropped if there is no demand from downstream. When bufferSize is 0 the overflowStrategy does not matter. An async boundary is added after this Source; as such, it is never safe to assume the downstream will always generate demand.

The stream can be completed successfully by sending the actor reference a message that is matched by completionMatcher in which case already buffered elements will be signaled before signaling completion.

The stream can be completed with failure by sending a message that is matched by failureMatcher. The extracted Throwable will be used to fail the stream. In case the Actor is still draining its internal buffer (after having received a message matched by completionMatcher) before signaling completion and it receives a message matched by failureMatcher, the failure will be signaled downstream immediately (instead of the completion signal).

The actor will be stopped when the stream is completed, failed or canceled from downstream, i.e. you can watch it to get notified when that happens.

See also akka.stream.scaladsl.Source.queue.