Messages can be sent downstream by sending them to the actor passed into the props function. This actor meets the contract of the actor returned by [[https://doc.akka.io/api/akka/2.6/akka/stream/scaladsl/Source$.html#actorRefT:akka.stream.scaladsl.Source[T,akka.actor.ActorRef]] akka.stream.scaladsl.Source.actorRef]].
The props function should return the props for an actor to handle the flow. This actor will be created using the passed in akka.actor.ActorRefFactory. Each message received will be sent to the actor - there is no back pressure, if the actor is unable to process the messages, they will queue up in the actors mailbox. The upstream can be cancelled by the actor terminating itself.
Value parameters
bufferSize
The maximum number of elements to buffer.
overflowStrategy
The strategy for how to handle a buffer overflow.
props
A function that creates the props for actor to handle the flow.