Hi all,
I've been looking at MergeHub and BroadcastHub for Akka Stream and I am a
bit confused.
In the beginning of the documentation the following is mentioned:
It is important to remember that even after constructing the RunnableGraph
> by connecting all the source, sink and different operators, no data will
> flow through it until it is materialized. Materialization is the process of
> allocating all resources needed to run the computation described by a Graph
> (in Akka Streams this will often involve starting up Actors).
> ...
> After running (materializing) the RunnableGraph[T] we get back the
> materialized value of type T.
>
This makes perfect sense. But I'm having issues uniting this with the code
sample from the MergeHub documentation.
// A simple producer that publishes a new "message" every second
val producer : Source[String, Cancellable]= Source.tick(1.second, 1.second,
"New message")
// Attach a BroadcastHub Sink to the producer. This will materialize to a
// corresponding Source.
// (We need to use toMat and Keep.right since by default the materialized
// value to the left is used)
val runnableGraph: RunnableGraph[Source[String, NotUsed]] =
producer.toMat(BroadcastHub.sink(bufferSize = 256))(Keep.right)
// By running/materializing the producer, we get back a Source, which
// gives us access to the elements published by the producer.
val fromProducer: Source[String, NotUsed] = runnableGraph.run()
// Print out messages from the producer in two independent consumers
fromProducer.runForeach(msg => println("consumer1: " + msg))
fromProducer.runForeach(msg => println("consumer2: " + msg))
In the above snippet a Source is created, and that is used as the Source
for a runnableGraph (meaning it has a source and a sink). Conceptually I
understand that the BroadcastHub is indeed a sink. But what I do not
understand is that when you run/materialize that RunnableGraph, you get
back a Source.
The way I see it, running a graph should return a future of the types of
values flowing through that graph. In this case Strings.
Can somebody shed some light on this, please?
Thanks,
Christophe
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