Composing Flow Graphs

Question

I've been playing around with Akka Streams and get the idea of creating Flows and wiring them together using FlowGraphs.

I know this part of Akka is still under development so some things may not be finished and some other bits may change, but is it possible to create a FlowGraph that isn't "complete" - i.e. isn't attached to a Sink - and pass it around to different parts of my code to be extended by adding Flow's to it and finally completed by adding a Sink?

Basically, I'd like to be able to compose FlowGraphs but don't understand how... Especially if a FlowGraph has split a stream by using a Broadcast.

Thanks

Answer 1

The next week (December) will be documentation writing for us, so I hope this will help you to get into akka streams more easily! Having that said, here's a quick answer:

Basically you need a PartialFlowGraph instead of FlowGraph. In those we allow the usage of UndefinedSink and UndefinedSource which you can then"attach" afterwards. In your case, we also provide a simple helper builder to create graphs which have exactly one "missing" sink – those can be treated exactly as if it was a Source, see below:

  // for akka-streams 1.0-M1

  val source = Source() { implicit b ⇒
    // prepare an undefined sink, which can be relpaced by a proper sink afterwards
    val sink = UndefinedSink[Int]

    // build your processing graph
    Source(1 to 10) ~> sink

    // return the undefined sink which you mean to "fill in" afterwards
    sink
  }

  // use the partial graph (source) multiple times, each time with a different sink
  source.runWith(Sink.ignore)
  source.runWith(Sink.foreach(x ⇒ println(x)))

Hope this helps!