Custom Akka Streams - scala

Question

I have a set of stream stages (Sources, Flows and Sinks) which I would like to add some MetaData information to.
Therefore rather than Sources producing A -> (A, StreamMetaData). I've managed to do this using custom stream stages whereby on grab(in) the element, I push(out, (elem, StreamMetaData)). In reality it is not 'converting' the existing Source but passing it to a flow to recreate a new source.
Now I'm trying to implement the below MetaStream stage:
Therefore given that the source is producing tuples of (A, StreamMetaData), I want to pass the A to an existing Flow for some computation to take place and then merge the output produced 'B' with the StreamMetaData. These will then be passed to a Sink which accepts (B, StreamMetaData).
How would you suggest I go about it. I've been informed partial graphs are the best bet and would help in completing such a task. UniformFanOut and UniformFanIn using Unzip((A streamMetaData), A, StreamMetaData) and Zip(A,B)
val fanOut = GraphDSL.create() { implicit b =>
val unzip = b.add(Unzip[T, StreamMetaData])
UniformFanOutShape(unzip.in, unzip.out0, unzip.out1)
}
val fanIn = GraphDSL.create() { implicit b =>
val zip = b.add(Zip[T ,StreamMetaData]())
UniformFanInShape(zip)
}
How can I connect the fanIn and fanOut so as to achieve the same behavior as in the picture?
I had something like this in mind;
def metaFlow[T, B, Mat](flow: Flow[T, B, Mat]): Unit = {
val wrappedFlow =
Flow.fromGraph(GraphDSL.create(){ implicit b =>
import GraphDSL.Implicits._
val unzip: FanOutShape2[(T, StreamMetaData), T, StreamMetaData] = b.add(Unzip[T, StreamMetaData])
val existingFlow = b.add(flow)
val zip: FanInShape2[B,StreamMetaData,(B,StreamMetaData)] = b.add(Zip[B, StreamMetaData])
unzip.out0 ~> existingFlow ~> zip.in0
unzip.out1 ~> zip.in1
FlowShape(unzip.in, zip.out)
})
}
Thanks in advance.

Answer 1

This aprox creating a new SourceShape stacking flow graph can work, a little bit different of your flowShape implementation.
def sourceGraph[A, B](f: A => B, source: Source[(A, StreamMetaData), NotUsed]) = Source.fromGraph(GraphDSL.create() { implicit builder: GraphDSL.Builder[NotUsed] =>
import GraphDSL.Implicits._
val unzip = builder.add(Unzip[A, StreamMetaData]())
val zip = builder.add(Zip[B, StreamMetaData]())
val flow0 = builder.add(Flow[A].map { f(_) })
val flow1 = source ~> unzip.in
unzip.out0 ~> flow0 ~> zip.in0
unzip.out1 ~> zip.in1
SourceShape(zip.out)
})
def flowGraph[A, B](f: A => B) = Flow.fromGraph(GraphDSL.create() { implicit builder =>
import GraphDSL.Implicits._
val unzip = builder.add(Unzip[A, StreamMetaData]())
val zip = builder.add(Zip[B, StreamMetaData]())
val flow0 = builder.add(Flow[A].map { f(_) })
unzip.out0 ~> flow0 ~> zip.in0
unzip.out1 ~> zip.in1
FlowShape(unzip.in, zip.out)
})