route.scala

val end: Sink[Foo, NotUsed] = ???

val route = Flow.fromGraph(GraphDSL.create(end) { implicit builder => dest =>
  import GraphDSL.Implicits._

  // todo: errors, request validation, etc.
  val entityFlow   = builder.add(Flow[HttpRequest].mapAsync(1)(r => Unmarshal(r.entity).to[Foo]))
  val responseFlow = builder.add(Flow[Foo].map(_ => HttpResponse(StatusCodes.Created)))

  val bcast = builder.add(Broadcast[Foo](2))

  entityFlow.out ~> bcast.in
  bcast.out(0) ~> dest.in
  bcast.out(1) ~> responseFlow.in

  FlowShape(entityFlow.in, responseFlow.out)
})
    
Http().bindAndHandle(route, "localhost", 8080)