How Spray benefit from Flow[HttpRequest, HttpResponse], if any?

From the scaladoc for trait Flow[-In, +Out] (package akka.stream.scaladsl):

A Flow is a set of stream processing steps that has one open input and one open output.

Therefore, Flow[HttpRequest, HttpResponse] is a set of stream processing steps that has one open HttpRequest input and one open HttpResponse output.

It appears that inputs are called Sources and outputs are Sinks. Connecting a Source to a Sink creates a flow graph.

That's pretty much the good ol' Spray.

There’s the companion object Flow

Actors

trait ActorPublisher[T] extends Actor
trait ActorSubscriber extends Actor

What does Props(new ManualSubscriber(probe)).withDispatcher("akka.test.stream-dispatcher”) do?

akka.stream.actor.ActorSubscriber

From the scaladoc:

Extend/mixin this trait in your akka.actor.Actor to make it a stream subscriber with full control of stream back pressure.

Questions:

What's a stream subscriber?
What's a stream backpressure?
How is the full control handled?
From the scaladoc: the RequestStrategy to control stream back pressure.

Reading the scaladoc further...

Attach the actor as a org.reactivestreams.Subscriber to the stream

Questions:

What's it to be a org.reactivestreams.Subscriber?

akka.http.server.Route

FIXME Where is the type used in Akka?

object Route seems to contain "the glue" between flows and the good ol' Spray.

type Route = RequestContext ⇒ Future[RouteResult]

def handlerFlow(route: Route)(implicit setup: RoutingSetup): Flow[HttpRequest, HttpResponse]

The term Flow[HttpRequest] is quite heavily used in akka.http.Http.ServerBinding.

Working example

Note the type of route - it's Flow[HttpRequest, HttpResponse] (!)

import akka.http.Http

val binding = Http().bind(interface, port)
import system.dispatcher
implicit val mat = FlowMaterializer()
val route: Flow[HttpRequest, HttpResponse] =
  get {
    path("hello") {
      complete("Hello back!")
    }
  }

binding startHandlingWith route

jaceklaskowski/Flow[HttpRequest, HttpResponse].md

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