How to bind akka http with akka streams? - scala

Question

I'm trying to use streams instead of pure actors to handle http requests and I came with the following code:
trait ImagesRoute {
val log = LoggerFactory.getLogger(this.getClass)
implicit def actorRefFactory: ActorRefFactory
implicit def materializer: ActorMaterializer
val source =
Source
.actorRef[Image](Int.MaxValue, OverflowStrategy.fail)
.via(Flow[Image].mapAsync(1)(ImageRepository.add))
.toMat(Sink.asPublisher(true))(Keep.both)
val route = {
pathPrefix("images") {
pathEnd {
post {
entity(as[Image]) { image =>
val (ref, publisher) = source.run()
val addFuture = Source.fromPublisher(publisher)
val future = addFuture.runWith(Sink.head[Option[Image]])
ref ! image
onComplete(future.mapTo[Option[Image]]) {
case Success(img) =>
complete(Created, img)
case Failure(e) =>
log.error("Error adding image resource", e)
complete(InternalServerError, e.getMessage)
}
}
}
}
}
}
}
I'm not sure if this is the correct way to do that, or even if this is a good approach or if I should use an actor to interact with the route, using the ask pattern and then inside the actor, stream everything.
Any ideas?

Answer 1

If you're only expecting 1 image from the entity then you don't need to create a Source from an ActorRef and you don't need Sink.asPublisher, you can simply use Source.single:
def imageToComplete(img : Option[Image]) : StandardRoute =
img.map(i => complete(Created, i))
.getOrElse {
log error ("Error adding image resource", e)
complete(InternalServerError, e.getMessage
}
...
entity(as[Image]) { image =>
val future : Future[StandardRoute] =
Source.single(image)
.via(Flow[Image].mapAsync(1)(ImageRepository.add))
.runWith(Sink.head[Option[Image]])
.map(imageToComplete)
onComplete(future)
}
Simplifying your code further, the fact that you are only processing 1 image means that Streams are unnecessary since there is no need for backpressure with just 1 element:
val future : Future[StandardRoute] = ImageRepository.add(image)
.map(imageToComplete)
onComplete(future)
In the comments you indicated
"this is just a simple example, but the stream pipeline should be
bigger doing a lot of things like contacting external resources and
eventually back pressure things"
This would only apply if your entity was a stream of images. If you're only ever processing 1 image per HttpRequest then backpressure never applies, and any stream you create will be a slower version of a Future.
If your entity is in fact a stream of Images, then you could use it as part of stream:
val byteStrToImage : Flow[ByteString, Image, _] = ???
val imageToByteStr : Flow[Image, Source[ByteString], _] = ???
def imageOptToSource(img : Option[Image]) : Source[Image,_] =
Source fromIterator img.toIterator
val route = path("images") {
post {
extractRequestEntity { reqEntity =>
val stream = reqEntity.via(byteStrToImage)
.via(Flow[Image].mapAsync(1)(ImageRepository.add))
.via(Flow.flatMapConcat(imageOptToSource))
.via(Flow.flatMapConcat(imageToByteStr))
complete(HttpResponse(status=Created,entity = stream))
}
}
}