akka-http How to use MergeHub to throttle requests from client side - akka-stream

Question

I use Source.queue to queue up HttpRequests and throttle it on the client side to download files from a remote server. I understand that Source.queue is not threadsafe and we need to use MergeHub to make it threadsafe. Following is the piece of code that uses Source.queue and uses cachedHostConnectionPool.
import java.io.File
import akka.actor.Actor
import akka.event.Logging
import akka.http.scaladsl.Http
import akka.http.scaladsl.client.RequestBuilding
import akka.http.scaladsl.model.{HttpResponse, HttpRequest, Uri}
import akka.stream._
import akka.stream.scaladsl._
import akka.util.ByteString
import com.typesafe.config.ConfigFactory
import scala.concurrent.{Promise, Future}
import scala.concurrent.duration._
import scala.util.{Failure, Success}
class HttpClient extends Actor with RequestBuilding {
implicit val system = context.system
val logger = Logging(system, this)
implicit lazy val materializer = ActorMaterializer()
val config = ConfigFactory.load()
val remoteHost = config.getString("pool.connection.host")
val remoteHostPort = config.getInt("pool.connection.port")
val queueSize = config.getInt("pool.queueSize")
val throttleSize = config.getInt("pool.throttle.numberOfRequests")
val throttleDuration = config.getInt("pool.throttle.duration")
import scala.concurrent.ExecutionContext.Implicits.global
val connectionPool = Http().cachedHostConnectionPool[Promise[HttpResponse]](host = remoteHost, port = remoteHostPort)
// Construct a Queue
val requestQueue =
Source.queue[(HttpRequest, Promise[HttpResponse])](queueSize, OverflowStrategy.backpressure)
.throttle(throttleSize, throttleDuration.seconds, 1, ThrottleMode.shaping)
.via(connectionPool)
.toMat(Sink.foreach({
case ((Success(resp), p)) => p.success(resp)
case ((Failure(error), p)) => p.failure(error)
}))(Keep.left)
.run()
// Convert Promise[HttpResponse] to Future[HttpResponse]
def queueRequest(request: HttpRequest): Future[HttpResponse] = {
val responsePromise = Promise[HttpResponse]()
requestQueue.offer(request -> responsePromise).flatMap {
case QueueOfferResult.Enqueued => responsePromise.future
case QueueOfferResult.Dropped => Future.failed(new RuntimeException("Queue overflowed. Try again later."))
case QueueOfferResult.Failure(ex) => Future.failed(ex)
case QueueOfferResult.QueueClosed => Future.failed(new RuntimeException("Queue was closed (pool shut down) while running the request. Try again later."))
}
}
def receive = {
case "download" =>
val uri = Uri(s"http://localhost:8080/file_csv.csv")
downloadFile(uri, new File("/tmp/compass_audience.csv"))
}
def downloadFile(uri: Uri, destinationFilePath: File) = {
def fileSink: Sink[ByteString, Future[IOResult]] =
Flow[ByteString].buffer(512, OverflowStrategy.backpressure)
.toMat(FileIO.toPath(destinationFilePath.toPath)) (Keep.right)
// Submit to queue and execute HttpRequest and write HttpResponse to file
Source.fromFuture(queueRequest(Get(uri)))
.flatMapConcat(_.entity.dataBytes)
.via(Framing.delimiter(ByteString("\n"), maximumFrameLength = 10000, allowTruncation = true))
.map(_.utf8String)
.map(d => s"$d\n")
.map(ByteString(_))
.runWith(fileSink)
}
}
However, when I use MergeHub, it returns Sink[(HttpRequest, Promise[HttpResponse]), NotUsed]. I need to extract the response.entity.dataBytes and write the response to a file using a filesink. I am not able figure out how to use MergeHub to achieve this. Any help will be appreciated.
val hub: Sink[(HttpRequest, Promise[HttpResponse]), NotUsed] =
MergeHub.source[(HttpRequest, Promise[HttpResponse])](perProducerBufferSize = queueSize)
.throttle(throttleSize, throttleDuration.seconds, 1, ThrottleMode.shaping)
.via(connectionPool)
.toMat(Sink.foreach({
case ((Success(resp), p)) => p.success(resp)
case ((Failure(error), p)) => p.failure(error)
}))(Keep.left)
.run()

Answer 1

Source.Queue is actually thread safe now. If you want to use MergeHub:
private lazy val poolFlow: Flow[(HttpRequest, Promise[HttpResponse]), (Try[HttpResponse], Promise[HttpResponse]), Http.HostConnectionPool] =
Http().cachedHostConnectionPool[Promise[HttpResponse]](host).tail.head, port, connectionPoolSettings)
val ServerSink =
poolFlow.toMat(Sink.foreach({
case ((Success(resp), p)) => p.success(resp)
case ((Failure(e), p)) => p.failure(e)
}))(Keep.left)
// Attach a MergeHub Source to the consumer. This will materialize to a
// corresponding Sink.
val runnableGraph: RunnableGraph[Sink[(HttpRequest, Promise[HttpResponse]), NotUsed]] =
MergeHub.source[(HttpRequest, Promise[HttpResponse])](perProducerBufferSize = 16).to(ServerSink)
val toConsumer: Sink[(HttpRequest, Promise[HttpResponse]), NotUsed] = runnableGraph.run()
protected[akkahttp] def executeRequest[T](httpRequest: HttpRequest, unmarshal: HttpResponse => Future[T]): Future[T] = {
val responsePromise = Promise[HttpResponse]()
Source.single((httpRequest -> responsePromise)).runWith(toConsumer)
responsePromise.future.flatMap(handleHttpResponse(_, unmarshal))
)
}
}