Akka http client blocked after max-open requests
I am taking this simple example from akka http doc: http://doc.akka.io/docs/akka-http/current/scala/http/client-side/request-level.html
And I modify it a bit to ask for one hundred requests. Application blocks after 32 requests (default setting max-open-requests
). Why?
import akka.actor.{Actor, ActorLogging, ActorSystem, Props}
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import akka.stream.{ActorMaterializer, ActorMaterializerSettings}
import akka.util.ByteString
import scala.io.StdIn
object AkkaClientExample extends App {
val system: ActorSystem = ActorSystem("BatchAkka")
try {
val unformattedAddresses = (1 to 100).map(i => s"Rue de la Gracieuse $i, Préverenges, Switzerland")
val googleGeocoder = system.actorOf(GoogleGeocoder.props, "GoogleGeocoder")
unformattedAddresses.foreach(e => googleGeocoder ! GoogleGeocoder.GeoCode(e))
println(">>> Press ENTER to exit <<<")
StdIn.readLine()
} finally {
system.terminate()
}
}
object GoogleGeocoder {
def props: Props = Props[GoogleGeocoder]
final case class GeoCode(unformattedAddress: String)
}
class GoogleGeocoder extends Actor with ActorLogging {
import GoogleGeocoder._
import akka.pattern.pipe
import context.dispatcher
final implicit val materializer: ActorMaterializer = ActorMaterializer(ActorMaterializerSettings(context.system))
val http = Http(context.system)
def receive = {
case GeoCode(unformattedAddress) =>
log.info(s"GeoCode $unformattedAddress")
http
.singleRequest(HttpRequest(uri = url(unformattedAddress)))
.map(r => (unformattedAddress, r))
.pipeTo(self)
case (unformattedAddress: String, resp @ HttpResponse(StatusCodes.OK, headers, entity, _)) =>
log.info(s"Success response comming for $unformattedAddress")
entity.dataBytes.runFold(ByteString(""))(_ ++ _).foreach { body =>
val response = body.utf8String.replaceAll("\\s+", " ").take(50)
log.info(s"Success response for $unformattedAddress: $response")
}
case (unformattedAddress: String, resp @ HttpResponse(code, _, _, _)) =>
log.info(s"Request failed, response code: $code for $unformattedAddress")
resp.discardEntityBytes()
}
def url(unformattedAddress: String): String =
//s"https://maps.googleapis.com/maps/api/geocode/json?address=${URLEncoder.encode(unformattedAddress, "UTF-8")}&key=${URLEncoder.encode(googleApiKey, "UTF-8")}"
s"https://www.epfl.ch/"
}
output:
[INFO] [07/28/2017 20:08:26.977] [BatchAkka-akka.actor.default-dispatcher-4] [akka://BatchAkka/user/GoogleGeocoder] GeoCode Rue de la Gracieuse 1, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.080] [BatchAkka-akka.actor.default-dispatcher-4] [akka://BatchAkka/user/GoogleGeocoder] GeoCode Rue de la Gracieuse 2, Préverenges, Switzerland
...
[INFO] [07/28/2017 20:08:27.098] [BatchAkka-akka.actor.default-dispatcher-13] [akka://BatchAkka/user/GoogleGeocoder] GeoCode Rue de la Gracieuse 99, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.098] [BatchAkka-akka.actor.default-dispatcher-13] [akka://BatchAkka/user/GoogleGeocoder] GeoCode Rue de la Gracieuse 100, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.615] [BatchAkka-akka.actor.default-dispatcher-11] [akka://BatchAkka/user/GoogleGeocoder] Success response comming for Rue de la Gracieuse 1, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.620] [BatchAkka-akka.actor.default-dispatcher-11] [akka://BatchAkka/user/GoogleGeocoder] Success response comming for Rue de la Gracieuse 4, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.668] [BatchAkka-akka.actor.default-dispatcher-17] [akka://BatchAkka/user/GoogleGeocoder] Success response for Rue de la Gracieuse 4, Préverenges, Switzerland: <!doctype html><html lang="fr" class="no-js"><head
[INFO] [07/28/2017 20:08:27.668] [BatchAkka-akka.actor.default-dispatcher-21] [akka://BatchAkka/user/GoogleGeocoder] Success response for Rue de la Gracieuse 1, Préverenges, Switzerland: <!doctype html><html lang="fr" class="no-js"><head
...
[INFO] [07/28/2017 20:08:27.787] [BatchAkka-akka.actor.default-dispatcher-5] [akka://BatchAkka/user/GoogleGeocoder] Success response comming for Rue de la Gracieuse 31, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.795] [BatchAkka-akka.actor.default-dispatcher-15] [akka://BatchAkka/user/GoogleGeocoder] Success response comming for Rue de la Gracieuse 32, Préverenges, Switzerland
[INFO] [07/28/2017 20:08:27.802] [BatchAkka-akka.actor.default-dispatcher-16] [akka://BatchAkka/user/GoogleGeocoder] Success response for Rue de la Gracieuse 31, Préverenges, Switzerland: <!doctype html><html lang="fr" class="no-js"><head
[INFO] [07/28/2017 20:08:27.806] [BatchAkka-akka.actor.default-dispatcher-17] [akka://BatchAkka/user/GoogleGeocoder] Success response for Rue de la Gracieuse 32, Préverenges, Switzerland: <!doctype html><html lang="fr" class="no-js"><head
blocked after the first 32 requests.
Update based on @shutty answer:
I modified the program as follows and it works:
class GoogleGeocoder extends Actor with ActorLogging {
import GoogleGeocoder._
import akka.pattern.pipe
import context.dispatcher
final implicit val materializer: ActorMaterializer = ActorMaterializer(ActorMaterializerSettings(context.system))
val http = Http(context.system)
val queue = new scala.collection.mutable.Queue[String]
var currentRequests = 0
val MaxCurrentRequest = 10
def receive = {
case GeoCode(unformattedAddress) =>
if (currentRequests < MaxCurrentRequest)
query(unformattedAddress)
else
queue += unformattedAddress
case (unformattedAddress: String, resp @ HttpResponse(StatusCodes.OK, headers, entity, _)) =>
log.info(s"Success response comming for $unformattedAddress")
entity.dataBytes.runFold(ByteString(""))(_ ++ _).foreach { body =>
currentRequests = currentRequests - 1
queryNext()
val response = body.utf8String.replaceAll("\\s+", " ").take(50)
log.info(s"Success response for $unformattedAddress: $response")
}
case (unformattedAddress: String, resp @ HttpResponse(code, _, _, _)) =>
log.info(s"Request failed, response code: $code for $unformattedAddress")
resp.discardEntityBytes()
currentRequests = currentRequests - 1
queryNext()
case f: Status.Failure =>
log.info("failure" + textSample(f))
case m =>
log.info("unexpected message: " + textSample(m))
}
def query(unformattedAddress: String) {
log.info(s"GeoCode $unformattedAddress")
http
.singleRequest(HttpRequest(uri = url(unformattedAddress)))
.map(r => (unformattedAddress, r))
.pipeTo(self)
}
def queryNext() {
if (queue.nonEmpty) {
query(queue.dequeue)
}
}
def url(unformattedAddress: String): String =
//s"https://maps.googleapis.com/maps/api/geocode/json?address=${URLEncoder.encode(unformattedAddress, "UTF-8")}&key=${URLEncoder.encode(googleApiKey, "UTF-8")}"
s"https://www.epfl.ch/"
}
So basically adding a queue.
However, is there a better way to achieve this?
I imagine cases where this implementation might fail: for example, if it http.singleRequest
creates a bad future, currentRequests
it won't shrink. I could handle this on case f: Status.Failure
, but still this solution looks very error prone.
Maybe akka already provides some mechanism for handling the queue?
Is there a way to add back pressure to the client (so AkkaClientExample
: unformattedAddresses.foreach(e => googleGeocoder ! GoogleGeocoder.GeoCode(e))
block on reaching MaxCurrentRequest
)?
source to share
If you run your example with akka.logging = DEBUG
, you will see the following output:
InputBuffer (max-open-requests = 32) now filled with 31 request after enqueuing GET / Empty
InputBuffer (max-open-requests = 32) now filled with 32 request after enqueuing GET / Empty
InputBuffer (max-open-requests = 32) exhausted when trying to enqueue GET / Empty
InputBuffer (max-open-requests = 32) exhausted when trying to enqueue GET / Empty
InputBuffer (max-open-requests = 32) exhausted when trying to enqueue GET / Empty
There is a pretty detailed description of how akka-http handles pooling client requests, but in short, if you overload the pool more than thant max-open-requests it will start dropping requests:
http
.singleRequest(HttpRequest(uri = url(unformattedAddress)))
.map(r => (unformattedAddress, r)) // <- HERE
.pipeTo(self)
When you make a map through the future in Scala, it will only execute your callback on success in the future, which is not consistent with your code. If you rewrite your code a little differently, for example:
http
.singleRequest(HttpRequest(uri = url(unformattedAddress)))
.onComplete {
case Success(r) =>
self ! (unformattedAddress, r)
case Failure(ex) =>
log.error(ex, "pool overflow")
}
You will see a bunch of exceptions complaining about a bad future.
Updated:
As for my own opinion, actors and streams are not very suitable when you need backpressure. As an option, you can completely rewrite your code without actors:
def url(addr: String) = "http://httpbin.org/headers"
implicit val system: ActorSystem = ActorSystem("BatchAkka")
implicit val mat: ActorMaterializer = ActorMaterializer()
import system.dispatcher
val http = Http()
val addresses = (1 to 100).map(i => s"Rue de la Gracieuse $i, Préverenges, Switzerland")
Source(addresses)
.mapAsync(4)(addr => http.singleRequest(HttpRequest(uri = url(addr))))
.map(response => println(response.status))
.runWith(Sink.seq)
.map(_ => println("done"))
In this solution, you will only have 4 concurrent requests to the server with back pressure, calls and whistles.
source to share