Sending a request to a server asynchronously involving akka - scala

Question

I want to make a request to a server asynchronously involving actors. Say I have 2 actors:
class SessionRetriever extends Actor {
import SessionRetriever._
def receiver = {
Get =>
val s = getSessionIdFromServer() // 1
sender ! Result(s) // 2
}
def getSessionIdFromServer(): String = { ... } // 3
}
object SessionRetriever {
object Get
object Result(s: String)
}
And
class RequestSender extends Actor {
val sActor = context actorOf Props[SessionRetriever]
def receiver = {
// get session id
val sesId = sActor ! SessionRetriever.Get
val res = sendRequestToServer(sesId)
logToFile(res)
context shutdown sActor
}
def sendRequestToServer(sessionId: String): String = { .... }
}
My questions:
val s = getSessionIdFromServer() // 1
sender ! Result(s) // 2
1) getSessionIdFromServer() does a synchronous request to the server. I think it would be much better make a request asynchronous, correct? So it will return Future[String] instead of a plain String.
2) How do I make asynchronous: by using an AsyncHttpClient (if I recall correctly its name) or by wrapping its synchronous body into Future { } ?
3) Should I use blocking { } block ? If yes, then where exactly: inside its body or here val s = blocking { getSessionIdFromServer() } ?
P.S. I'd like not to use async { } and await { } at this point because they are quite high level functions and after all they are build on top on Futures.

Answer 1

you might try this non-blocking way
def receive = {
Get =>
//assume getSessionIdFromServer() run aysnchronize
val f: Future[String] = getSessionIdFromServer()
val client = sender //keep it local to use when future back
f onComplete {
case Success(rep) => client ! Result(rep)
case Failure(ex) => client ! Failed(ex)
}
}

Answer 2

1) If getSessionIdFromServer() is blocking then you should execute it asynchronously from your receive function, otherwise your actor will block each time it receives a new request and will always wait until it receives a new session before processing the next request.
2) Using a Future will "move" the blocking operation to a different thread. So, your actor will not be blocked and will be able to keep processing incoming requests - that's good -, however you are still blocking a thread - not so great. Using the AsyncHttpClient is a good idea. You can explore other non-blocking httpClient, like PlayWebService.
3) I am not quite familiar with blocking so not sure I should advise anything here. From what I understand, it will tell the thread pool that the operation is blocking and that it should spawn a temporary new thread to handle it - this avoids having all your workers being blocked. Again, if you do that your actor will not blocked, but you are still blocking a thread while getting the session from the server.
To summarize: just use an async http client in getSessionIdFromServer if it is possible. Otherwise, use either Future{} or blocking.

Answer 3

To do an asynchronous call with AsyncHttpClient you could deal with the java Future via a scala Promise.
import scala.concurrent.Future
import com.ning.http.client.AsyncHttpClient
import scala.concurrent.Promise
import java.util.concurrent.Executor
object WebClient {
private val client = new AsyncHttpClient
case class BadStatus(status: Int) extends RuntimeException
def get(url: String)(implicit exec: Executor): Future[String] = {
val f = client.prepareGet(url).execute();
val p = Promise[String]()
f.addListener(new Runnable {
def run = {
val response = f.get
if (response.getStatusCode / 100 < 4)
p.success(response.getResponseBodyExcerpt(131072))
else p.failure(BadStatus(response.getStatusCode))
}
}, exec)
p.future
}
def shutdown(): Unit = client.close()
}
object WebClientTest extends App {
import scala.concurrent.ExecutionContext.Implicits.global
WebClient get "http://www.google.com/" map println foreach (_ => WebClient.shutdown())
}
And then deal with the future completion via a callback.
Credit for the code to the awesome reactive programming course at Coursera.