Async call inside actor's receive method

[newuser001]

Hi i have been trying to use the async-http-client in actor's receive method with no success, my method looks like,

class HttpActor extends Actor {

  def receive = {

    case GET => {

          asyncHttpClient.prepareGet("http://www.ning.com/").execute(new AsyncCompletionHandler[Response]{

              override def onCompleted(response:Response) = {

                  sender ! "ok"

                  response

              }

         }

  }

}

case class GET

But i never get a reply for ( httpactor ? GET ) and it always timesout? Does the methods inside actors needs to be synchronous?

[Andrew Headrick]

You need to extract out the sender to make it thread safe.

class HttpActor extends Actor {

  def receive = {

    case GET => {

          val sendTo = context.sender

          asyncHttpClient.prepareGet("http://www.ning.com/").execute(new AsyncCompletionHandler[Response]{

              override def onCompleted(response:Response) = {

                  sendTo ! "ok"

                  response

[Derek Wyatt]

There are a couple of things wrong here.

1) Obviously you need to send back the response, not "ok", but I'll assume you have been playing with it and left it in a weird state.

2) You're closing over 'sender' directly.  'sender' is a method, not a value.  Freeze the value first.

i.e. for number 2:

// freeze

val requester = sender

asyncHttpClient.prepareGet(…).execute(… {

  override def onCompleted(response: Response) = {

    // use

    requester ! response

  }

}

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[√iktor Ҡlang]

All the replies are correct.

But I'd recommend you to instead of doing that, create a bridge between async-http listeners and Akka Futures instead, then you can use all the goodness of Akka Futures and compose freely.

sendSomeHttpRequest(...) pipeTo sender

Cheers,

√

--
Viktor Klang

Akka Tech Lead

Typesafe - The software stack for applications that scale

Twitter: @viktorklang

[Havoc Pennington]

Agree with Viktor (having tried it both ways myself). There's an old
(akka 1.2) asynchttpclient-to-Future bridge here:
https://github.com/typesafehub/webwords/blob/heroku-devcenter/common/src/main/scala/com/typesafe/webwords/common/URLFetcher.scala#L41

This code could be improved quite a bit but maybe it's a starting point for you.

Havoc

[newuser001]

Thanks all ! "freezing" the actorref seems to work although it sounds odd to me as i dont understand how does assigning a reference outside the callback makes a difference. I have not yet tried the bridge method as it sounds lot of work :P.

[Roland Kuhn]

Scala closures are aggressively opposed to the “by value” concept: closing over “sender” means that that method will be called when the callback is run, which will be on a different thread at a much later time, when the actual value you want is long gone. The only trick you have at your disposal is to reference a val on the stack (i.e. a local name in a method), because the stack frame where that lives cannot be referenced and hence the value must be copied.

If this sounds outlandish to you, don’t worry: just make it a habit to create a “useless” copy of the sender.

Regards,

Roland

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Roland Kuhn
Typesafe – The software stack for applications that scale.
twitter: @rolandkuhn