I'm dealing with a 3rd-party library which provides me with asynchronous method calls like this:
def doSomething1(input:String, callback:String => Any)
def doSomething2(input:Double, callback:String => Any)
The library is running stuff on some thread it creates.
I'd like to wrap an actor around it so I can ask it for junk, but I'm not sure how to get access to the promise so that I can fulfill the request.
The naive approach:
class Wrapper extends Actor {
def receive {
case s:String => doSomething1(s, sender ! _)
case d:Double => doSomething2(d, sender ! _)
}
}
val wrapper = system.actorOf(Props[Wrapper], "wrapper")
Then ask it for results:
(wrapper ? "hello").mapTo[String].foreach(println)
(wrapper ? 123.456).mapTo[String].foreach(println)
But the result never comes back, presumably because the callback isn't coming from the actor it asked.
Is there some way to get access to the promise so the callback can success it?
Please note that I haven't tested this, but is this about what you're looking for?:
class Wrapper extends Actor {
def receive = {
case s : String =>
val response = Promise[String]()
val originator = sender
doSomething1(s, response.success _)
response.future.foreach(originator ! _)
case d : Double =>
val response = Promise[Double]()
val originator = sender
doSomething2(d, response.success _)
response.future.foreach(originator ! _)
}
}
Related
I have an actorA that takes data from dataActor, and responds data to requestActors:
class A extends Actor {
var data = Option.empty[Int]
val requestors = mutable.Set[ActorRef]
def receive = {
case data: Int =>
this.data = Some(data)
requestors.foreach {_ ! data}
case Request =>
if (data.isEmpty) {
requestors.add(sender)
//What should I send back to sender here?
} else {
sender ! data.get
}
}
}
In the sender actor, I use the ask pattern:
(actorA ? Request()).map {
//if the data is not ready here, how can I wait for data?
}
In the data provider actor, it sends the data to the actorA:
dataActor ! 100
The problem is the data may not be ready when the requestor is asking for it, so the Future from the ask may fail. I need to send the sender something to promise there is data, but apparently Akka does not seem to have promise AKAIK.
How to implement it here?
A couple of possibilities.
First, you could return the actual Option (data.get is "code smell" anyway, and better be avoided):
case Request() => data
And then just keep trying on the client end:
def result: Future[Int] = (actor ? Request()).map {
case Some(n) => n
case None => after(1 second, system.scheduler)(result)
}
Alternatively, return a Future:
val ready = Promise[Int]()
case data: Int =>
this.data = data
ready.complete(data)
case Request() =>
data.fold(ready.future)(Future.successful)
Now, you just need to flatten it on the caller side:
val result: Future[Int] = (actor ? Request()).flatMap {
case f: Future[Int] => f
}
I'm currently trying to rewrite an existing untyped actor into a typed one. Since the actor is talking to a MySQL database using ScalikeJDBC, and since I'd like to have that done asynchronously, I'm dealing with Futures coming out of a separate (non-actor) repository class.
With untyped Akka, in an actor's receive method, I could do this:
import akka.pattern.pipe
val horseList : Future[Seq[Horse]] = horseRepository.listHorses(...)
horseList pipeTo sender()
And the sender actor would eventually receive a list of horses. I can't figure out how to do this inside a Behaviour, like:
val behaviour : Behavior[ListHorses] = Behaviors.receive {
(ctx,msg) => msg match {
case ListHorses(replyTo) =>
val horseListF : Future[Seq[Horse]] = horseRepository.listHorses(...)
// -> how do I make horseListF's content end up at replyTo? <-
Behaviors.same
}
}
The pipe pattern doesn't work (as it expects an untyped ActorRef), and so far I haven't found anything else in the akka-actor-typed (2.5.12) dependency I'm using to make this work.
How do I do this?
In Akka 2.5.22 (maybe earlier) there is context.pipeToSelf:
def pipeToSelf[Value](future: Future[Value])(mapResult: Try[Value] => T): Unit
You still have to provide a pattern match for Success and Failure, which in my code I've reduced with this sugar:
def mapPipe[A, T](success: A => T, failure: Throwable => T): Try[A] => T = {
case Success(value) => success(value)
case Failure(e) => failure(e)
}
Resulting in a call like this:
case class Horses(horses: Seq[Horse]) extends Command
case class HorseFailure(e: Throwable) extends Command
...
context.pipeToSelf(horseList) {
mapPipe(Horses,HorseFailure)
}
You can simply send a message to replyTo when the future completes successfully:
case ListHorses(replyTo) =>
horseRepository.listHorses(...) foreach { horses => replyTo ! horses }
Behaviors.same
Or if you want to handle errors as well:
case ListHorses(replyTo) =>
horseRepository.listHorses(...) onComplete {
case Success(horses) => replyTo ! horses
case Failure(e) => // error handling
}
Behaviors.same
In order for this to work, you need an ExecutionContext. It usually makes sense to use the same one as the actor, so you will have to make it available to onComplete or foreach first:
implicit val ec = ctx.executionContext
I have a system that spawns a single actor who will spawn many futures. Some of these futures will run into scenarios that need to spawn more futures (but tell the actor about it). How do I send a message from a future to an actor on the completion of the future's operations?
I've looked at the pipeTo documentation but I am having trouble referencing the actors in my system in my future class.
Here is what my Future class looks like:
class crawler(string: String) {
val status: Future[Boolean] = Future[Boolean] {
//Do something with content
println("I am a future working on cert crawling. My cert contents are: " + cert.content)
true
}
status onComplete {
case Success(true) =>
for(chars <- string.toCharArray) {
//send actor a message for each character of the string.
}
case Failure(t) => println("An error has occured: " + t.getMessage)
}
}
Where the actor's receive method does the following:
def receive = {
case c:Char => if(!certCache.containsKey(c)){
println("actor >>>> Need to begin crawl on " + c + ".")
sender() ! new crawler("give sender the future")
case _ => println("That's not the right input!")
}
And, my Actor is spawned like:
object Main extends App {
val system = ActorSystem("MySystem")
val actor = system.actorOf(Props[actorClass], name = "actor")
actor ! 'a'
}
Directly
You could dependency inject the ActorRef into your Future (not recommended, see Abstracted) :
import akka.actor.ActorRef
//dependency injection of the ActorRef with a default value of noSender
class crawler(string : String, actorRef : ActorRef = ActorRef.noSender) {
...
status OnComplete {
//send each Char in string to the actorRef
case Success(true) => string.foreach(actorRef ! _)
...
}
Then in your Actor you can use self to pass the ActorRef into the crawler:
def receive = {
case c : Char => if(!certCache.containsKey(c)) {
sender() ! new crawler("give sender the future", self)
}
}
Abstracted
Further, you could abstract away the use of ActorRef entirely so that crawler doesn't need to know the details of messaging passing. This is the more "functional" approach which has the benefit of being extendable if you ever switch to Futures or even akka.stream.scaladsl.Source for reactive streams (see example):
//no akka imports or dependencies
class crawler(string : String, sendChar : (Char) => Unit) {
...
case Success(true) => string foreach sendChar
}
And in your Actor you can pass an anonymous function to crawler which sends a Char to the Actor via self:
def receive = {
case c : Char => if(!certCache.containsKey(c)) {
sender ! new crawler("give sender the future", self ! _)
}
}
You can even get robust and provide default "do nothing" behavior for your sendChar function:
class crawler(string : String, sendChar : (Char) => Unit = {_=>}) {
...
}
val crawler = crawler("foo") //still get regular Future behavior for status
I'm trying to execute the following code
trait CustomHttpService extends HttpService {
import MyJsonProtocol._
import spray.httpx.SprayJsonSupport._
implicit def executionContext = actorRefFactory.dispatcher
implicit val timeout = Timeout(5 seconds)
val offerActor = actorRefFactory.actorOf(Props[OfferActor], "offer-actor")
val defaultRoute = {
path("offer" / JavaUUID) { uuid =>
get {
respondWithMediaType(`application/json`) {
complete {
(offerActor ? Get(uuid)).mapTo[Offer]
}
}
}
}
}
}
class OfferActor extends Actor {
override def receive = {
case Get(id) =>
val future = OfferService.genericService.getById(id)
future.onComplete {
case Success(s) =>
s match {
case Some(offer) => sender ! offer
case None => println("none")
}
case Failure(f) => f
}
case id: String => println("received string id: " + id)
case _ => println("receive nothing")
}
}
Initially I was trying to return directly the future, but it was giving me an error, complaining about the promise that I was trying to cast to my Offer object.
Then I just ugly solve my future inside my actor to finally get the Offer and then return it to the sender.
Doing this I'm getting the following:
[06/09/2015 15:16:43.056]
[spray-system-akka.actor.default-dispatcher-4]
[akka://spray-system/deadLetters] Message
[com.spray.entity.Offer] from
Actor[akka://spray-system/user/spray-actor/offer-actor#-617290326] to
Actor[akka://spray-system/deadLetters] was not delivered. [2] dead
letters encountered. This logging can be turned off or adjusted with
configuration settings 'akka.log-dead-letters' and
'akka.log-dead-letters-during-shutdown'.
Indeed, I'm sending a msg with an Offer that I got from the database.
Instead if I simply create an Offer like this, works perfectly.
case Get(id) => sender ! Offer(Some(id), "offer", new DateTime())
I'm believing the future.onComplete inside the actor is causing something wrong.
Any thoughts?
sender is really a function, so you could write sender() to show that it is not just accessing an immutable value. When you call sender inside the future.onComplete the value of sender isn't valid anymore.
I've run into this problem before and the way I worked around was by saving the value of sender outside of the future:
class OfferActor extends Actor {
override def receive = {
case Get(id) =>
val future = OfferService.genericService.getById(id)
val replyTo = sender
future.onComplete {
case Success(s) =>
s match {
case Some(offer) => replyTo ! offer
case None => println("none")
}
case Failure(f) => f
}
case id: String => println("received string id: " + id)
case _ => println("receive nothing")
}
}
Well, just solved it trying to block my future.
I just created a blocked version of
OfferService.genericService.getByIdBlocking(id)
Where I blocked it with
Await.result
then it worked!
So basically I had to let akka embrace my call with a future using the ask pattern but do blocking operations inside the actor.
I am calling an Actor using the ask pattern within a Spray application, and returning the result as the HTTP response. I map failures from the actor to a custom error code.
val authActor = context.actorOf(Props[AuthenticationActor])
callService((authActor ? TokenAuthenticationRequest(token)).mapTo[LoggedInUser]) { user =>
complete(StatusCodes.OK, user)
}
def callService[T](f: => Future[T])(cb: T => RequestContext => Unit) = {
onComplete(f) {
case Success(value: T) => cb(value)
case Failure(ex: ServiceException) => complete(ex.statusCode, ex.errorMessage)
case e => complete(StatusCodes.InternalServerError, "Unable to complete the request. Please try again later.")
//In reality this returns a custom error object.
}
}
This works correctly when the authActor sends a failure, but if the authActor throws an exception, nothing happens until the ask timeout completes. For example:
override def receive: Receive = {
case _ => throw new ServiceException(ErrorCodes.AuthenticationFailed, "No valid session was found for that token")
}
I know that the Akka docs say that
To complete the future with an exception you need send a Failure message to the sender. This is not done automatically when an actor throws an exception while processing a message.
But given that I use asks for a lot of the interface between the Spray routing actors and the service actors, I would rather not wrap the receive part of every child actor with a try/catch. Is there a better way to achieve automatic handling of exceptions in child actors, and immediately resolve the future in the event of an exception?
Edit: this is my current solution. However, it's quite messy to do this for every child actor.
override def receive: Receive = {
case default =>
try {
default match {
case _ => throw new ServiceException("")//Actual code would go here
}
}
catch {
case se: ServiceException =>
logger.error("Service error raised:", se)
sender ! Failure(se)
case ex: Exception =>
sender ! Failure(ex)
throw ex
}
}
That way if it's an expected error (i.e. ServiceException), it's handled by creating a failure. If it's unexpected, it returns a failure immediately so the future is resolved, but then throws the exception so it can still be handled by the SupervisorStrategy.
If you want a way to provide automatic sending of a response back to the sender in case of an unexpected exception, then something like this could work for you:
trait FailurePropatingActor extends Actor{
override def preRestart(reason:Throwable, message:Option[Any]){
super.preRestart(reason, message)
sender() ! Status.Failure(reason)
}
}
We override preRestart and propagate the failure back to the sender as a Status.Failure which will cause an upstream Future to be failed. Also, it's important to call super.preRestart here as that's where child stopping happens. Using this in an actor looks something like this:
case class GetElement(list:List[Int], index:Int)
class MySimpleActor extends FailurePropatingActor {
def receive = {
case GetElement(list, i) =>
val result = list(i)
sender() ! result
}
}
If I was to call an instance of this actor like so:
import akka.pattern.ask
import concurrent.duration._
val system = ActorSystem("test")
import system.dispatcher
implicit val timeout = Timeout(2 seconds)
val ref = system.actorOf(Props[MySimpleActor])
val fut = ref ? GetElement(List(1,2,3), 6)
fut onComplete{
case util.Success(result) =>
println(s"success: $result")
case util.Failure(ex) =>
println(s"FAIL: ${ex.getMessage}")
ex.printStackTrace()
}
Then it would properly hit my Failure block. Now, the code in that base trait works well when Futures are not involved in the actor that is extending that trait, like the simple actor here. But if you use Futures then you need to be careful as exceptions that happen in the Future don't cause restarts in the actor and also, in preRestart, the call to sender() will not return the correct ref because the actor has already moved into the next message. An actor like this shows that issue:
class MyBadFutureUsingActor extends FailurePropatingActor{
import context.dispatcher
def receive = {
case GetElement(list, i) =>
val orig = sender()
val fut = Future{
val result = list(i)
orig ! result
}
}
}
If we were to use this actor in the previous test code, we would always get a timeout in the failure situation. To mitigate that, you need to pipe the results of futures back to the sender like so:
class MyGoodFutureUsingActor extends FailurePropatingActor{
import context.dispatcher
import akka.pattern.pipe
def receive = {
case GetElement(list, i) =>
val fut = Future{
list(i)
}
fut pipeTo sender()
}
}
In this particular case, the actor itself is not restarted because it did not encounter an uncaught exception. Now, if your actor needed to do some additional processing after the future, you can pipe back to self and explicitly fail when you get a Status.Failure:
class MyGoodFutureUsingActor extends FailurePropatingActor{
import context.dispatcher
import akka.pattern.pipe
def receive = {
case GetElement(list, i) =>
val fut = Future{
list(i)
}
fut.to(self, sender())
case d:Double =>
sender() ! d * 2
case Status.Failure(ex) =>
throw ex
}
}
If that behavior becomes common, you can make it available to whatever actors need it like so:
trait StatusFailureHandling{ me:Actor =>
def failureHandling:Receive = {
case Status.Failure(ex) =>
throw ex
}
}
class MyGoodFutureUsingActor extends FailurePropatingActor with StatusFailureHandling{
import context.dispatcher
import akka.pattern.pipe
def receive = myReceive orElse failureHandling
def myReceive:Receive = {
case GetElement(list, i) =>
val fut = Future{
list(i)
}
fut.to(self, sender())
case d:Double =>
sender() ! d * 2
}
}