scala,future , All Scala futures completed


All Scala futures completed

Question:

Tag: scala,future

I have an akka actor in which I have several futures in scala that do just some extra work and save the computation into the shared variable. After the main work is done, I want to wait for these 'extra' futures and reply with an answer. The simple code is like this:

....
val replyTo = sender()
var informationCollector = InformationCollector()
val mainFuture: Future[GetMainInformation] = ...
val aFuture: Future[GetExtraInformationA] =...
val bFuture: Future[GetExtraInformationB] =...
val cFuture: Future[GetExtraInformationC] =...

// now I want all the futures to write their result
// and save it into the informationCollector and then
// after all have executed, reply back to Sender all the collected Data.

With this approach, I have 2 issues. I am not sure that acessing the InformationCollector is safe within the onSuccess calls of the futures and I also would like to make this easily extensible (so other collectors may be easily added).

Until now, I came up with this solution, however I am not sure it is correct

// IMPORTANT: firstly add map for all of the extraInfoFutures to add that information into collector
// this will create aFutureMapped, bFutureMapped, cFutureMapped
roomFuture onComplete {
      case Success(mainInfo: GetMainInformation) => {
           informationCollector = informationCollector.copy(mainInformation=mainInfo)
           Future.sequence(List(aFutureMapped,bFutureMapped,cFutureMapped)) onComplete { _ =>
               replyTo ! informationCollector
           }

      }
}

Any help highly appreciated.


Answer:

I'm not sure if I fully understood your question but I can provide an answer at least to the first issue.

It is potentially unsafe to access mutable state in onComplete. Actually it voids the whole purpose of the actor. If you access the mutable state you should always do it through the mailbox of the actor e.g. self ! ModifyState(newValue).

The problem here is that the onComplete method is not in the same thread with the actor causing a situation where potentially two threads are concurrently modifying same data and thus mutating the state has all the same problems that there is generally in concurrent mutation of state (race condition etc).

For the second issue I would just collect the values from the different types of extra Futures through self ! TypeAExtraValue(someValue) and then send the collector (e.g. TypeAInfoCollector) after receiving all the extra values. When used correctly actors remove this problem since they process only one message at a time.

The code could look something like this:

val extraInfoFuture = getExtraInfo()
val infoCollector = new Collector()

extraInfoFuture onComplete {
  case Success(extraInfo) => self ! ExtraInfo(extraInfo) 
}

...

def receive = {
  case ExtraInfo(info) => {
    infoCollector.collect(info)
    if(infoCollector.collectedAll) sender() ! infoCollector
  }
}

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