Migrating systems which use multiple nested reacts to effect initialization

[oxbow_lakes]

More from me I'm afraid :-(

Another common pattern for me in scala's actors is to do this. The situation is that my actor has three dependencies from which it requires data from in order to go into the main process loop; it makes requests of all 3 and then uses nested reacts to ensure it can only hit the main loop when everything has arrived:

class MyActor(dep1: D1, dep2: D2, dep3: D3) {

  def act() {

     dep1 ! Request1

     dep2 ! Request2

     dep3 ! Request3

     react {

       case Reply1(r1) =>

         react {

           case Reply2(r2) =>

             react {

               case Reply3(r3) =>

                 //main loop here

Now, I cannot do this in Akka and neither can I use "become", because I don't know which one of the replies will get sent first (and I don't want to wait for the first reply to send the second request). Using Akka, I can think of doing the following:

class MyActor(dep1: ActorRef, dep2:  ActorRef , dep3: ActorRef) {

  override def preStart() {

    (dep1 ? Request1) zip (dep2 ? Request2) zip (dep3 ? Request3) pipeTo self

  }

  def receive = { case ((Reply1(r1), Reply2(r2)), Reply3(r3)) => become(looping(r1, r2, r3)) }

  def looping(r1: R1, r2: R2, r3: R3): Receive =  { //main loop }

}

Is this a valid mechanism for replacing the behaviour? Is there a better one? (Note - I am doing migrations, so want to avoid as much as possible major application re-writing. I'm trying to determine some mechanical rules for replacing a scala system with an Akka one)

Chris

[√iktor Ҡlang]

Was answered this week on the ML, see archives.
Thx.
V

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[Samuel Tardieu]

You need to break the sequencing: you do not need to receive Reply1
before Reply2 and before Reply3.

Let's rephrase what you want: you want to have received all three
results before doing anything (your main loop). If you had used Future
instances to hold the replies, then you want in fact to do something
when all of them are fullfilled.

Let's assume you work with Future1, Future2 and Future3 (instead of
waiting for Reply1..3), then you can do something like:

for (r1 <- Future1; r2 <- Future2; r3 <- Future3) {
… // main loop here
}

If your action *must* be performed by your actor, instead of the
composition context used by the futures, then you can do something like:

val r13 = for (r1 <- Future1; r2 <- Future2; r3 <- Future3) yield (r1, r2, r3)
r13 pipeTo self

using the "pipeTo" pattern.

Now, to get futures instead of Reply(…), you may want to look at the
"ask" pattern which gives you a future resolving to the reply you want.

Sam

[oxbow_lakes]

Isn't flatMap on Akka's futures non-concurrent? That is; the second request is actually waiting for the first to complete in this example

Other than that, isn't this precisely what I just laid out?

Chris

[oxbow_lakes]

I asked earlier in the week about piping the results of a future to an actor; is that what you mean? (Was not the same question). I can't find anything else on rested reacts

C

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[Samuel Tardieu]

On 25/05, oxbow_lakes wrote:

| Isn't flatMap on Akka's futures non-concurrent? That is; the second request
| is actually waiting for the first to complete in this example

Yes, when I started this answer, I was planning on directing you to
sequence and missed your use of zip, I read it too fast :)

[Jonas Boner]

Does this help?
http://doc.akka.io/api/akka/2.0/akka/actor/Stash.html

On May 25, 2012 6:34 PM, "oxbow_lakes" <oxbow...@gmail.com> wrote:

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[oxbow_lakes]

It may do; I can't see exactly how I'd use it though. In my case, there's no "done" message; there is simply the fact that all 3 initialization replies are received. I'm afraid the usage example in the documentation doesn't really explain what it going on

Chris

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[oxbow_lakes]

The answer is, "yes" - to some extent; I could do this (using scalaz), or something like it

object state {

  var maybeR1 = none[R1]

  var maybeR2 = none[R2] 

  var maybeR3 = none[R3] 

  def invokeIfReady(fn: (R1, R2, R3) => Unit) = (maybeR1 <***> (maybeR2, maybeR3) foreach fn

}

def tryLoop() {

  val transition = (r1: R1, r2: R2, r3: R3) => { unstashAll(); context become looping(r1, r2, r3) }

  state invokeIfReady transition 

}

def receive {

  case Reply(r1) => state.maybeR1 = some(r1); tryLoop()

  case Reply(r2) => state.maybeR2 = some(r2); tryLoop()

  case Reply(r3) => state.maybeR3 = some(r3); tryLoop()

  case other => stash()

}

def looping(r1: R1, r2: R2, r3: R3) : Receive = { /* main loop here */ }