UnboundedMailbox - One at a Time?

[Kevin Meredith]

Looking at the Mailboxes docs, for the UnboundedMailbox, is it true that an Actor with this mailbox will only read a single message from its mailbox at a time?

In other words, if mutating a field within an Actor (and within that Actor alone), is it necessary to use a thread-safe value, i.e. java.util.concurrent.atomic.AtomicLong versus Scala's native Long?

Further, in the following example using Scala 2.11.8:

import akka.actor._

class SimpleActor extends Actor {

  var x = 0L;

  def receive =  ??? // does not matter for this question

}

using the UnboundedMailbox, would `x` ever run into a race condition as-is?

[Viktor Klang]

Hi Kevin,

have you read this: http://doc.akka.io/docs/akka/2.4/general/jmm.html ?

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Cheers,

√

[Kevin Meredith]

Thanks, Viktor, for that link. 

So, in the following:

1. class MyActor extends Actor {
2. var state = ...
3. def receive = {
4. case _ =>
5. //Wrongs
6.  
7. // Very bad, shared mutable state,
8. // will break your application in weird ways
9. Future { state = NewState }
10. anotherActor ? message onSuccess { r => state = r }
11.  
12. // Very bad, "sender" changes for every message,
13. // shared mutable state bug
14. Future { expensiveCalculation(sender()) }
15.  
16. //Rights
17.  
18. // Completely safe, "self" is OK to close over
19. // and it's an ActorRef, which is thread-safe
20. Future { expensiveCalculation() } onComplete { f => self ! f.value.get }
21.  
22. // Completely safe, we close over a fixed value
23. // and it's an ActorRef, which is thread-safe
24. val currentSender = sender()
25. Future { expensiveCalculation(currentSender) }
26. }
27. }

Why is the following call bad?

> Future { state = NewState }

Thanks,

Kevin

On Monday, November 28, 2016 at 2:49:26 PM UTC-5, √ wrote:

Hi Kevin,

have you read this: http://doc.akka.io/docs/akka/2.4/general/jmm.html ?

On Mon, Nov 28, 2016 at 8:44 PM, Kevin Meredith <kevin.m....@gmail.com> wrote:

Looking at the Mailboxes docs, for the UnboundedMailbox, is it true that an Actor with this mailbox will only read a single message from its mailbox at a time?

In other words, if mutating a field within an Actor (and within that Actor alone), is it necessary to use a thread-safe value, i.e. java.util.concurrent.atomic.AtomicLong versus Scala's native Long?

Further, in the following example using Scala 2.11.8:

import akka.actor._

class SimpleActor extends Actor {

  var x = 0L;

  def receive =  ??? // does not matter for this question

}

using the UnboundedMailbox, would `x` ever run into a race condition as-is?

--
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ: http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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Cheers,

√

[Viktor Klang]

Hi Kevin,

this is explained in the following section: http://doc.akka.io/docs/akka/2.4/general/jmm.html#Futures_and_the_Java_Memory_Model

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Cheers,

√