Why are messages received by an actor unordered?

Question

I've been studying the actor model (specifically the implementation in Scala) but I can't understand why there's a requirement that messages arrive in no particular order.

It seems like there are at least some elegant, actor-based solutions to concurrency problems that would work if only the messages arrived in order (eg producer-consumer variants, deferred database writes, concurrency-safe caches).

So why don't actor messages arrive in order? Is it to permit efficient implementation or maybe to prevent some kind of deadlock that would arise when messages are ordered?

Answer 1

My impression is that if two threads send a message to an actor a , there is no particular guarantee about which will be received by the actor first. But if you have code that looks like

a ! "one"
a ! "two"

then a will always get "one" before "two" (though who knows what else might have arrived in between from other threads).

Thus, I don't think it is the case that messages arrive in no particular order at all. Multiple messages from within one thread will (as far as I can tell from the code or from experience) arrive in order.

Answer 2

I'm not privy to the reasons why Scala's Actors (those in the standard library, at any rate -- there are also Akka, Lift and Scalaz implementations of Actors) chose that particular implementation. Probably as a copy of Erlang's own restrictions -- but without the guarantees for communication between two single threads. Or maybe with that guarantee as well -- I wish Phillip Haller was here to comment.

BUT, I do question your statement about concurrency problems. When studying asynchronous distributed algorithms, a basic tenet is that you can't guarantee any ordering of message receipt.

To quote Distributed Computing: Fundamentals, Simulation and Advanced Topics , by Hagit Attiya and Jennifer Welch,

A system is said to be asynchronous if there is no fixed upper bound on how long it takes for a message to be delivered or how much time elapses between consecutive steps of a processor.

The actor model is an asynchronous one. That enables it to work over distributed hardware -- be it different computers communicating through a network, or different processors on a system that does not provide synchronous guarantees.

Furthermore, even the multi-threading model on a multi-core processor is mostly asynchronous, with the primitives that enable synchronism being extremely expensive.

So a simple answer to the question might be:

Messages are not guaranteed to arrive in order because that's an underlying limitation of asynchronous systems, which is the basic model of computation used by actors.

This model is the one we actually have on any system distributed over TCP/IP, and the most efficient over i386/x64 multicore/multiprocessor hardware.

Answer 3

The following simple example shows messages arriving out of order to a very simple actor:

import scala.actors._
import scala.actors.Actor._
import scala.collection.mutable._

val adder = actor {
  loop {
    react {
      case x: Int => println(" Computing " + x); reply(x+2)
      case Exit => println("Exiting"); exit
    }
  }
}

actor {
  for (i <- 1 to 5) {
    println("Sending " + i)
    adder !! (i, { case answer => println("Computed " + i + " -> " + answer) })
  }

  println("Sending Exit")
  adder !! Exit
}

Here is the output from one run of the above code with Scala 2.9.0 final on Windows 64-bit with Sun JDK 1.6.0u25:

Sending 1
Sending 2
Sending 3
Sending 4
Sending 5
Sending Exit
 Computing 1
Computed 1 -> 3
 Computing 4
Computed 4 -> 6
 Computing 3
Computed 3 -> 5
Exiting

Answer 4

What order would you choose? Should it be by when they were sent or when they were recieved? Should we freeze the entire mailbox whilst we sort the messages? Imagine sorting a large and nearly full mailbox, wouldn't that put an arbitrary lock on the queue? I think the messages don't arrive in order because there is no guaranteed way to enforce such an order. We have latency in networks and between processors.

We have no idea where the messages are coming from, only that they have arrived. So how about this, we make the guarantee that we have no ordering and don't even try to think about ordering. Instead of having to come up with some impressive logic to keep things organized while remaining as contention-free as possible we can just focus on keeping things as contention-free as possible.

Someone else probably has an even better answer than I on this.

Edit:

Now that I've had time to sleep on it, I think it's a stipulation that allows for a much more vibrant Actor ecosystem to be created. Hence, why restrict one Actor or one thread or partial ownership of a thread from a thread pool? What if someone wanted to have an Actor which could grab as many threads as possible to process as many messages in its mailbox as it could?

If you made the stipulation up front that messages had to be done in the order they proceeded you'd never be able to allow for this. The minute multiple threads could be assigned by an Actor to process messages within the mailbox you'd be in the situation whereby you had no control over which message was processed first.

Phew, what your dreams say about your mind as you sleep.