Question

As it was said several times before here, you can spwan a new thread (putting a long-processing computation into actor{} block) inside an actor, and spawned computation will be safely run on the same thread pool (used by the actor scheduler).

actor{
    var i = 0
    case msg => actor {
        // computation
        i = i + 1  // is `i` still thread safe?
                   // looks like it can be access simultaneosly from 2 two threads now
                   // should I make it @volatile?
    } 
    reply(i) 
}

However, will it be thread-safe, and does it follow, in general, the original design, which states that only one thread in a moment of time can work with one actor?

Answer 1

Spawning an actor from another is completely safe.

However, sharing mutable state between actors isn’t, regardless of how & where they were spawned.

The whole point of actors is that they should communicate with messages, via their mailboxes. Abuse this model and actors offer no more protection from concurrency issues than you get with raw threads.