Async Method calls

As actors are sequential programs and can only do one thing at a time, it is important not to spend time waiting in a blocking fashion. Acton leverages asynchronous style programming to allow actors to react and run only when necessary. Async is at the core of Acton!

A method is called asynchronously when the return value is not used.

Source:

def nsieve(n: int):
    """Sieve of Erathostenes to find primes up to n
    """
    count = 0
    flags = [True] * n
    for i in range(2, n, 1):
        if flags[i]:
            count += 1
            for j in range(i, n, i):
                flags[j] = False
    return count

actor Simon(idx):
    def say(msg, n):
        # Simon likes to compute primes and will tell you how many there are under a given number
        count = nsieve(n)
        print("Simon%d says: %s.... oh and there are %d primes under %d" % (idx, msg, count, n))

actor main(env):
    s1 = Simon(1)
    s2 = Simon(2)

    s1.say("foo", 1000000)
    s2.say("bar", 5)

    def exit():
        env.exit(0)
    after 0.2: exit()

Compile and run:

actonc async.act

Output:

Simon2 says: bar.... oh and there are 2 primes under 5
Simon1 says: foo.... oh and there are 78498 primes under 1000000

A method call like s1.say("foo", 100000) does not use the return value of and is thus called asynchronously. We ask s1 to compute primes under 1000000 while s2 only gets to compute primes up to 5 which will invariably run faster. Thus, s2 despite being called after s1, will print out its result before s1. The s1 and s2 actors are called asynchronously and are executed concurrently and in parallel.

The call flow can be illustrated like this. We can see how main asynchronously calls s1 and s2 that will be scheduled to run concurrently. The run time system (RTS) will run s1.say() and s2.say() in parallel if there are 2 worker threads available. Per default, there are as many worker threads as available CPU threads.

In addition we see how the call to after 2 schedules the main actor to run again after 2 seconds, specifically it will run the main.exit() method, which in turn exists the whole program.