Supervision

[drozzy]

Currently, the only way to know that the "child" actor failed, is to employ the 'ask' pattern:

actor = MyActor.start()
try:
      actor.ask({'command':'hey'})
except Exception:
      # actor failed!

However, if we are using tell, the is no way to "detect" a failure:

actor.tell({'command':'throw me an exception!'})
# Everything is great!

Akka handles this by registering an actor for "death watch": http://doc.akka.io/docs/akka/snapshot/general/supervision.html

In pykka we can implement a similar thing manually by providing an actor with a parent argument and overriding on_failure in the MyActor:

class MyActor(ThreadingActor):
    def __init__(self, parent):
        super(MyActor, self).__init__()
        self.parent = parent

    def on_failure(self, exception_type, exception_value, traceback):
        self.parent.tell({'command': 'death', 'exception': exception_value})

Then the parent actor would simply have to handle it, like so:

class Parent(ThreadingActor):
    def __init__(self, parent):
        super(Parent, self).__init__()
        self.child = MyActor.start(parent=self.actor_ref)

    def on_receive(self, message):
        if message.get('command') == 'death:            
            # Child actor died....
            if not self.child.is_alive():
                # restart it...
                self.child = MyActor.start(parent=self.actor_ref)

P.S.: On the readme page, where it says what "pykka is not" --- supervision is one of the mentions. Is there any specific reasons (aside from time/effort) that its not? I.e. are you opposed to the idea?

[jodal]

Thanks for providing a good howto on how to implement a supervision pattern yourself. Supervision is mostly lacking from Pykka because of time/effort/need, and not because I oppose it. I'm interested in including it in a future Pykka 2.

I'll leave this bug open to either add the pattern to docs or to use it as basis for some future supervision support