Pykka

Pykka is a Python implementation of the actor model. The actor model introduces some simple rules to control the sharing of state and cooperation between execution units, which makes it easier to build concurrent applications.

Rules of the actor model

  • An actor is an execution unit that executes concurrently with other actors.

  • An actor does not share state with anybody else, but it can have its own state.

  • An actor can only communicate with other actors by sending and receiving messages. It can only send messages to actors whose address it has.

  • When an actor receives a message it may take actions like:

    • altering its own state, e.g. so that it can react differently to a future message,
    • sending messages to other actors, or
    • starting new actors.

    None of the actions are required, and they may be applied in any order.

  • An actor only processes one message at a time. In other words, a single actor does not give you any concurrency, and it does not need to use locks internally to protect its own state.

The actor implementations

Pykka’s actor API comes with the following implementations:

  • Threads: Each ThreadingActor is executed by a regular thread, i.e. threading.Thread. As handles for future results, it uses ThreadingFuture which is a thin wrapper around a Queue.Queue. It has no dependencies outside Python itself. ThreadingActor plays well together with non-actor threads.

    Note: If you monkey patch the standard library with gevent or eventlet you can still use ThreadingActor and ThreadingFuture. Python’s threads will transparently use the underlying implementation provided by gevent or Eventlet.

  • gevent: Each GeventActor is executed by a gevent greenlet. gevent is a coroutine-based Python networking library built on top of a libevent (in 0.13) or libev (in 1.0) event loop. GeventActor is generally faster than ThreadingActor, but as of gevent 0.13 it doesn’t work in processes with other threads, which limits when it can be used. With gevent 1.0, which is currently available as a release candidate, this is no longer an issue. Pykka works with both gevent 0.13 and 1.0.

  • Eventlet: Each EventletActor is executed by a Eventlet greenlet. Pykka is tested with Eventlet 0.12.1.

Pykka has an extensive test suite, and is tested on CPython 2.6, 2.7, and 3.2+, as well as PyPy. gevent and eventlet are currently not available for CPython 3.x or PyPy.

A basic actor

In its most basic form, a Pykka actor is a class with an on_receive(message) method:

import pykka

class Greeter(pykka.ThreadingActor):
    def on_receive(self, message):
        print('Hi there!')

To start an actor, you call the class’ method start(), which starts the actor and returns an actor reference which can be used to communicate with the running actor:

actor_ref = Greeter.start()

If you need to pass arguments to the actor upon creation, you can pass them to the start() method, and receive them using the regular __init__() method:

import pykka

class Greeter(pykka.ThreadingActor):
    def __init__(self, greeting='Hi there!'):
        super(Greeter, self).__init__()
        self.greeting = greeting

    def on_receive(self, message):
        print(self.greeting)

actor_ref = Greeter.start(greeting='Hi you!')

It can be useful to know that the init method is run in the execution context that starts the actor. There are also hooks for running code in the actor’s own execution context when the actor starts, when it stops, and when an unhandled exception is raised. Check out the full API docs for the details.

To stop an actor, you can either call stop() on the actor_ref:

actor_ref.stop()

Or, if an actor wants to stop itself, it can simply do so:

self.stop()

Once an actor has been stopped, it cannot be restarted.

Sending messages

To send a message to the actor, you can either use the tell() method or the ask() method on the actor_ref object. tell() will fire of a message without waiting for an answer. In other words, it will never block. ask() will by default block until an answer is returned, potentially forever. If you provide a timeout keyword argument to ask(), you can specify for how long it should wait for an answer. If you want an answer, but don’t need it right away because you have other stuff you can do first, you can pass block=False, and ask() will immediately return a “future” object.

The message itself must always be a dict, but you’re mostly free to use whatever dict keys you want to.

Summarized in code:

actor_ref.tell({'msg': 'Hi!'})
# => Returns nothing. Will never block.

answer = actor_ref.ask({'msg': 'Hi?'})
# => May block forever waiting for an answer

answer = actor_ref.ask({'msg': 'Hi?'}, timeout=3)
# => May wait 3s for an answer, then raises exception if no answer.

future = actor_ref.ask({'msg': 'Hi?'}, block=False)
# => Will return a future object immediately.
answer = future.get()
# => May block forever waiting for an answer
answer = future.get(timeout=0.1)
# => May wait 0.1s for an answer, then raises exception if no answer.

For performance reasons, Pykka does not clone the dict you send before delivering it to the receiver. You are yourself responsible for either using immutable data structures or to copy.deepcopy() the data you’re sending off to other actors.

Replying to messages

If a message is sent using actor_ref.ask() you can reply to the sender of the message by simply returning a value from on_receive method:

import pykka

class Greeter(pykka.ThreadingActor):
    def on_receive(self, message):
        return 'Hi there!'

actor_ref = Greeter.start()

answer = actor_ref.ask('Hi?')
print(answer)
# => 'Hi there!'

None is a valid response so if you return None explicitly, or don’t return at all, a response containing None will be returned to the sender.

From the point of view of the actor it doesn’t matter whether the message was sent using actor_ref.tell() or actor_ref.ask() . When the sender doesn’t expect a response the on_receive return value will be ignored.

The situation is similar in regard to exceptions: when actor_ref.ask() is used and you raise an exception from within on_receive method it will propagate to the sender:

import pykka

class Raiser(pykka.ThreadingActor):
    def on_receive(self, message):
        raise Exception('Oops')

actor_ref = Raiser.start()

try:
    actor_ref.ask('How are you?')
except Exception as e:
    print(repr(e))
    # => Exception('Oops')

Actor proxies

With the basic building blocks provided by actors and futures, we got everything we need to build more advanced abstractions. Pykka provides a single abstraction on top of the basic actor model, named “actor proxies”. You can use Pykka without proxies, but we’ve found it to be a very convenient abstraction when builing Mopidy.

Let’s create an actor and start it:

import pykka

class Calculator(pykka.ThreadingActor):
    def __init__(self):
        super(Calculator, self).__init__()
        self.last_result = None

    def add(self, a, b=None):
        if b is not None:
            self.last_result = a + b
        else:
            self.last_result += a
        return self.last_result

    def sub(self, a, b=None):
        if b is not None:
            self.last_result = a - b
        else:
            self.last_result -= a
        return self.last_result

actor_ref = Calculator.start()

You can create a proxy from any reference to a running actor:

proxy = actor_ref.proxy()

The proxy object will use introspection to figure out what public attributes and methods the actor has, and then mirror the full API of the actor. Any attribute or method prefixed with underscore will be ignored, which is the convention for keeping stuff private in Python.

When we access attributes or call methods on the proxy, it will ask the actor to access the given attribute or call the given method, and return the result to us. All results are wrapped in “future” objects, so you must use the get() method to get the actual data:

future = proxy.add(1, 3)
future.get()
# => 4

proxy.last_result.get()
# => 4

Since an actor only processes one message at the time and all messages are kept in order, you don’t need to add the call to get() just to block processing until the actor has completed processing your last message:

proxy.sub(5)
proxy.add(3)
proxy.last_result.get()
# => 2

Since assignment doesn’t return anything, it works just like on regular objects:

proxy.last_result = 17
proxy.last_result.get()
# => 17

Under the hood, the proxy does everything by sending messages to the actor using the regular actor_ref.ask() method we talked about previously. By doing so, it maintains the actor model restrictions. The only “magic” happening here is some basic introspection and automatic building of three different message types; one for method calls, one for attribute reads, and one for attribute writes.

Traversable attributes on proxies

Sometimes you’ll want to access an actor attribute’s methods or attributes through a proxy. For this case, Pykka supports “traversable attributes”. By marking an actor attribute as traversable, Pykka will not return the attribute when accessed, but wrap it in a new proxy which is returned instead.

To mark an attribute as traversable, simply set the pykka_traversable attribute to True:

import pykka

class AnActor(pykka.ThreadingActor):
    playback = Playback()

class Playback(object):
    pykka_traversable = True

    def play(self):
        # ...
        return True

proxy = AnActor.start().proxy()
play_success = proxy.playback.play().get()

You can access methods and attributes nested as deep as you like, as long as all attributes on the path between the actor and the method or attribute on the end is marked as traversable.

Examples

See the examples/ dir in Pykka’s Git repo for some runnable examples.

What Pykka is not

Much of the naming of concepts and methods in Pykka is taken from the Akka project which implements actors on the JVM. Though, Pykka does not aim to be a Python port of Akka, and supports far fewer features.

Notably, Pykka does not support the following features:

  • Supervision: Linking actors, supervisors, or supervisor groups.
  • Remoting: Communicating with actors running on other hosts.
  • Routers: Pykka does not come with a set of predefined message routers, though you may make your own actors for routing messages.

Installation

Install Pykka’s dependencies:

  • Python 2.6, 2.7, or 3.2+
  • Optionally, Python 2.6/2.7 only:
    • gevent, if you want to use gevent based actors from pykka.gevent.
    • eventlet, if you want to use eventlet based actors from pykka.eventlet. Eventlet is known to work with PyPy 2.0 as well but Pykka is not tested with it yet.

To install Pykka you can use pip:

pip install pykka

To upgrade your Pykka installation to the latest released version:

pip install --upgrade pykka

To install the latest development snapshot:

pip install pykka==dev

License

Pykka is licensed under the Apache License, Version 2.0.

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