tractor/README.rst

tractor

An async-native actor model built on trio and multiprocessing.

tractor is an attempt to bring trionic structured concurrency to distributed multi-core Python.

tractor lets you run and spawn actors: processes which each run a trio scheduler and task tree (also known as an async sandwich). Actors communicate by sending messages over channels and avoid sharing any local state. This actor model allows for highly distributed software architecture which works just as well on multiple cores as it does over many hosts. tractor takes much inspiration from pulsar and execnet but attempts to be much more focussed on sophistication of the lower level distributed architecture as well as have first class support for modern async Python.

The first step to grok tractor is to get the basics of trio down. A great place to start is the trio docs and this blog post.

Table of Contents

Philosophy

tractor's tenets non-comprehensively include:

• no spawning of processes willy-nilly; causality is paramount!
• shared nothing architecture
• remote errors always propagate back to the caller
• verbatim support for trio's cancellation system
• no use of proxy objects to wrap RPC calls
• an immersive debugging experience
• anti-fragility through chaos engineering

Warning

tractor is in alpha-alpha and is expected to change rapidly! Expect nothing to be set in stone. Your ideas about where it should go are greatly appreciated!

Install

No PyPi release yet!

pip install git+git://github.com/tgoodlet/tractor.git

A trynamic first scene

Let's direct a couple actors and have them run their lines for the hip new film we're shooting:

import tractor
from functools import partial

_this_module = __name__
the_line = 'Hi my name is {}'


async def hi():
    return the_line.format(tractor.current_actor().name)


async def say_hello(other_actor):
    async with tractor.wait_for_actor(other_actor) as portal:
        return await portal.run(_this_module, 'hi')


async def main():
    """Main tractor entry point, the "master" process (for now
    acts as the "director").
    """
    async with tractor.open_nursery() as n:
        print("Alright... Action!")

        donny = await n.run_in_actor(
            'donny',
            say_hello,
            other_actor='gretchen',
        )
        gretchen = await n.run_in_actor(
            'gretchen',
            say_hello,
            other_actor='donny',
        )
        print(await gretchen.result())
        print(await donny.result())
        print("CUTTTT CUUTT CUT!!! Donny!! You're supposed to say...")


tractor.run(main)

We spawn two actors, donny and gretchen. Each actor starts up and executes their main task defined by an async function, say_hello(). The function instructs each actor to find their partner and say hello by calling their partner's hi() function using something called a portal. Each actor receives a response and relays that back to the parent actor (in this case our "director" executing main()).

Actor spawning and causality

tractor tries to take trio's concept of causal task lifetimes to multi-process land. Accordingly, tractor's actor nursery behaves similar to trio's nursery. That is, tractor.open_nursery() opens an ActorNursery which waits on spawned actors to complete (or error) in the same causal way trio waits on spawned subtasks. This includes errors from any one actor causing all other actors spawned by the same nursery to be cancelled.

To spawn an actor and run a function in it, open a nursery block and use the run_in_actor() method:

import tractor


    def cellar_door():
        return "Dang that's beautiful"


    async def main():
        """The main ``tractor`` routine.
        """
        async with tractor.open_nursery() as n:

            portal = await n.run_in_actor('frank', movie_theatre_question)

        # The ``async with`` will unblock here since the 'frank'
        # actor has completed its main task ``movie_theatre_question()``.

        print(await portal.result())


tractor.run(main)

What's going on?

• an initial actor is started with tractor.run() and told to execute its main task: main()
• inside main() an actor is spawned using an ActorNusery and is told to run a single function: cellar_door()
• a portal instance (we'll get to what it is shortly) returned from nursery.run_in_actor() is used to communicate with the newly spawned sub-actor
• the second actor, frank, in a new process running a new trio task then executes cellar_door() and returns its result over a channel back to the parent actor
• the parent actor retrieves the subactor's (frank) final result using portal.result() much like you'd expect from a future.

This run_in_actor() API should look very familiar to users of asyncio's run_in_executor which uses a concurrent.futures Executor.

Since you might also want to spawn long running worker or daemon actors, each actor's lifetime can be determined based on the spawn method:

• if the actor is spawned using run_in_actor() it terminates when its main task completes (i.e. when the (async) function submitted to it returns). The with tractor.open_nursery() exits only once all actors' main function/task complete (just like the nursery in trio)
• actors can be spawned to live forever using the start_actor() method and act like an RPC daemon that runs indefinitely (the with tractor.open_nursery() won't exit) until cancelled

Had we wanted the latter form in our example it would have looked like:

def movie_theatre_question():
    """A question asked in a dark theatre, in a tangent
    (errr, I mean different) process.
    """
    return 'have you ever seen a portal?'


async def main():
    """The main ``tractor`` routine.
    """
    async with tractor.open_nursery() as n:

        portal = await n.start_actor(
            'frank',
            # enable the actor to run funcs from this current module
            rpc_module_paths=[__name__],
        )

        print(await portal.run(__name__, 'movie_theatre_question'))
        # call the subactor a 2nd time
        print(await portal.run(__name__, 'movie_theatre_question'))

        # the async with will block here indefinitely waiting
        # for our actor "frank" to complete, but since it's an
        # "outlive_main" actor it will never end until cancelled
        await portal.cancel_actor()

The rpc_module_paths kwarg above is a list of module path strings that will be loaded and made accessible for execution in the remote actor through a call to Portal.run(). For now this is a simple mechanism to restrict the functionality of the remote (and possibly daemonized) actor and uses Python's module system to limit the allowed remote function namespace(s).

tractor is opinionated about the underlying threading model used for each actor. Since Python has a GIL and an actor model by definition shares no state between actors, it fits naturally to use a multiprocessing Process. This allows tractor programs to leverage not only multi-core hardware but also distribute over many hardware hosts (each actor can talk to all others with ease over standard network protocols).

Async IPC using portals

tractor introduces the concept of a portal which is an API borrowed from trio. A portal may seem similar to the idea of a RPC future except a portal allows invoking remote async functions and generators and intermittently blocking to receive responses. This allows for fully async-native IPC between actors.

When you invoke another actor's routines using a portal it looks as though it was called locally in the current actor. So when you see a call to await portal.run() what you get back is what you'd expect to if you'd called the function directly in-process. This approach avoids the need to add any special RPC proxy objects to the library by instead just relying on the built-in (async) function calling semantics and protocols of Python.

Depending on the function type Portal.run() tries to correctly interface exactly like a local version of the remote built-in Python function type. Currently async functions, generators, and regular functions are supported. Inspiration for this API comes from the way execnet does remote function execution but without the client code (necessarily) having to worry about the underlying channels system or shipping code over the network.

This portal approach turns out to be paricularly exciting with the introduction of asynchronous generators in Python 3.6! It means that actors can compose nicely in a data processing pipeline.

As an example here's an actor that streams for 1 second from a remote async generator function running in a separate actor:

from itertools import repeat
import trio
import tractor


async def stream_forever():
    for i in repeat("I can see these little future bubble things"):
        # each yielded value is sent over the ``Channel`` to the
        # parent actor
        yield i
        await trio.sleep(0.01)


async def main():
    # stream for at most 1 seconds
    with trio.move_on_after(1) as cancel_scope:
        async with tractor.open_nursery() as n:
            portal = await n.start_actor(
                f'donny',
                rpc_module_paths=[__name__],
            )

            # this async for loop streams values from the above
            # async generator running in a separate process
            async for letter in await portal.run(__name__, 'stream_forever'):
                print(letter)

    # we support trio's cancellation system
    assert cancel_scope.cancelled_caught
    assert n.cancelled


tractor.run(main)

Alright, let's get fancy.

Say you wanted to spawn two actors which each pull data feeds from two different sources (and wanted this work spread across 2 cpus). You also want to aggregate these feeds, do some processing on them and then deliver the final result stream to a client (or in this case parent) actor and print the results to your screen:

import time
import trio
import tractor


# this is the first 2 actors, streamer_1 and streamer_2
async def stream_data(seed):
    for i in range(seed):
        yield i
        await trio.sleep(0)  # trigger scheduler


# this is the third actor; the aggregator
async def aggregate(seed):
    """Ensure that the two streams we receive match but only stream
    a single set of values to the parent.
    """
    async with tractor.open_nursery() as nursery:
        portals = []
        for i in range(1, 3):
            # fork point
            portal = await nursery.start_actor(
                name=f'streamer_{i}',
                rpc_module_paths=[__name__],
            )

            portals.append(portal)

        q = trio.Queue(500)

        async def push_to_q(portal):
            async for value in await portal.run(
                __name__, 'stream_data', seed=seed
            ):
                # leverage trio's built-in backpressure
                await q.put(value)

            await q.put(None)
            print(f"FINISHED ITERATING {portal.channel.uid}")

        # spawn 2 trio tasks to collect streams and push to a local queue
        async with trio.open_nursery() as n:
            for portal in portals:
                n.start_soon(push_to_q, portal)

            unique_vals = set()
            async for value in q:
                if value not in unique_vals:
                    unique_vals.add(value)
                    # yield upwards to the spawning parent actor
                    yield value

                    if value is None:
                        break

                assert value in unique_vals

            print("FINISHED ITERATING in aggregator")

        await nursery.cancel()
        print("WAITING on `ActorNursery` to finish")
    print("AGGREGATOR COMPLETE!")


# this is the main actor and *arbiter*
async def main():
    # a nursery which spawns "actors"
    async with tractor.open_nursery() as nursery:

        seed = int(1e3)
        import time
        pre_start = time.time()

        portal = await nursery.run_in_actor(
            'aggregator',
            aggregate,
            seed=seed,
        )

        start = time.time()
        # the portal call returns exactly what you'd expect
        # as if the remote "aggregate" function was called locally
        result_stream = []
        async for value in await portal.result():
            result_stream.append(value)

        print(f"STREAM TIME = {time.time() - start}")
        print(f"STREAM + SPAWN TIME = {time.time() - pre_start}")
        assert result_stream == list(range(seed)) + [None]
        return result_stream


final_stream = tractor.run(main, arbiter_addr=('127.0.0.1', 1616))

Here there's four actors running in separate processes (using all the cores on you machine). Two are streaming by yielding values from the stream_data() async generator, one is aggregating values from those two in aggregate() (also an async generator) and shipping the single stream of unique values up the parent actor (the 'MainProcess' as multiprocessing calls it) which is running main().

Cancellation

tractor supports trio's cancellation system verbatim. Cancelling a nursery block cancels all actors spawned by it. Eventually tractor plans to support different supervision strategies like erlang.

Remote error propagation

Any task invoked in a remote actor should ship any error(s) back to the calling actor where it is raised and expected to be dealt with. This way remote actors are never cancelled unless explicitly asked or there's a bug in tractor itself.

async def assert_err():
    assert 0


async def main():
    async with tractor.open_nursery() as n:
        real_actors = []
        for i in range(3):
            real_actors.append(await n.start_actor(
                f'actor_{i}',
                rpc_module_paths=[__name__],
            ))

        # start one actor that will fail immediately
        await n.run_in_actor('extra', assert_err)

    # should error here with a ``RemoteActorError`` containing
    # an ``AssertionError`` and all the other actors have been cancelled

try:
    # also raises
    tractor.run(main)
except tractor.RemoteActorError:
    print("Look Maa that actor failed hard, hehhh!")

You'll notice the nursery cancellation conducts a one-cancels-all supervisory strategy exactly like trio. The plan is to add more erlang strategies in the near future by allowing nurseries to accept a Supervisor type.

Actor local variables

Although tractor uses a shared-nothing architecture between processes you can of course share state between tasks running within an actor. trio tasks spawned via multiple RPC calls to an actor can access global state using the per actor statespace dictionary:

statespace = {'doggy': 10}


def check_statespace():
    # Remember this runs in a new process so no changes
    # will propagate back to the parent actor
    assert tractor.current_actor().statespace == statespace


async def main():
    async with tractor.open_nursery() as n:
        await n.run_in_actor(
            'checker',
            check_statespace,
            statespace=statespace
        )

Of course you don't have to use the statespace variable (it's mostly a convenience for passing simple data to newly spawned actors); building out a state sharing system per-actor is totally up to you.

How do actors find each other (a poor man's service discovery)?

Though it will be built out much more in the near future, tractor currently keeps track of actors by (name: str, id: str) using a special actor called the arbiter. Currently the arbiter must exist on a host (or it will be created if one can't be found) and keeps a simple dict of actor names to sockets for discovery by other actors. Obviously this can be made more sophisticated (help me with it!) but for now it does the trick.

To find the arbiter from the current actor use the get_arbiter() function and to find an actor's socket address by name use the find_actor() function:

import tractor


async def main(service_name):

    async with tractor.get_arbiter() as portal:
        print(f"Arbiter is listening on {portal.channel}")

    async with tractor.find_actor(service_name) as sockaddr:
        print(f"my_service is found at {my_service}")


tractor.run(main, service_name)

The name value you should pass to find_actor() is the one you passed as the first argument to either tractor.run() or ActorNursery.start_actor().

Streaming using channels and contexts

Channel is the API which wraps an underlying transport and interchange format to enable inter-actor-communication. In its present state tractor uses TCP and msgpack.

If you aren't fond of having to write an async generator to stream data between actors (or need something more flexible) you can instead use a Context. A context wraps an actor-local spawned task and a Channel so that tasks executing across multiple processes can stream data to one another using a low level, request oriented API.

As an example if you wanted to create a streaming server without writing an async generator that yields values you instead define an async function:

async def streamer(ctx, rate=2):
   """A simple web response streaming server.
   """
   while True:
      val = await web_request('http://data.feed.com')

      # this is the same as ``yield`` in the async gen case
      await ctx.send_yield(val)

      await trio.sleep(1 / rate)

All that's required is declaring a ctx argument name somewhere in your function signature and tractor will treat the async function like an async generator - as a streaming function from the client side. This turns out to be handy particularly if you have multiple tasks streaming responses concurrently:

async def streamer(ctx, url, rate=2):
   """A simple web response streaming server.
   """
   while True:
      val = await web_request(url)

      # this is the same as ``yield`` in the async gen case
      await ctx.send_yield(val)

      await trio.sleep(1 / rate)


async def stream_multiple_sources(ctx, sources):
   async with trio.open_nursery() as n:
      for url in sources:
         n.start_soon(streamer, ctx, url)

The context notion comes from the context in nanomsg.

Running actors standalone

You don't have to spawn any actors using open_nursery() if you just want to run a single actor that connects to an existing cluster. All the comms and arbiter registration stuff still works. This can somtimes turn out being handy when debugging mult-process apps when you need to hop into a debugger. You just need to pass the existing arbiter's socket address you'd like to connect to:

tractor.run(main, arbiter_addr=('192.168.0.10', 1616))

Enabling logging

Considering how complicated distributed software can become it helps to know what exactly it's doing (even at the lowest levels). Luckily tractor has tons of logging throughout the core. tractor isn't opinionated on how you use this information and users are expected to consume log messages in whichever way is appropriate for the system at hand. That being said, when hacking on tractor there is a prettified console formatter which you can enable to see what the heck is going on. Just put the following somewhere in your code:

from tractor.log import get_console_log
log = get_console_log('trace')

What the future holds

Stuff I'd like to see tractor do real soon:

• erlang-like supervisors
• native support for nanomsg as a channel transport
• native gossip protocol support for service discovery and arbiter election
• a distributed log ledger for tracking cluster behaviour
• a slick multi-process aware debugger much like in celery but with better pdb++ support
• an extensive chaos engineering test suite
• support for reactive programming primitives and native support for asyncitertools like libs

Feel like saying hi?

This project is very much coupled to the ongoing development of trio (i.e. tractor gets all its ideas from that brilliant community). If you want to help, have suggestions or just want to say hi, please feel free to ping me on the trio gitter channel!