tractor ======= A minimalist `actor model`_ built on multiprocessing_ and trio_. ``tractor`` is an attempt to take trionic_ concurrency concepts and apply them to distributed-multicore Python. ``tractor`` lets you run and spawn Python *actors*: separate processes which are internally running a ``trio`` scheduler and task tree (also known as an `async sandwich`_). Actors communicate with each other by sending *messages* over channels_, but the details of this in ``tractor`` is by default hidden and *actors* can instead easily invoke remote asynchronous functions using *portals*. ``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 - be simple, be small .. warning:: ``tractor`` is in alpha-alpha and is expected to change rapidly! Expect nothing to be set in stone and your ideas about where it should go to be greatly appreciated! .. _actor model: https://en.wikipedia.org/wiki/Actor_model .. _trio: https://github.com/python-trio/trio .. _multiprocessing: https://docs.python.org/3/library/multiprocessing.html .. _trionic: https://trio.readthedocs.io/en/latest/design.html#high-level-design-principles .. _async sandwich: https://trio.readthedocs.io/en/latest/tutorial.html#async-sandwich .. _always propagate: https://trio.readthedocs.io/en/latest/design.html#exceptions-always-propagate .. _causality: https://vorpus.org/blog/some-thoughts-on-asynchronous-api-design-in-a-post-asyncawait-world/#c-c-c-c-causality-breaker .. _shared nothing architecture: https://en.wikipedia.org/wiki/Shared-nothing_architecture .. _cancellation: https://trio.readthedocs.io/en/latest/reference-core.html#cancellation-and-timeouts .. _channels: https://en.wikipedia.org/wiki/Channel_(programming) Install ------- No PyPi release yet! :: pip install git+git://github.com/tgoodlet/tractor.git What's this? Spawning event loops in subprocesses? -------------------------------------------------- Close, but not quite. 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`_ by njsmith_. ``tractor`` takes much inspiration from pulsar_ and execnet_ but attempts to be much more minimal, focus on sophistication of the lower level distributed architecture, and of course does **not** use ``asyncio``, hence **no** event loops. .. _trio docs: https://trio.readthedocs.io/en/latest/ .. _pulsar: http://quantmind.github.io/pulsar/design.html .. _execnet: https://codespeak.net/execnet/ .. _blog post: https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/ .. _njsmith: https://github.com/njsmith/ A trynamic first scene ---------------------- As a first example let's spawn a couple *actors* and have them run their lines: .. code:: python 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): await trio.sleep(0.4) # wait for other actor to spawn async with tractor.find_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.start_actor( 'donny', main=partial(say_hello, 'gretchen'), rpc_module_paths=[_this_module], outlive_main=True ) gretchen = await n.start_actor( 'gretchen', main=partial(say_hello, 'donny'), rpc_module_paths=[_this_module], ) print(await gretchen.result()) print(await donny.result()) await donny.cancel_actor() print("CUTTTT CUUTT CUT!!?! Donny!! You're supposed to say...") tractor.run(main) Here, we've spawned two actors, *donny* and *gretchen* in separate processes. Each starts up and begins executing 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 a something called a *portal*. Each actor receives a response and relays that back to the parent actor (in this case our "director"). To gain more insight as to how ``tractor`` accomplishes all this please read on! 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 the nursery_ in ``trio``. That is, an ``ActorNursery`` created with ``tractor.open_nursery()`` waits on spawned sub-actors to complete (or error) in the same causal_ way ``trio`` waits on spawned subtasks. This includes errors from any one sub-actor causing all other actors spawned by the nursery to be cancelled_. To spawn an actor open a *nursery block* and use the ``start_actor()`` method: .. code:: python 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__], outlive_main=True, ) print(await portal.run(__name__, 'movie_theatre_question')) # calls 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() Notice the ``portal`` instance returned from ``nursery.start_actor()``, we'll get to that shortly. Spawned actor lifetimes can be configured in one of two ways: - the actor terminates when its *main* task completes (the default if the ``main`` kwarg is provided) - the actor can be told to ``outlive_main=True`` and thus act like an RPC daemon where it runs indefinitely until cancelled Had we wanted the former in our example it would have been much simpler: .. code:: python 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.start_actor('some_linguist', main=cellar_door) # The ``async with`` will unblock here since the 'some_linguist' # actor has completed its main task ``cellar_door``. print(await portal.result()) Note that the main task's *final result(s)* (returned from the provided ``main`` function) is **always** accessed using ``Portal.result()`` much like you'd expect from a future_. 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 (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, there is no reason to use anything other then a multiprocessing_ ``Process`` for execution. This makes ``tractor`` programs able 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). Eventually ``tractor`` plans to support different `supervision strategies`_ like ``erlang``. .. _nursery: https://trio.readthedocs.io/en/latest/reference-core.html#nurseries-and-spawning .. _supervision strategies: http://erlang.org/doc/man/supervisor.html#sup_flags .. _causal: https://vorpus.org/blog/some-thoughts-on-asynchronous-api-design-in-a-post-asyncawait-world/#causality .. _cancelled: https://trio.readthedocs.io/en/latest/reference-core.html#child-tasks-and-cancellation Transparent function calling using *portals* -------------------------------------------- ``tractor`` introdces the concept of a *portal* which is an API borrowed_ from ``trio``. A portal may seems 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. Say you wanted to spawn two actors which each pulling 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: .. code:: python 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__], outlive_main=True, # daemonize these actors ) 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.start_actor( name='aggregator', # executed in the actor's "main task" immediately main=partial(aggregate, seed), ) start = time.time() # the portal call returns exactly what you'd expect # as if the remote "main" 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 in ``stream_data()``, one is aggregating values from those two in ``aggregate()`` and shipping the single stream of unique values up the parent actor (the ``'MainProcess'`` as ``multiprocessing`` calls it) which is running ``main()``. There has also been some discussion about adding support for reactive programming primitives and native support for asyncitertools_ like libs - so keep an eye out for that! .. _future: https://en.wikipedia.org/wiki/Futures_and_promises .. _borrowed: https://trio.readthedocs.io/en/latest/reference-core.html#getting-back-into-the-trio-thread-from-another-thread .. _asynchronous generators: https://www.python.org/dev/peps/pep-0525/ .. _remote function execution: https://codespeak.net/execnet/example/test_info.html#remote-exec-a-function-avoiding-inlined-source-part-i .. _asyncitertools: https://github.com/vodik/asyncitertools Cancellation ------------ ``tractor`` supports ``trio``'s cancellation_ system verbatim: .. code:: python import trio import tractor from itertools import repeat async def stream_forever(): for i in repeat("I can see these little future bubble things"): yield i await trio.sleep(0.01) async def main(): # stream for at most 1 second 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__], outlive_main=True ) async for letter in await portal.run(__name__, 'stream_forever'): print(letter) assert cancel_scope.cancelled_caught assert n.cancelled tractor.run(main) Cancelling a nursery block cancels all actors spawned by it. 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 actor's are never cancelled unless explicitly asked or there's a bug in ``tractor`` itself. .. code:: python 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__], outlive_main=True )) # start one actor that will fail immediately await n.start_actor('extra', main=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. .. _exactly like trio: https://trio.readthedocs.io/en/latest/reference-core.html#cancellation-semantics .. _erlang strategies: http://learnyousomeerlang.com/supervisors Shared task state ----------------- Although ``tractor`` uses a *shared-nothing* architecture between processes you can of course share state within an actor. ``trio`` tasks spawned via multiple RPC calls to an actor can access global data using the per actor ``statespace`` dictionary: .. code:: python 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.start_actor( 'checker', main=check_statespace, statespace=statespace ) 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: .. code:: python 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()``. Using ``Channel`` directly (undocumented) ----------------------------------------- You can use the ``Channel`` api if necessary by simply defining a ``chan`` and ``cid`` *kwarg* in your async function definition. ``tractor`` will treat such async functions like async generators on the calling side (for now anyway) such that you can push stream values a little more granularly if you find *yielding* values to be restrictive. I am purposely not documenting this feature with code because I'm not yet sure yet how it should be used correctly. If you'd like more details please feel free to ask me on the `trio gitter channel`_. Running actors standalone (without spawning) -------------------------------------------- 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: .. code:: python 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: .. code:: python from tractor.log import get_console_log log = get_console_log('trace') What the future holds --------------------- Stuff I'd like to see ``tractor`` do one day: - erlang-like supervisors_ - native support for zeromq_ 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 If you're interested in tackling any of these please do shout about it on the `trio gitter channel`_! .. _supervisors: http://learnyousomeerlang.com/supervisors .. _zeromq: https://en.wikipedia.org/wiki/ZeroMQ .. _gossip protocol: https://en.wikipedia.org/wiki/Gossip_protocol .. _trio gitter channel: https://gitter.im/python-trio/general .. _celery: http://docs.celeryproject.org/en/latest/userguide/debugging.html .. _pdb++: https://github.com/antocuni/pdb