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A Celery-like Python Task Queue in 55 Lines of Code

Update: you can check this out on GitHub here.

Celery is probably the best known task queuing Python package around. It makes asynchronous execution of Python code both possible and reasonably straightforward. It does, however, come with a good deal of complexity, and it's not as simple to use as I would like (i.e. for many use cases it's overkill). So I wrote a distributed Python task queue. In 55 lines of code (caveat: using two awesome libraries).

Background

What does a distributed task queue do? It takes code like the following (taken from the documentation for RQ, another Celery alternative):

import requests

def count_words_in_page(url):
    resp = requests.get(url)
    return len(resp.text.split())

and allows it to be sent to a worker process (possibly on another machine) for execution. The worker process then sends back the results after the calculation is complete. In the meantime, the sender doesn't have to block waiting for the (possibly expensive) calculation to complete. They can just periodically check if the results are ready.

So what's the absolute simplest way we could do this? I submit to you, brokest.py:

"""Broker-less distributed task queue."""
import pickle

import zmq
import cloud

HOST = '127.0.0.1'
PORT = 9090
TASK_SOCKET = zmq.Context().socket(zmq.REQ)
TASK_SOCKET.connect('tcp://{}:{}'.format(HOST, PORT))

class Worker(object):
    """A remote task executor."""

    def __init__(self, host=HOST, port=PORT):
        """Initialize worker."""
        self.host = host
        self.port = port
        self._context = zmq.Context()
        self._socket = self._context.socket(zmq.REP)

    def start(self):
        """Start listening for tasks."""
        self._socket.bind('tcp://{}:{}'.format(self.host, self.port))
        while True:
            runnable_string = self._socket.recv_pyobj()
            runnable = pickle.loads(runnable_string)
            self._socket.send_pyobj('')
            args = self._socket.recv_pyobj()
            self._socket.send_pyobj('')
            kwargs = self._socket.recv_pyobj()
            response = self._do_work(runnable, args, kwargs)
            self._socket.send_pyobj(response)

    def _do_work(self, task, args, kwargs):
        """Return the result of executing the given task."""
        print('Running [{}] with args [{}] and kwargs [{}]'.format(
            task, args, kwargs))
        return task(*args, **kwargs)

def queue(runnable, *args, **kwargs):
    """Return the result of running the task *runnable* with the given 
    arguments."""
    runnable_string = cloud.serialization.cloudpickle.dumps(runnable)
    TASK_SOCKET.send_pyobj(runnable_string)
    TASK_SOCKET.recv()
    TASK_SOCKET.send_pyobj(args)
    TASK_SOCKET.recv()
    TASK_SOCKET.send_pyobj(kwargs)
    results = TASK_SOCKET.recv_pyobj()
    return results

if __name__ == '__main__':
    w = Worker()
    w.start()

And to use it? Here's the complete contents of app.py:

import requests
from brokest import queue

def count_words_in_page(url):
    resp = requests.get(url)
    return len(resp.text.split())

result = queue(count_words_in_page, 'http://www.jeffknupp.com')
print result

Rather than calling the function directly, you simply call brokest.queue with the function and it arguments as arguments. While the current implementation is blocking, it would be trivially easy to make it non-blocking. Adding multiple workers is just a matter of adding code to make use of a config file with their locations.

Clearly, the stars here are zmq and cloud. ZeroMQ makes creating distributed systems a lot easier, and the documentation is chock full of ZeroMQ design patterns (ours is probably the simplest one, Request-Reply).

cloud is the LGPL'd PiCloud library. PiCloud is a company whose value proposition is that they let you seamlessly run computationally intensive Python code using Amazon EC2 for computing resources. Part of making that a reality, though, required a way to pickle functions and their dependencies (functions are not normally directly pickle-able). In our example, the Worker is able to make use of code using requests library despite not having imported it. It's the secret sauce that makes this all possible.

Is This For Real?

The code works, but my intention was not to create a production quality distributed task queue. Rather, it was to show how new libraries are making it easier than ever to create distributed systems. Having a way to pickle code objects and their dependencies is a huge win, and I'm angry I hadn't heard of PiCloud earlier.

One of the best things about being a programmer is the ability to tinker not just with things, but with ideas. I can take existing ideas and tweak them, or combine existing ideas in new ways. I think brokest is an interesting example of how easy it has become to create distributed systems.

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