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A job is a Python object, representing a function that is invoked asynchronously in a worker (background) process. Any Python function can be invoked asynchronously, by simply pushing a reference to the function and its arguments onto a queue. This is called enqueueing.

Enqueueing jobs

To put jobs on queues, first declare a function:

import requests

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

Noticed anything? There's nothing special about this function! Any Python function call can be put on an RQ queue.

To put this potentially expensive word count for a given URL in the background, simply do this:

from rq import Connection, Queue
from redis import Redis
from somewhere import count_words_at_url

# Tell RQ what Redis connection to use
redis_conn = Redis()
q = Queue(connection=redis_conn)  # no args implies the default queue

# Delay calculation of the multiplication
job = q.enqueue(count_words_at_url, 'http://nvie.com')
print job.result   # => None

# Now, wait a while, until the worker is finished
time.sleep(2)
print job.result   # => 889

If you want to put the work on a specific queue, simply specify its name:

q = Queue('low', connection=redis_conn)
q.enqueue(count_words_at_url, 'http://nvie.com')

Notice the Queue('low') in the example above? You can use any queue name, so you can quite flexibly distribute work to your own desire. A common naming pattern is to name your queues after priorities (e.g. high, medium, low).

For cases where you want to pass in options to .enqueue() itself (rather than to the job function), use .enqueue_call(), which is the more explicit counterpart. A typical use case for this is to pass in a timeout argument:

q = Queue('low', connection=redis_conn)
q.enqueue_call(func=count_words_at_url,
               args=('http://nvie.com',),
               timeout=30)

For cases where the web process doesn't have access to the source code running in the worker (i.e. code base X invokes a delayed function from code base Y), you can pass the function as a string reference, too.

q = Queue('low', connection=redis_conn)
q.enqueue('my_package.my_module.my_func', 3, 4)

On the Design

With RQ, you don't have to set up any queues upfront, and you don't have to specify any channels, exchanges, routing rules, or whatnot. You can just put jobs onto any queue you want. As soon as you enqueue a job to a queue that does not exist yet, it is created on the fly.

RQ does not use an advanced broker to do the message routing for you. You may consider this an awesome advantage or a handicap, depending on the problem you're solving.

Lastly, it does not speak a portable protocol, since it depends on pickle to serialize the jobs, so it's a Python-only system.

The delayed result

When jobs get enqueued, the queue.enqueue() method returns a Job instance. This is nothing more than a proxy object that can be used to check the outcome of the actual job.

For this purpose, it has a convenience result accessor property, that will return None when the job is not yet finished, or a non-None value when the job has finished (assuming the job has a return value in the first place, of course).

The @job decorator

If you're familiar with Celery, you might be used to its @task decorator. Starting from RQ >= 0.3, there exists a similar decorator:

from rq.decorators import job

@job('low', connection=my_redis_conn, timeout=5)
def add(x, y):
    return x + y

job = add.delay(3, 4)
time.sleep(1)
print job.result

Bypassing workers

For testing purposes, you can enqueue jobs without delegating the actual execution to a worker (available since version 0.3.1). To do this, pass the async=False argument into the Queue constructor:

>>> q = Queue('low', async=False)
>>> job = q.enqueue(fib, 8)
>>> job.result
21

The above code runs without an active worker and executes fib(8) synchronously within the same process. You may know this behaviour from Celery as ALWAYS_EAGER.

Job dependencies

New in RQ 0.4.0 is the ability to chain the execution of multiple jobs. To execute a job that depends on another job, use the depends_on argument:

q = Queue('low', async=False)
report_job = q.enqueue(generate_report)
q.enqueue(send_report, depends_on=report_job)

The ability to handle job dependencies allows you to split a big job into several smaller ones. A job that is dependent on another is enqueued only when it's dependency finishes successfully.

The worker

To learn about workers, see the workers documentation.

Considerations for jobs

Technically, you can put any Python function call on a queue, but that does not mean it's always wise to do so. Some things to consider before putting a job on a queue:

Limitations

RQ workers will only run on systems that implement fork(). Most notably, this means it is not possible to run the workers on Windows.