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A simple framework for backgrounding tasks with Django, built on top of ZeroMQ.

branch: master
README.markdown

IMPORTANT: READ ME

In version 0.1.4, we are introducing two major changes:

  1. Tasks now have a created datetime field. This was added to make sure --replayfailed replayed tasks in the appropriate order
  2. Introduced South migrations.

IF YOU HAVE ALREADY INSTALLED django-ztask - you can "fake" the first migration, and then run the second migration:

./manage.py migrate django_ztask --fake 0001
./manage.py migrate django_ztask

If you are not using South in your Django project, it is strongly recommended you do. If you are not, you will have to add the "created" field to your database manually.

Installing

Download and install 0MQ version 2.1.3 or better from http://www.zeromq.org

Install pyzmq and django-ztask using PIP:

pip install pyzmq
pip install -e git+git@github.com:dmgctrl/django-ztask.git#egg=django_ztask

Add django_ztask to your INSTALLED_APPS setting in settings.py

INSTALLED_APPS = (
    ...,
    'django_ztask',
)

Then run syncdb

python manage.py syncdb

Running the server

Run django-ztask using the manage.py command:

python manage.py ztaskd

Command-line arguments

The ztaskd command takes a series of command-line arguments:

  • --noreload

    By default, ztaskd will use the built-in Django reloader to reload the server whenever a change is made to a python file. Passing in --noreload will prevent it from listening for changed files. (Good to use in production.)

  • -l or --loglevel

    Choose from the standard CRITICAL, ERROR, WARNING, INFO, DEBUG, or NOTSET. If this argument isn't passed in, INFO is used by default.

  • -f or --logfile

    The file to log messages to. By default, all messages are logged to stdout

  • --replayfailed

    If a command has failed more times than allowed in the ZTASKD_RETRY_COUNT (see below for more), the task is logged as failed. Passing in --replayfailed will cause all failed tasks to be re-run.

Settings

There are several settings that you can put in your settings.py file in your Django project. These are the settings and their defaults

ZTASKD_URL = 'tcp://127.0.0.1:5555'

By default, ztaskd will run over TCP, listening on 127.0.0.1 port 5555.

ZTASKD_ALWAYS_EAGER = False

If set to True, all .async and .after tasks will be run in-process and not sent to the ztaskd process. Good for task debugging.

ZTASKD_DISABLED = False

If set, all tasks will be logged, but not executed. This setting is often used during testing runs. If you set ZTASKD_DISABLED before running python manage.py test, tasks will be logged, but not executed.

ZTASKD_RETRY_COUNT = 5

The number of times a task should be reattempted before it is considered failed.

ZTASKD_RETRY_AFTER = 5

The number, in seconds, to wait in-between task retries.

ZTASKD_ON_LOAD = ()

This is a list of callables - either classes or functions - that are called when the server first starts. This is implemented to support several possible Django setup scenarios when launching ztask - for an example, see the section below called Implementing with Johnny Cache.

Running in production

A recommended way to run in production would be to put something similar to the following in to your rc.local file. This example has been tested on Ubuntu 10.04 and Ubuntu 10.10:

#!/bin/bash -e
pushd /var/www/path/to/site
sudo -u www-data python manage.py ztaskd --noreload -f /var/log/ztaskd.log &
popd

Making functions in to tasks

Decorators and function extensions make tasks able to run. Unlike some solutions, tasks can be in any file anywhere. When the file is imported, ztaskd will register the task for running.

Important note: all functions and their arguments must be able to be pickled.

(Read more about pickling here)

It is a recommended best practice that instead of passing a Django model object to a task, you intead pass along the model's ID or primary key, and re-get the object in the task function.

The @task Decorator

from django_ztask.decorators import task

The @task() decorator will turn any normal function in to a django_ztask task if called using one of the function extensions.

Function extensions

Any function can be called in one of three ways:

  • func(*args, *kwargs)

    Calling a function normally will bypass the decorator and call the function directly

  • func.async(*args, **kwargs)

    Calling a function with .async will cause the function task to be called asyncronously on the ztaskd server. For backwards compatability, .delay will do the same thing as .async, but is deprecated.

  • func.after(seconds, *args, **kwargs)

    This will cause the task to be sent to the ztaskd server, which will wait seconds seconds to execute.

Example

from django_ztask.decorators import task

@task()
def print_this(what_to_print):
    print what_to_print

if __name__ == '__main__':

    # Call the function directly
    print_this('Hello world!')

    # Call the function asynchronously
    print_this.async('This will print to the ztaskd log')

    # Call the function asynchronously
    # after a 5 second delay
    print_this.after(5, 'This will print to the ztaskd log')

Implementing with Johnny Cache

Because Johnny Cache monkey-patches all the Django query compilers, any changes to models in django-ztask that aren't properly patched won't reflect on your site until the cache is cleared. Since django-ztask doesn't concern itself with Middleware, you must put Johnny Cache's query cache middleware in as a callable in the ZTASKD_ON_LOAD setting.

ZTASKD_ON_LOAD = (
    'johnny.middleware.QueryCacheMiddleware',
    ...
)

If you wanted to do this and other things, you could write your own function, and pass that in to ZTASKD_ON_LOAD, as in this example:

myutilities.py

def ztaskd_startup_stuff():
    '''
    Stuff to run every time the ztaskd server 
    is started or reloaded
    '''
    from johnny import middleware
    middleware.QueryCacheMiddleware()
    ... # Other setup stuff

settings.py

ZTASKD_ON_LOAD = (
    'myutilities.ztaskd_startup_stuff',
    ...
)

TODOs and BUGS

See: http://github.com/dmgctrl/django-ztask/issues

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