About

django-ztask is a lightweight asynchronous task queue for Django based on zeromq. It appeared as a quick hack but proved to be a simple, solid and highly hackable alternative to celery.

This is a fork of of the original project which seems unmaintained for almost an year.

New features:

• stop command and init script helper: ztask_server.sh

• parallel processing of tasks

• checking the server status before sending a task (the task is executed in-process if ztaskd is not available)

• daemonize support

Installing

Install the required packages (preferably using your distribution's package manager):

• zeromq

• pyzmq

• South

Install django-ztask.

Add django_ztask to your INSTALLED_APPS setting in settings.py

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

Then apply the South migrations:

./manage.py migrate django-ztask

Running the server

Run django-ztask using the manage.py command:

./manage.py ztaskd

Command-line arguments

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

• -f or --logfile

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

• -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.

• --pidfile

  The file to write the PID to. Required for stopping the daemon.

• --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.)

• --daemonize

  Make the server a daemon. Disabled by default.

• --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.

• --workers

  Number of worker processes. Defaults to the number of available CPUs/cores as reported by the operating system.

• --stop

  Stop the ztaskd instance indicated by --pidfile. The pidfile should be the same passed as an argument when the ztaskd server was started. This command will wait until the server is stopped.

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_ALIVE_URL = 'tcp://127.0.0.1:5556'

The URL on which ztaskd listens for 'alive' requests (used by the decorator to decide whether to send the task or run it in-process).

ZTASKD_ALIVE_TIMEOUT = 1000

Number of milliseconds to wait for an 'alive' reply. If the timeout is reached and no reply was received, the server is considered dead.

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

The recommended way to run ztaskd in production is through the ztask_server.sh shell script:

./ztask_server.sh start
./ztask_server.sh restart
./ztask_server.sh stop

Make sure that the logging and pidfile directories exist and can be written into by the user running the script.

The stop command (and implicitly restart) is synchronous. Usually this is what you want when stopping the server: wait until the server has been stopped. Restarts, however, can be asynchronous (like when restarting ztaskd from a git hook) and in this case we would run the script in background:

./ztask_server.sh restart &

You can run many 'restart' jobs in parallel. There's file locking to ensure proper behavior.

Making functions into 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 instead 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 asynchronously on the ztaskd server. If the server is not available (not responding to 'alive' requests within ZTASKD_ALIVE_TIMEOUT milliseconds), the function will be called directly, in-process. It is your responsibility to make sure that func won't block the worker process.

  For backwards compatibility, .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. If the server is not available, the requested delay is ignored and the function is called directly just like it's done for func.async()

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/odeoncg/django-ztask-odeon/issues