Tasks

.. py:currentmodule:: django_q

.. _async:

async_task()

.. warning:: Since Python 3.7 async became a reserved keyword and was refactored to async_task

Use :func:async_task from your code to quickly offload tasks to the :class:Cluster:

.. code:: python

from django_q.tasks import async_task, result

# create the task
async_task('math.copysign', 2, -2)

# or with import and storing the id
import math.copysign

task_id = async_task(copysign, 2, -2)

# get the result
task_result = result(task_id)

# result returns None if the task has not been executed yet
# you can wait for it
task_result = result(task_id, 200)

# but in most cases you will want to use a hook:

async_task('math.modf', 2.5, hook='hooks.print_result')

# hooks.py
def print_result(task):
    print(task.result)

:func:async_task can take the following optional keyword arguments:

hook """" The function to call after the task has been executed. This function gets passed the complete :class:Task object as its argument.

group """"" A group label. Check :doc:group for group functions.

save """" Overrides the result backend's save setting for this task.

timeout """"""" Overrides the cluster's timeout setting for this task.

See :ref:retry for details how to set values for timeout.

ack_failure """"""""""" Overrides the cluster's :ref:ack_failures setting for this task.

sync """" Simulates a task execution synchronously. Useful for testing. Can also be forced globally via the :ref:sync configuration option.

cached """""" Redirects the result to the cache backend instead of the database if set to True or to an integer indicating the cache timeout in seconds. e.g. cached=60. Especially useful with large and group operations where you don't need the all results in your database and want to take advantage of the speed of your cache backend.

broker """""" A broker instance, in case you want to control your own connections.

task_name """""""""

Optionally overwrites the auto-generated task name.

q_options """"""""" None of the option keywords get passed on to the task function. As an alternative you can also put them in a single keyword dict named q_options. This enables you to use these keywords for your function call::

# Async options in a dict

opts = {'hook': 'hooks.print_result',
        'group': 'math',
        'timeout': 30}

async_task('math.modf', 2.5, q_options=opts)

Please note that this will override any other option keywords.

.. note:: For tasks to be processed you will need to have a worker cluster running in the background using python manage.py qcluster or you need to configure Django Q to run in synchronous mode for testing using the :ref:sync option.

AsyncTask

Optionally you can use the :class:AsyncTask class to instantiate a task and keep everything in a single object.:

.. code-block:: python

# AsyncTask class instance example
from django_q.tasks import AsyncTask

# instantiate an async task
a = AsyncTask('math.floor', 1.5, group='math')

# you can set or change keywords afterwards
a.cached = True

# run it
a.run()

# wait indefinitely for the result and print it
print(a.result(wait=-1))

# change the args
a.args = (2.5,)

# run it again
a.run()

# wait max 10 seconds for the result and print it

print(a.result(wait=10))

.. code-block:: python

1
2

Once you change any of the parameters of the task after it has run, the result is invalidated and you will have to :func:AsyncTask.run it again to retrieve a new result.

Cached operations

You can run your tasks results against the Django cache backend instead of the database backend by either using the global :ref:cached setting or by supplying the cached keyword to individual functions. This can be useful if you are not interested in persistent results or if you run large group tasks where you only want the final result. By using a cache backend like Redis or Memcached you can speed up access to your task results significantly compared to a relational database.

When you set cached=True, results will be saved permanently in the cache and you will have to rely on your backend's cleanup strategies (like LRU) to manage stale results. You can also opt to set a manual timeout on the results, by setting e.g. cached=60. Meaning the result will be evicted from the cache after 60 seconds. This works both globally or on individual async executions.::

# simple cached example
from django_q.tasks import async_task, result

# cache the result for 10 seconds
id = async_task('math.floor', 100, cached=10)

# wait max 50ms for the result to appear in the cache
result(id, wait=50, cached=True)

# or fetch the task object
task = fetch(id, cached=True)

# and then save it to the database
task.save()

As you can see you can easily turn a cached result into a permanent database result by calling save() on it.

This also works for group actions::

# cached group example
from django_q.tasks import async_task, result_group
from django_q.brokers import get_broker

# set up a broker instance for better performance
broker = get_broker()

# Async a hundred functions under a group label
for i in range(100):
    async_task('math.frexp',
            i,
            group='frexp',
            cached=True,
            broker=broker)

# wait max 50ms for one hundred results to return
result_group('frexp', wait=50, count=100, cached=True)

If you don't need hooks, that exact same result can be achieved by using the more convenient :func:async_iter.

Synchronous testing

:func:async_task can be instructed to execute a task immediately by setting the optional keyword sync=True. The task will then be injected straight into a worker and the result saved by a monitor instance::

from django_q.tasks import async_task, fetch

# create a synchronous task
task_id = async_task('my.buggy.code', sync=True)

# the task will then be available immediately
task = fetch(task_id)

# and can be examined
if not task.success:
    print('An error occurred: {}'.format(task.result))

.. code:: bash

An error occurred: ImportError("No module named 'my'",)

Note that :func:async_task will block until the task is executed and saved. This feature bypasses the broker and is intended for debugging and development. Instead of setting sync on each individual async_task you can also configure :ref:sync as a global override.

Connection pooling

Django Q tries to pass broker instances around its parts as much as possible to save you from running out of connections. When you are making individual calls to :func:async_task a lot though, it can help to set up a broker to reuse for :func:async_task:

.. code:: python

# broker connection economy example
from django_q.tasks import async_task
from django_q.brokers import get_broker

broker = get_broker()
for i in range(50):
    async_task('math.modf', 2.5, broker=broker)

.. tip::

If you are using `django-redis <https://github.com/niwinz/django-redis>`__  and the redis broker, you can :ref:`configure <django_redis>` Django Q to use its connection pool.

Reference

.. py:function:: async_task(func, *args, hook=None, group=None, timeout=None,
save=None, sync=False, cached=False, broker=None, q_options=None, **kwargs)

Puts a task in the cluster queue

:param object func: The task function to execute :param tuple args: The arguments for the task function :param object hook: Optional function to call after execution :param str group: An optional group identifier :param int timeout: Overrides global cluster :ref:timeout. :param bool save: Overrides global save setting for this task. :param bool ack_failure: Overrides the global :ref:ack_failures setting for this task. :param bool sync: If set to True, async_task will simulate a task execution :param cached: Output the result to the cache backend. Bool or timeout in seconds :param broker: Optional broker connection from :func:brokers.get_broker :param dict q_options: Options dict, overrides option keywords :param dict kwargs: Keyword arguments for the task function :returns: The uuid of the task :rtype: str

.. py:function:: result(task_id, wait=0, cached=False)

Gets the result of a previously executed task

:param str task_id: the uuid or name of the task
:param int wait: optional milliseconds to wait for a result. -1 for indefinite
:param bool cached: run this against the cache backend.
:returns: The result of the executed task

.. py:function:: fetch(task_id, wait=0, cached=False)

Returns a previously executed task

:param str task_id: the uuid or name of the task
:param int wait: optional milliseconds to wait for a result. -1 for indefinite
:param bool cached: run this against the cache backend.
:returns: A task object
:rtype: Task

.. versionchanged:: 0.2.0

Renamed from get_task

.. py:function:: queue_size()

Returns the size of the broker queue.
Note that this does not count tasks currently being processed.

:returns: The amount of task packages in the broker
:rtype: int

.. py:function:: delete_cached(task_id, broker=None)

Deletes a task from the cache backend

:param str task_id: the uuid of the task
:param broker: an optional broker instance

.. py:class:: Task

Database model describing an executed task

.. py:attribute:: id

An  :func:`uuid.uuid4()` identifier

.. py:attribute:: name

The name of the task as a humanized version of the :attr:`id`

    .. note::

        This is for convenience and can be used as a parameter for most functions that take a `task_id`.
        Keep in mind that it is not guaranteed to be unique if you store very large amounts of tasks in the database.

.. py:attribute:: func

The function or reference that was executed

.. py:attribute:: hook


The function to call after execution.

.. py:attribute:: args

Positional arguments for the function.

.. py:attribute:: kwargs


Keyword arguments for the function.

.. py:attribute:: result

The result object. Contains the error if any occur.

.. py:attribute:: started

The moment the task was created by an async command

.. py:attribute:: stopped

The moment a worker finished this task

.. py:attribute:: success

Was the task executed without problems?

.. py:method:: time_taken

Calculates the difference in seconds between started and stopped.

    .. note::

        Time taken represents the time a task spends in the cluster, this includes any time it may have waited in the queue.

.. py:method:: group_result(failures=False)

Returns a list of results from this task's group.
Set failures to ``True`` to include failed results.

.. py:method:: group_count(failures=False)

Returns a count of the number of task results in this task's group.
Returns the number of failures when ``failures=True``

.. py:method:: group_delete(tasks=False)

Resets the group label on all the tasks in this task's group.
If ``tasks=True`` it will also delete the tasks in this group from the database, including itself.

.. py:classmethod:: get_result(task_id)

Gets a result directly by task uuid or name.

.. py:classmethod:: get_result_group(group_id, failures=False)

Returns a list of results from a task group.
Set failures to ``True`` to include failed results.

.. py:classmethod:: get_task(task_id)

Fetches a single task object by uuid or name.

.. py:classmethod:: get_task_group(group_id, failures=True)

Gets a queryset of tasks with this group id.
Set failures to ``False`` to exclude failed tasks.

.. py:classmethod::  get_group_count(group_id, failures=False)

Returns a count of the number of tasks results in a group.
Returns the number of failures when ``failures=True``

.. py:classmethod:: delete_group(group_id, objects=False)

Deletes a group label only, by default.
If ``objects=True`` it will also delete the tasks in this group from the database.

.. py:class:: Success

A proxy model of :class:`Task` with the queryset filtered on :attr:`Task.success` is ``True``.

.. py:class:: Failure

 A proxy model of :class:`Task` with the queryset filtered on :attr:`Task.success` is ``False``.

.. py:class:: AsyncTask(func, *args, **kwargs)

A class wrapper for the :func:`async_task` function.

:param object func: The task function to execute
:param tuple args: The arguments for the task function
:param dict kwargs: Keyword arguments for the task function, including async_task options

.. py:attribute:: id

The task unique identifier. This will only be available after it has been :meth:`run`

.. py:attribute:: started

Bool indicating if the task has been run with the current parameters

.. py:attribute:: func

The task function to execute

.. py:attribute:: args

A tuple of arguments for the task function

.. py:attribute:: kwargs

Keyword arguments for the function. Can include any of the optional async_task keyword attributes directly or in a `q_options` dictionary.

.. py:attribute:: broker

Optional :class:`Broker` instance to use

.. py:attribute:: sync

Run this task inline instead of asynchronous.

.. py:attribute:: save

Overrides the global save setting.

.. py:attribute:: hook

Optional function to call after a result is available. Takes the result :class:`Task` as the first argument.

.. py:attribute:: group

Optional group identifier

.. py:attribute:: cached

Run the task against the cache result backend.

.. py:method:: run()

Send the task to a worker cluster for execution

.. py:method:: result(wait=0)

 The task result. Always returns None if the task hasn't been run with the current parameters.

    :param int wait: the number of milliseconds to wait for a result. -1 for indefinite


.. py:method:: fetch(wait=0)

Returns the full :class:`Task` result instance.

    :param int wait: the number of milliseconds to wait for a result. -1 for indefinite

.. py:method:: result_group(failures=False, wait=0, count=None)

Returns a list of results from this task's group.

    :param bool failures: set this to ``True`` to include failed results
    :param int wait: optional milliseconds to wait for a result or count. -1 for indefinite
    :param int count: block until there are this many results in the group

.. py:method:: fetch_group(failures=True, wait=0, count=None)

Returns a list of task results from this task's group

    :param bool failures: set this to ``False`` to exclude failed tasks
    :param int wait: optional milliseconds to wait for a task or count. -1 for indefinite
    :param int count: block until there are this many tasks in the group