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b931156c20
This removes the ability to configure Task enqueueing via a setting, since the proposed `ENQUEUE_ON_COMMIT` did not support multi-database setups. Thanks to Simon Charette for the report. Follow-up to 4289966d1b8e848e5e460b7c782dac009d746b20.
432 lines
13 KiB
Plaintext
432 lines
13 KiB
Plaintext
=====
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Tasks
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=====
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.. versionadded:: 6.0
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.. module:: django.tasks
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:synopsis: Django's built-in background Task system.
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Task definition
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===============
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The ``task`` decorator
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----------------------
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.. function:: task(*, priority=0, queue_name="default", backend="default", takes_context=False)
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The ``@task`` decorator defines a :class:`Task` instance. This has the
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following optional arguments:
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* ``priority``: Sets the :attr:`~Task.priority` of the ``Task``. Defaults
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to 0.
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* ``queue_name``: Sets the :attr:`~Task.queue_name` of the ``Task``.
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Defaults to ``"default"``.
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* ``backend``: Sets the :attr:`~Task.backend` of the ``Task``. Defaults to
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``"default"``.
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* ``takes_context``: Controls whether the ``Task`` function accepts a
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:class:`TaskContext`. Defaults to ``False``. See :ref:`Task context
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<task-context>` for details.
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If the defined ``Task`` is not valid according to the backend,
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:exc:`~django.tasks.exceptions.InvalidTask` is raised.
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See :ref:`defining tasks <defining-tasks>` for usage examples.
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``Task``
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--------
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.. class:: Task
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Represents a Task to be run in the background. Tasks should be defined
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using the :func:`task` decorator.
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Attributes of ``Task`` cannot be modified. See :ref:`modifying Tasks
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<modifying-tasks>` for details.
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.. attribute:: Task.priority
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The priority of the ``Task``. Priorities must be between -100 and 100,
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where larger numbers are higher priority, and will be run sooner.
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The backend must have :attr:`.supports_priority` set to ``True`` to use
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this feature.
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.. attribute:: Task.backend
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The alias of the backend the ``Task`` should be enqueued to. This must
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match a backend defined in :setting:`BACKEND <TASKS-BACKEND>`.
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.. attribute:: Task.queue_name
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The name of the queue the ``Task`` will be enqueued on to. Defaults to
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``"default"``. This must match a queue defined in
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:setting:`QUEUES <TASKS-QUEUES>`, unless
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:setting:`QUEUES <TASKS-QUEUES>` is set to ``[]``.
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.. attribute:: Task.run_after
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The earliest time the ``Task`` will be executed. This can be a
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:class:`timedelta <datetime.timedelta>`, which is used relative to the
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current time, a timezone-aware :class:`datetime <datetime.datetime>`,
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or ``None`` if not constrained. Defaults to ``None``.
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The backend must have :attr:`.supports_defer` set to ``True`` to use
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this feature. Otherwise,
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:exc:`~django.tasks.exceptions.InvalidTask` is raised.
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.. attribute:: Task.name
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The name of the function decorated with :func:`task`. This name is not
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necessarily unique.
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.. method:: Task.using(*, priority=None, backend=None, queue_name=None, run_after=None)
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Creates a new ``Task`` with modified defaults. The existing ``Task`` is
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left unchanged.
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``using`` allows modifying the following attributes:
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* :attr:`priority <Task.priority>`
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* :attr:`backend <Task.backend>`
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* :attr:`queue_name <Task.queue_name>`
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* :attr:`run_after <Task.run_after>`
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See :ref:`modifying Tasks <modifying-tasks>` for usage examples.
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.. method:: Task.enqueue(*args, **kwargs)
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Enqueues the ``Task`` to the ``Task`` backend for later execution.
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Arguments are passed to the ``Task``'s function after a round-trip
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through a :func:`json.dumps`/:func:`json.loads` cycle. Hence, all
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arguments must be JSON-serializable and preserve their type after the
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round-trip.
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If the ``Task`` is not valid according to the backend,
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:exc:`~django.tasks.exceptions.InvalidTask` is raised.
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See :ref:`enqueueing Tasks <enqueueing-tasks>` for usage examples.
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.. method:: Task.aenqueue(*args, **kwargs)
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The ``async`` variant of :meth:`enqueue <Task.enqueue>`.
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.. method:: Task.get_result(result_id)
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Retrieves a result by its id.
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If the result does not exist, :exc:`TaskResultDoesNotExist
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<django.tasks.exceptions.TaskResultDoesNotExist>` is raised. If the
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result is not the same type as the current Task,
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:exc:`TaskResultMismatch <django.tasks.exceptions.TaskResultMismatch>`
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is raised. If the backend does not support ``get_result()``,
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:exc:`NotImplementedError` is raised.
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.. method:: Task.aget_result(*args, **kwargs)
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The ``async`` variant of :meth:`get_result <Task.get_result>`.
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Task context
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============
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.. class:: TaskContext
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Contains context for the running :class:`Task`. Context only passed to a
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``Task`` if it was defined with ``takes_context=True``.
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Attributes of ``TaskContext`` cannot be modified.
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.. attribute:: TaskContext.task_result
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The :class:`TaskResult` currently being run.
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.. attribute:: TaskContext.attempt
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The number of the current execution attempts for this Task, starting at
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1.
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Task results
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============
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.. class:: TaskResultStatus
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An Enum representing the status of a :class:`TaskResult`.
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.. attribute:: TaskResultStatus.READY
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The :class:`Task` has just been enqueued, or is ready to be executed
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again.
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.. attribute:: TaskResultStatus.RUNNING
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The :class:`Task` is currently being executed.
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.. attribute:: TaskResultStatus.FAILED
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The :class:`Task` raised an exception during execution, or was unable
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to start.
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.. attribute:: TaskResultStatus.SUCCESSFUL
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The :class:`Task` has finished executing successfully.
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.. class:: TaskResult
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The ``TaskResult`` stores the information about a specific execution of a
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:class:`Task`.
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Attributes of ``TaskResult`` cannot be modified.
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.. attribute:: TaskResult.task
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The :class:`Task` the result was enqueued for.
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.. attribute:: TaskResult.id
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A unique identifier for the result, which can be passed to
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:meth:`Task.get_result`.
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The format of the id will depend on the backend being used. Task result
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ids are always strings less than 64 characters.
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See :ref:`Task results <task-results>` for more details.
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.. attribute:: TaskResult.status
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The :class:`status <TaskResultStatus>` of the result.
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.. attribute:: TaskResult.enqueued_at
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The time when the ``Task`` was enqueued.
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.. attribute:: TaskResult.started_at
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The time when the ``Task`` began execution, on its first attempt.
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.. attribute:: TaskResult.last_attempted_at
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The time when the most recent ``Task`` run began execution.
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.. attribute:: TaskResult.finished_at
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The time when the ``Task`` finished execution, whether it failed or
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succeeded.
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.. attribute:: TaskResult.backend
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The backend the result is from.
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.. attribute:: TaskResult.errors
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A list of :class:`TaskError` instances for the errors raised as part of
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each execution of the Task.
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.. attribute:: TaskResult.return_value
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The return value from the ``Task`` function.
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If the ``Task`` did not finish successfully, :exc:`ValueError` is
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raised.
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See :ref:`return values <task-return-values>` for usage examples.
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.. method:: TaskResult.refresh
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Refresh the result's attributes from the queue store.
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.. method:: TaskResult.arefresh
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The ``async`` variant of :meth:`TaskResult.refresh`.
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.. attribute:: TaskResult.is_finished
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Whether the ``Task`` has finished (successfully or not).
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.. attribute:: TaskResult.attempts
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The number of times the Task has been run.
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If the task is currently running, it does not count as an attempt.
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.. attribute:: TaskResult.worker_ids
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The ids of the workers which have executed the Task.
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Task errors
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-----------
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.. class:: TaskError
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Contains information about the error raised during the execution of a
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``Task``.
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.. attribute:: TaskError.traceback
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The traceback (as a string) from the raised exception when the ``Task``
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failed.
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.. attribute:: TaskError.exception_class
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The exception class raised when executing the ``Task``.
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Backends
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========
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Base backend
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------------
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.. module:: django.tasks.backends.base
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.. class:: BaseTaskBackend
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``BaseTaskBackend`` is the parent class for all Task backends.
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.. attribute:: BaseTaskBackend.options
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A dictionary of extra parameters for the Task backend. These are
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provided using the :setting:`OPTIONS <TASKS-OPTIONS>` setting.
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.. method:: BaseTaskBackend.enqueue(task, args, kwargs)
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Task backends which subclass ``BaseTaskBackend`` should implement this
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method as a minimum.
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When implemented, ``enqueue()`` enqueues the ``task``, a :class:`.Task`
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instance, for later execution. ``args`` are the positional arguments
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and ``kwargs`` are the keyword arguments to be passed to the ``task``.
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Returns a :class:`~django.tasks.TaskResult`.
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.. method:: BaseTaskBackend.aenqueue(task, args, kwargs)
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The ``async`` variant of :meth:`BaseTaskBackend.enqueue`.
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.. method:: BaseTaskBackend.get_result(result_id)
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Retrieve a result by its id. If the result does not exist,
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:exc:`TaskResultDoesNotExist
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<django.tasks.exceptions.TaskResultDoesNotExist>` is raised.
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If the backend does not support ``get_result()``,
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:exc:`NotImplementedError` is raised.
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.. method:: BaseTaskBackend.aget_result(result_id)
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The ``async`` variant of :meth:`BaseTaskBackend.get_result`.
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.. method:: BaseTaskBackend.validate_task(task)
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Validates whether the provided ``Task`` is able to be enqueued using
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the backend. If the Task is not valid,
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:exc:`InvalidTask <django.tasks.exceptions.InvalidTask>`
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is raised.
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Feature flags
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~~~~~~~~~~~~~
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Some backends may not support all features Django provides. It's possible to
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identify the supported functionality of a backend, and potentially change
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behavior accordingly.
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.. attribute:: BaseTaskBackend.supports_defer
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Whether the backend supports enqueueing Tasks to be executed after a
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specific time using the :attr:`~django.tasks.Task.run_after` attribute.
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.. attribute:: BaseTaskBackend.supports_async_task
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Whether the backend supports enqueueing async functions (coroutines).
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.. attribute:: BaseTaskBackend.supports_get_result
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Whether the backend supports retrieving ``Task`` results from another
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thread after they have been enqueued.
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.. attribute:: BaseTaskBackend.supports_priority
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Whether the backend supports executing Tasks as ordered by their
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:attr:`~django.tasks.Task.priority`.
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The below table notes which of the :ref:`built-in backends
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<task-available-backends>` support which features:
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============================ ======================= ===========================
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Feature :class:`.DummyBackend` :class:`.ImmediateBackend`
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============================ ======================= ===========================
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:attr:`.supports_defer` Yes No
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:attr:`.supports_async_task` Yes Yes
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:attr:`.supports_get_result` No No [#fnimmediateresult]_
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:attr:`.supports_priority` Yes [#fndummypriority]_ Yes [#fnimmediatepriority]_
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============================ ======================= ===========================
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.. _task-available-backends:
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Available backends
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------------------
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Immediate backend
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~~~~~~~~~~~~~~~~~
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.. module:: django.tasks.backends.immediate
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.. class:: ImmediateBackend
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The :ref:`immediate backend <immediate-task-backend>` executes Tasks
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immediately, rather than in the background.
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Dummy backend
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~~~~~~~~~~~~~
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.. module:: django.tasks.backends.dummy
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.. class:: DummyBackend
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The :ref:`dummy backend <dummy-task-backend>` does not execute enqueued
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Tasks. Instead, it stores task results for later inspection.
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.. attribute:: DummyBackend.results
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A list of results for the enqueued Tasks, in the order they were
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enqueued.
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.. method:: DummyBackend.clear
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Clears the list of stored results.
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Exceptions
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==========
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.. module:: django.tasks.exceptions
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.. exception:: InvalidTask
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Raised when the :class:`.Task` attempting to be enqueued
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is invalid.
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.. exception:: InvalidTaskBackend
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Raised when the requested :class:`.BaseTaskBackend` is invalid.
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.. exception:: TaskResultDoesNotExist
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Raised by :meth:`~django.tasks.backends.base.BaseTaskBackend.get_result`
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when the provided ``result_id`` does not exist.
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.. exception:: TaskResultMismatch
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Raised by :meth:`~django.tasks.Task.get_result` when the provided
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``result_id`` is for a different Task than the current Task.
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.. rubric:: Footnotes
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.. [#fnimmediateresult] The :class:`.ImmediateBackend` doesn't officially
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support ``get_result()``, despite implementing the API, since the result
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cannot be retrieved from a different thread.
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.. [#fndummypriority] The :class:`.DummyBackend` has ``supports_priority=True``
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so that it can be used as a drop-in replacement in tests. Since this
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backend never executes Tasks, the ``priority`` value has no effect.
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.. [#fnimmediatepriority] The :class:`.ImmediateBackend` has
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``supports_priority=True`` so that it can be used as a drop-in replacement
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in tests. Because Tasks run as soon as they are scheduled, the ``priority``
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value has no effect.
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