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Tasks

The Task framework provides the contract and plumbing for background work, not the engine that runs it. The Tasks API defines how work is described, queued, and tracked, but leaves actual execution to external infrastructure.

Task definition

The task decorator

task(*, priority=0, queue_name='default', backend='default', takes_context=False)

The @task decorator defines a Task instance. This has the following optional arguments:

  • priority: Sets the priority of the Task. Defaults to 0.

  • queue_name: Sets the queue_name of the Task. Defaults to "default".

  • backend: Sets the backend of the Task. Defaults to "default".

  • takes_context: Controls whether the Task function accepts a TaskContext. Defaults to False. See Task context for details.

If the defined Task is not valid according to the backend, InvalidTask is raised.

See defining tasks for usage examples.

Task

class Task

Represents a Task to be run in the background. Tasks should be defined using the task() decorator.

Attributes of Task cannot be modified. See modifying Tasks for details.

priority

The priority of the Task. Priorities must be between -100 and 100, where larger numbers are higher priority, and will be run sooner.

The backend must have supports_priority set to True to use this feature.

backend

The alias of the backend the Task should be enqueued to. This must match a backend defined in BACKEND.

queue_name

The name of the queue the Task will be enqueued on to. Defaults to "default". This must match a queue defined in QUEUES, unless QUEUES is set to [].

run_after

The earliest time the Task will be executed. This can be a timedelta, which is used relative to the current time, a timezone-aware datetime, or None if not constrained. Defaults to None.

The backend must have supports_defer set to True to use this feature. Otherwise, InvalidTask is raised.

name

The name of the function decorated with task(). This name is not necessarily unique.

using(*, priority=None, backend=None, queue_name=None, run_after=None)

Creates a new Task with modified defaults. The existing Task is left unchanged.

using allows modifying the following attributes:

See modifying Tasks for usage examples.

enqueue(*args, **kwargs)

Enqueues the Task to the Task backend for later execution.

Arguments are passed to the Task’s function after a round-trip through a json.dumps()/json.loads() cycle. Hence, all arguments must be JSON-serializable and preserve their type after the round-trip.

If the Task is not valid according to the backend, InvalidTask is raised.

See enqueueing Tasks for usage examples.

aenqueue(*args, **kwargs)

The async variant of enqueue.

get_result(result_id)

Retrieves a result by its id.

If the result does not exist, TaskResultDoesNotExist is raised. If the result is not the same type as the current Task, TaskResultMismatch is raised. If the backend does not support get_result(), NotImplementedError is raised.

aget_result(*args, **kwargs)

The async variant of get_result.

Task context

class TaskContext

Contains context for the running Task. Context only passed to a Task if it was defined with takes_context=True.

Attributes of TaskContext cannot be modified.

task_result

The TaskResult currently being run.

attempt

The number of the current execution attempts for this Task, starting at 1.

Task results

class TaskResultStatus

An Enum representing the status of a TaskResult.

READY

The Task has just been enqueued, or is ready to be executed again.

RUNNING

The Task is currently being executed.

FAILED

The Task raised an exception during execution, or was unable to start.

SUCCESSFUL

The Task has finished executing successfully.

class TaskResult

The TaskResult stores the information about a specific execution of a Task.

Attributes of TaskResult cannot be modified.

task

The Task the result was enqueued for.

id

A unique identifier for the result, which can be passed to Task.get_result().

The format of the id will depend on the backend being used. Task result ids are always strings less than 64 characters.

See Task results for more details.

status

The status of the result.

enqueued_at

The time when the Task was enqueued.

started_at

The time when the Task began execution, on its first attempt.

last_attempted_at

The time when the most recent Task run began execution.

finished_at

The time when the Task finished execution, whether it failed or succeeded.

backend

The backend the result is from.

errors

A list of TaskError instances for the errors raised as part of each execution of the Task.

return_value

The return value from the Task function.

If the Task did not finish successfully, ValueError is raised.

See return values for usage examples.

refresh()

Refresh the result’s attributes from the queue store.

arefresh()

The async variant of TaskResult.refresh().

is_finished

Whether the Task has finished (successfully or not).

attempts

The number of times the Task has been run.

If the task is currently running, it does not count as an attempt.

worker_ids

The ids of the workers which have executed the Task.

Task errors

class TaskError

Contains information about the error raised during the execution of a Task.

traceback

The traceback (as a string) from the raised exception when the Task failed.

exception_class

The exception class raised when executing the Task.

Backends

Backends handle how Tasks are stored and executed. All backends share a common interface defined by BaseTaskBackend, which specifies the core methods for enqueueing Tasks and retrieving results.

Base backend

class BaseTaskBackend

BaseTaskBackend is the parent class for all Task backends.

options

A dictionary of extra parameters for the Task backend. These are provided using the OPTIONS setting.

enqueue(task, args, kwargs)

Task backends which subclass BaseTaskBackend should implement this method as a minimum.

When implemented, enqueue() enqueues the task, a Task instance, for later execution. args are the positional arguments and kwargs are the keyword arguments to be passed to the task. Returns a TaskResult.

aenqueue(task, args, kwargs)

The async variant of BaseTaskBackend.enqueue().

get_result(result_id)

Retrieve a result by its id. If the result does not exist, TaskResultDoesNotExist is raised.

If the backend does not support get_result(), NotImplementedError is raised.

aget_result(result_id)

The async variant of BaseTaskBackend.get_result().

validate_task(task)

Validates whether the provided Task is able to be enqueued using the backend. If the Task is not valid, InvalidTask is raised.

Feature flags

Some backends may not support all features Django provides. It’s possible to identify the supported functionality of a backend, and potentially change behavior accordingly.

BaseTaskBackend.supports_defer

Whether the backend supports enqueueing Tasks to be executed after a specific time using the run_after attribute.

BaseTaskBackend.supports_async_task

Whether the backend supports enqueueing async functions (coroutines).

BaseTaskBackend.supports_get_result

Whether the backend supports retrieving Task results from another thread after they have been enqueued.

BaseTaskBackend.supports_priority

Whether the backend supports executing Tasks as ordered by their priority.

The below table notes which of the built-in backends support which features:

Feature

DummyBackend

ImmediateBackend

supports_defer

Ja

Nej

supports_async_task

Ja

Ja

supports_get_result

Nej

No [1]

supports_priority

Yes [2]

Yes [3]

Available backends

Django includes only development and testing backends. These support local execution and inspection, for production ready backends refer to Configuring a Task backend.

Immediate backend

class ImmediateBackend

The immediate backend executes Tasks immediately, rather than in the background.

Dummy backend

class DummyBackend

The dummy backend does not execute enqueued Tasks. Instead, it stores task results for later inspection.

results

A list of results for the enqueued Tasks, in the order they were enqueued.

clear()

Clears the list of stored results.

Undantag

exception InvalidTask

Raised when the Task attempting to be enqueued is invalid.

exception InvalidTaskBackend

Raised when the requested BaseTaskBackend is invalid.

exception TaskResultDoesNotExist

Raised by get_result() when the provided result_id does not exist.

exception TaskResultMismatch

Raised by get_result() when the provided result_id is for a different Task than the current Task.

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