signals.rst

Signals

• Basics
• Signals
  • Task Signals
    • before_task_publish
    • after_task_publish
    • task_prerun
    • task_postrun
    • task_retry
    • task_success
    • task_failure
    • task_revoked
  • App Signals
    • import_modules
  • Worker Signals
    • celeryd_after_setup
    • celeryd_init
    • worker_init
    • worker_ready
    • worker_process_init
    • worker_process_shutdown
    • worker_shutdown
  • Beat Signals
    • beat_init
    • beat_embedded_init
  • Eventlet Signals
    • eventlet_pool_started
    • eventlet_pool_preshutdown
    • eventlet_pool_postshutdown
    • eventlet_pool_apply
  • Logging Signals
    • setup_logging
    • after_setup_logger
    • after_setup_task_logger
  • Command signals
    • user_preload_options
  • Deprecated Signals
    • task_sent

Signals allows decoupled applications to receive notifications when certain actions occur elsewhere in the application.

Celery ships with many signals that you application can hook into to augment behavior of certain actions.

Basics

Several kinds of events trigger signals, you can connect to these signals to perform actions as they trigger.

Example connecting to the :signal:`after_task_publish` signal:

from celery.signals import after_task_publish

@after_task_publish.connect
def task_sent_handler(sender=None, headers=None, body=None, **kwargs):
    # information about task are located in headers for task messages
    # using the task protocol version 2.
    info = headers if 'task' in headers else body
    print('after_task_publish for task id {info[id]}'.format(
        info=info,
    ))

Some signals also have a sender which you can filter by. For example the :signal:`after_task_publish` signal uses the task name as a sender, so by providing the sender argument to :class:`~celery.utils.dispatch.signal.Signal.connect` you can connect your handler to be called every time a task with name "proj.tasks.add" is published:

@after_task_publish.connect(sender='proj.tasks.add')
def task_sent_handler(sender=None, headers=None, body=None, **kwargs):
    # information about task are located in headers for task messages
    # using the task protocol version 2.
    info = headers if 'task' in headers else body
    print('after_task_publish for task id {info[id]}'.format(
        info=info,
    ))

Signals use the same implementation as django.core.dispatch. As a result other keyword parameters (e.g. signal) are passed to all signal handlers by default.

The best practice for signal handlers is to accept arbitrary keyword arguments (i.e. **kwargs). That way new celery versions can add additional arguments without breaking user code.

Signals

Task Signals

.. signal:: before_task_publish

before_task_publish

.. versionadded:: 3.1

Dispatched before a task is published. Note that this is executed in the process sending the task.

Sender is the name of the task being sent.

Provides arguments:

• body

  Task message body.

  This is a mapping containing the task message fields (see :ref:`message-protocol-task-v1`).

• exchange

  Name of the exchange to send to or a :class:`~kombu.Exchange` object.

• routing_key

  Routing key to use when sending the message.

• headers

  Application headers mapping (can be modified).

• properties

  Message properties (can be modified)

• declare

  List of entities (:class:`~kombu.Exchange`, :class:`~kombu.Queue` or :class:~`kombu.binding` to declare before publishing the message. Can be modified.

• retry_policy

  Mapping of retry options. Can be any argument to :meth:`kombu.Connection.ensure` and can be modified.

.. signal:: after_task_publish

after_task_publish

Dispatched when a task has been sent to the broker. Note that this is executed in the process that sent the task.

Sender is the name of the task being sent.

Provides arguments:

• headers

  The task message headers, see :ref:`message-protocol-task-v2` and :ref:`message-protocol-task-v1`. for a reference of possible fields that can be defined.

• body

  The task message body, see :ref:`message-protocol-task-v2` and :ref:`message-protocol-task-v1`. for a reference of possible fields that can be defined.

• exchange

  Name of the exchange or :class:`~kombu.Exchange` object used.

• routing_key

  Routing key used.

.. signal:: task_prerun

task_prerun

Dispatched before a task is executed.

Sender is the task object being executed.

Provides arguments:

• task_id

  Id of the task to be executed.

• task

  The task being executed.

• args

  the tasks positional arguments.

• kwargs

  The tasks keyword arguments.

.. signal:: task_postrun

task_postrun

Dispatched after a task has been executed.

Sender is the task object executed.

Provides arguments:

• task_id

  Id of the task to be executed.

• task

  The task being executed.

• args

  The tasks positional arguments.

• kwargs

  The tasks keyword arguments.

• retval

  The return value of the task.

• state

  Name of the resulting state.

.. signal:: task_retry

task_retry

Dispatched when a task will be retried.

Sender is the task object.

Provides arguments:

• request

  The current task request.

• reason

  Reason for retry (usually an exception instance, but can always be coerced to :class:`str`).

• einfo

  Detailed exception information, including traceback (a :class:`billiard.einfo.ExceptionInfo` object).

.. signal:: task_success

task_success

Dispatched when a task succeeds.

Sender is the task object executed.

Provides arguments

• result

  Return value of the task.

.. signal:: task_failure

task_failure

Dispatched when a task fails.

Sender is the task object executed.

Provides arguments:

• task_id

  Id of the task.

• exception

  Exception instance raised.

• args

  Positional arguments the task was called with.

• kwargs

  Keyword arguments the task was called with.

• traceback

  Stack trace object.

• einfo

  The :class:`celery.datastructures.ExceptionInfo` instance.

.. signal:: task_revoked

task_revoked

Dispatched when a task is revoked/terminated by the worker.

Sender is the task object revoked/terminated.

Provides arguments:

• request

  This is a :class:`~celery.worker.request.Request` instance, and not task.request. When using the prefork pool this signal is dispatched in the parent process, so task.request is not available and should not be used. Use this object instead, which should have many of the same fields.

• terminated

  Set to :const:`True` if the task was terminated.

• signum

  Signal number used to terminate the task. If this is :const:`None` and terminated is :const:`True` then :sig:`TERM` should be assumed.

• expired Set to :const:`True` if the task expired.

App Signals

.. signal:: import_modules

import_modules

This signal is sent when a program (worker, beat, shell) etc, asks for modules in the :setting:`CELERY_INCLUDE` and :setting:`CELERY_IMPORTS` settings to be imported.

Sender is the app instance.

Worker Signals

.. signal:: celeryd_after_setup

celeryd_after_setup

This signal is sent after the worker instance is set up, but before it calls run. This means that any queues from the :option:`-Q` option is enabled, logging has been set up and so on.

It can be used to e.g. add custom queues that should always be consumed from, disregarding the :option:`-Q` option. Here's an example that sets up a direct queue for each worker, these queues can then be used to route a task to any specific worker:

from celery.signals import celeryd_after_setup

@celeryd_after_setup.connect
def setup_direct_queue(sender, instance, **kwargs):
    queue_name = '{0}.dq'.format(sender)  # sender is the nodename of the worker
    instance.app.amqp.queues.select_add(queue_name)

Provides arguments:

• sender Hostname of the worker.

• instance

  This is the :class:`celery.apps.worker.Worker` instance to be initialized. Note that only the :attr:`app` and :attr:`hostname` (nodename) attributes have been set so far, and the rest of __init__ has not been executed.

• conf

  The configuration of the current app.

.. signal:: celeryd_init

celeryd_init

This is the first signal sent when :program:`celery worker` starts up. The sender is the host name of the worker, so this signal can be used to setup worker specific configuration:

from celery.signals import celeryd_init

@celeryd_init.connect(sender='[email protected]')
def configure_worker12(conf=None, **kwargs):
    conf.CELERY_DEFAULT_RATE_LIMIT = '10/m'

or to set up configuration for multiple workers you can omit specifying a sender when you connect:

from celery.signals import celeryd_init

@celeryd_init.connect
def configure_workers(sender=None, conf=None, **kwargs):
    if sender in ('[email protected]', '[email protected]'):
        conf.CELERY_DEFAULT_RATE_LIMIT = '10/m'
    if sender == '[email protected]':
        conf.CELERYD_PREFETCH_MULTIPLIER = 0

Provides arguments:

• sender Nodename of the worker.

• instance

  This is the :class:`celery.apps.worker.Worker` instance to be initialized. Note that only the :attr:`app` and :attr:`hostname` (nodename) attributes have been set so far, and the rest of __init__ has not been executed.

• conf

  The configuration of the current app.

• options

  Options passed to the worker from command-line arguments (including defaults).

.. signal:: worker_init

worker_init

Dispatched before the worker is started.

.. signal:: worker_ready

worker_ready

Dispatched when the worker is ready to accept work.

.. signal:: worker_process_init

worker_process_init

Dispatched in all pool child processes when they start.

Note that handlers attached to this signal must not be blocking for more than 4 seconds, or the process will be killed assuming it failed to start.

.. signal:: worker_process_shutdown

worker_process_shutdown

Dispatched in all pool child processes just before they exit.

Note: There is no guarantee that this signal will be dispatched, similarly to finally blocks it's impossible to guarantee that handlers will be called at shutdown, and if called it may be interrupted during.

Provides arguments:

• pid

  The pid of the child process that is about to shutdown.

• exitcode

  The exitcode that will be used when the child process exits.

.. signal:: worker_shutdown

worker_shutdown

Dispatched when the worker is about to shut down.

Beat Signals

.. signal:: beat_init

beat_init

Dispatched when :program:`celery beat` starts (either standalone or embedded). Sender is the :class:`celery.beat.Service` instance.

.. signal:: beat_embedded_init

beat_embedded_init

Dispatched in addition to the :signal:`beat_init` signal when :program:`celery beat` is started as an embedded process. Sender is the :class:`celery.beat.Service` instance.

Eventlet Signals

.. signal:: eventlet_pool_started

eventlet_pool_started

Sent when the eventlet pool has been started.

Sender is the :class:`celery.concurrency.eventlet.TaskPool` instance.

.. signal:: eventlet_pool_preshutdown

eventlet_pool_preshutdown

Sent when the worker shutdown, just before the eventlet pool is requested to wait for remaining workers.

Sender is the :class:`celery.concurrency.eventlet.TaskPool` instance.

.. signal:: eventlet_pool_postshutdown

eventlet_pool_postshutdown

Sent when the pool has been joined and the worker is ready to shutdown.

Sender is the :class:`celery.concurrency.eventlet.TaskPool` instance.

.. signal:: eventlet_pool_apply

eventlet_pool_apply

Sent whenever a task is applied to the pool.

Sender is the :class:`celery.concurrency.eventlet.TaskPool` instance.

Provides arguments:

• target

  The target function.

• args

  Positional arguments.

• kwargs

  Keyword arguments.

Logging Signals

.. signal:: setup_logging

setup_logging

Celery won't configure the loggers if this signal is connected, so you can use this to completely override the logging configuration with your own.

If you would like to augment the logging configuration setup by Celery then you can use the :signal:`after_setup_logger` and :signal:`after_setup_task_logger` signals.

Provides arguments:

• loglevel

  The level of the logging object.

• logfile

  The name of the logfile.

• format

  The log format string.

• colorize

  Specify if log messages are colored or not.

.. signal:: after_setup_logger

after_setup_logger

Sent after the setup of every global logger (not task loggers). Used to augment logging configuration.

Provides arguments:

• logger

  The logger object.

• loglevel

  The level of the logging object.

• logfile

  The name of the logfile.

• format

  The log format string.

• colorize

  Specify if log messages are colored or not.

.. signal:: after_setup_task_logger

after_setup_task_logger

Sent after the setup of every single task logger. Used to augment logging configuration.

Provides arguments:

• logger

  The logger object.

• loglevel

  The level of the logging object.

• logfile

  The name of the logfile.

• format

  The log format string.

• colorize

  Specify if log messages are colored or not.

Command signals

.. signal:: user_preload_options

user_preload_options

This signal is sent after any of the Celery command line programs are finished parsing the user preload options.

It can be used to add additional command-line arguments to the :program:`celery` umbrella command:

from celery import Celery
from celery import signals
from celery.bin.base import Option

app = Celery()
app.user_options['preload'].add(Option(
    '--monitoring', action='store_true',
    help='Enable our external monitoring utility, blahblah',
))

@signals.user_preload_options.connect
def handle_preload_options(options, **kwargs):
    if options['monitoring']:
        enable_monitoring()

Sender is the :class:`~celery.bin.base.Command` instance, which depends on what program was called (e.g. for the umbrella command it will be a :class:`~celery.bin.celery.CeleryCommand`) object).

Provides arguments:

• app

  The app instance.

• options

  Mapping of the parsed user preload options (with default values).

Deprecated Signals

.. signal:: task_sent

task_sent

This signal is deprecated, please use :signal:`after_task_publish` instead.