Signals

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

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

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

Example connecting to the 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 you can filter by. For example the after_task_publish signal uses the task name as a sender, so by providing the sender argument to 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.

Added in version 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 Version 2 and Version 1 for a reference of possible fields that can be defined.
exchange

Name of the exchange to send to or a 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 (Exchange, Queue, or binding to declare before publishing the message. Can be modified.
retry_policy

Mapping of retry options. Can be any argument to kombu.Connection.ensure() and can be modified.

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 Version 2 and Version 1 for a reference of possible fields that can be defined.
body

The task message body, see Version 2 and Version 1 for a reference of possible fields that can be defined.
exchange

Name of the exchange or Exchange object used.
routing_key

Routing key used.

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.

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.

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 str).
einfo

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

Note

Only the request argument is guaranteed to be provided in all cases. The reason and einfo arguments may be None or not provided in certain scenarios, such as when a task is cancelled and retried. Signal handlers should not assume these arguments are always present.

Dispatched when a task succeeds.

Sender is the task object executed.

Provides arguments

result

Return value of the task.

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 billiard.einfo.ExceptionInfo instance.

Dispatched when an internal Celery error occurs while executing the task.

Sender is the task object executed.

Provides arguments:

task_id

Id of the task.
args

Positional arguments the task was called with.
kwargs

Keyword arguments the task was called with.
request

The original request dictionary. This is provided as the task.request may not be ready by the time the exception is raised.
exception

Exception instance raised.
traceback

Stack trace object.
einfo

The billiard.einfo.ExceptionInfo instance.

Dispatched when a task is received from the broker and is ready for execution.

Sender is the consumer object.

Provides arguments:

request

This is a Request instance, and not task.request. When using the prefork pool this signal is dispatched in the parent process, so task.request isn’t available and shouldn’t be used. Use this object instead, as they share many of the same fields.

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

Sender is the task object revoked/terminated.

Provides arguments:

request

This is a Context instance, and not task.request. When using the prefork pool this signal is dispatched in the parent process, so task.request isn’t available and shouldn’t be used. Use this object instead, as they share many of the same fields.
terminated

Set to True if the task was terminated.
signum

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

Set to True if the task expired.

Dispatched when a worker receives a message for a task that’s not registered.

Sender is the worker Consumer.

Provides arguments:

name

Name of task not found in registry.
id

The task id found in the message.
message

Raw message object.
exc

The error that occurred.

Dispatched when a worker receives an unknown type of message to one of its task queues.

Sender is the worker Consumer.

Provides arguments:

message

Raw message object.
exc

The error that occurred (if any).

This signal is sent when a program (worker, beat, shell) etc, asks for modules in the include and imports settings to be imported.

Sender is the app instance.

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

It can be used to add custom queues that should always be consumed from, disregarding the celery worker -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

Node name of the worker.
instance

This is the celery.apps.worker.Worker instance to be initialized. Note that only the app and hostname (nodename) attributes have been set so far, and the rest of __init__ hasn’t been executed.
conf

The configuration of the current app.

This is the first signal sent when 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='worker12@example.com')
def configure_worker12(conf=None, **kwargs):
    conf.task_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 ('worker1@example.com', 'worker2@example.com'):
        conf.task_default_rate_limit = '10/m'
    if sender == 'worker3@example.com':
        conf.worker_prefetch_multiplier = 0

Provides arguments:

sender

Nodename of the worker.
instance

This is the celery.apps.worker.Worker instance to be initialized. Note that only the app and hostname (nodename) attributes have been set so far, and the rest of __init__ hasn’t been executed.
conf

The configuration of the current app.
options

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

Dispatched before the worker is started.

Dispatched in the parent process, just before new child process is created in the prefork pool. It can be used to clean up instances that don’t behave well when forking.

@signals.worker_before_create_process.connect
def clean_channels(**kwargs):
    grpc_singleton.clean_channel()

Dispatched when the worker is ready to accept work.

Dispatched when Celery sends a worker heartbeat.

Sender is the celery.worker.heartbeat.Heart instance.

Dispatched when the worker begins the shutdown process.

Provides arguments:

sig

The POSIX signal that was received.
how

The shutdown method, warm or cold.
exitcode

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

Dispatched in all pool child processes when they start.

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

Dispatched in all pool child processes just before they exit.

Note: There’s 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’s about to shutdown.
exitcode

The exitcode that’ll be used when the child process exits.

Dispatched when the worker is about to shut down.

Dispatched when celery beat starts (either standalone or embedded).

Sender is the celery.beat.Service instance.

Dispatched in addition to the beat_init signal when celery beat is started as an embedded process.

Sender is the celery.beat.Service instance.

Sent when the eventlet pool has been started.

Sender is the celery.concurrency.eventlet.TaskPool instance.

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

Sender is the celery.concurrency.eventlet.TaskPool instance.

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

Sender is the celery.concurrency.eventlet.TaskPool instance.

Sent whenever a task is applied to the pool.

Sender is the celery.concurrency.eventlet.TaskPool instance.

Provides arguments:

target

The target function.
args

Positional arguments.
kwargs

Keyword arguments.

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’d like to augment the logging configuration setup by Celery then you can use the after_setup_logger and 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.

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.

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.

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 celery umbrella command:

from celery import Celery, signals
from click import Option

app = Celery()

# Celery 5.0+ uses click for its command-line interface.
# Use click.option to add new command-line arguments.
app.user_options['preload'].add(Option(
    ('--monitoring',), is_flag=True,
    help='Enable our external monitoring utility, blahblah',
))

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

Sender is the Command instance, and the value depends on the program that was called (e.g., for the umbrella command it’ll be a CeleryCommand) object).

Provides arguments:

app

The app instance.
options

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

This signal is deprecated, please use after_task_publish instead.

Copyright © 2017-2026 Asif Saif Uddin, core team & contributors. All rights reserved.
Celery is licensed under The BSD License (3 Clause, also known as the new BSD license). The license is an OSI approved Open Source license and is GPL-compatible.
https://docs.celeryq.dev/en/stable/userguide/signals.html