This document describes Celery 2.2. For development docs, go here.

Workers Guide

Starting the worker

You can start celeryd to run in the foreground by executing the command:

$ celeryd --loglevel=INFO

You probably want to use a daemonization tool to start celeryd in the background. See Running celeryd as a daemon for help using celeryd with popular daemonization tools.

For a full list of available command line options see celeryd, or simply do:

$ celeryd --help

You can also start multiple workers on the same machine. If you do so be sure to give a unique name to each individual worker by specifying a host name with the --hostname|-n argument:

$ celeryd --loglevel=INFO --concurrency=10 -n worker1.example.com
$ celeryd --loglevel=INFO --concurrency=10 -n worker2.example.com
$ celeryd --loglevel=INFO --concurrency=10 -n worker3.example.com

Stopping the worker

Shutdown should be accomplished using the TERM signal.

When shutdown is initiated the worker will finish all currently executing tasks before it actually terminates, so if these tasks are important you should wait for it to finish before doing anything drastic (like sending the KILL signal).

If the worker won’t shutdown after considerate time, for example because of tasks stuck in an infinite-loop, you can use the KILL signal to force terminate the worker, but be aware that currently executing tasks will be lost (unless the tasks have the acks_late option set).

Also as processes can’t override the KILL signal, the worker will not be able to reap its children, so make sure to do so manually. This command usually does the trick:

$ ps auxww | grep celeryd | awk '{print $2}' | xargs kill -9

Restarting the worker

Other than stopping then starting the worker to restart, you can also restart the worker using the HUP signal:

$ kill -HUP $pid

The worker will then replace itself with a new instance using the same arguments as it was started with.

Concurrency

By default multiprocessing is used to perform concurrent execution of tasks, but you can also use Eventlet. The number of worker processes/threads can be changed using the --concurrency argument and defaults to the number of CPUs available on the machine.

Number of processes (multiprocessing)

More worker processes are usually better, but there’s a cut-off point where adding more processes affects performance in negative ways. There is even some evidence to support that having multiple celeryd’s running, may perform better than having a single worker. For example 3 celeryd’s with 10 worker processes each. You need to experiment to find the numbers that works best for you, as this varies based on application, work load, task run times and other factors.

Persistent revokes

Revoking tasks works by sending a broadcast message to all the workers, the workers then keep a list of revoked tasks in memory.

If you want tasks to remain revoked after worker restart you need to specify a file for these to be stored in, either by using the –statedb argument to celeryd or the CELERYD_STATE_DB setting. See CELERYD_STATE_DB for more information.

Time limits

New in version 2.0.

A single task can potentially run forever, if you have lots of tasks waiting for some event that will never happen you will block the worker from processing new tasks indefinitely. The best way to defend against this scenario happening is enabling time limits.

The time limit (–time-limit) is the maximum number of seconds a task may run before the process executing it is terminated and replaced by a new process. You can also enable a soft time limit (–soft-time-limit), this raises an exception the task can catch to clean up before the hard time limit kills it:

from celery.task import task
from celery.exceptions import SoftTimeLimitExceeded

@task()
def mytask():
    try:
        do_work()
    except SoftTimeLimitExceeded:
        clean_up_in_a_hurry()

Time limits can also be set using the CELERYD_TASK_TIME_LIMIT / CELERYD_SOFT_TASK_TIME_LIMIT settings.

Note

Time limits do not currently work on Windows.

Max tasks per child setting

With this option you can configure the maximum number of tasks a worker can execute before it’s replaced by a new process.

This is useful if you have memory leaks you have no control over for example from closed source C extensions.

The option can be set using the –maxtasksperchild argument to celeryd or using the CELERYD_MAX_TASKS_PER_CHILD setting.

Remote control

New in version 2.0.

Workers have the ability to be remote controlled using a high-priority broadcast message queue. The commands can be directed to all, or a specific list of workers.

Commands can also have replies. The client can then wait for and collect those replies. Since there’s no central authority to know how many workers are available in the cluster, there is also no way to estimate how many workers may send a reply, so the client has a configurable timeout — the deadline in seconds for replies to arrive in. This timeout defaults to one second. If the worker doesn’t reply within the deadline it doesn’t necessarily mean the worker didn’t reply, or worse is dead, but may simply be caused by network latency or the worker being slow at processing commands, so adjust the timeout accordingly.

In addition to timeouts, the client can specify the maximum number of replies to wait for. If a destination is specified, this limit is set to the number of destination hosts.

See also

The celeryctl program is used to execute remote control commands from the command line. It supports all of the commands listed below. See celeryctl: Management Utility for more information.

The broadcast() function.

This is the client function used to send commands to the workers. Some remote control commands also have higher-level interfaces using broadcast() in the background, like rate_limit() and ping().

Sending the rate_limit command and keyword arguments:

>>> from celery.task.control import broadcast
>>> broadcast("rate_limit", arguments={"task_name": "myapp.mytask",
...                                    "rate_limit": "200/m"})

This will send the command asynchronously, without waiting for a reply. To request a reply you have to use the reply argument:

>>> broadcast("rate_limit", {"task_name": "myapp.mytask",
...                          "rate_limit": "200/m"}, reply=True)
[{'worker1.example.com': 'New rate limit set successfully'},
 {'worker2.example.com': 'New rate limit set successfully'},
 {'worker3.example.com': 'New rate limit set successfully'}]

Using the destination argument you can specify a list of workers to receive the command:

>>> broadcast
>>> broadcast("rate_limit", {"task_name": "myapp.mytask",
...                          "rate_limit": "200/m"}, reply=True,
...           destination=["worker1.example.com"])
[{'worker1.example.com': 'New rate limit set successfully'}]

Of course, using the higher-level interface to set rate limits is much more convenient, but there are commands that can only be requested using broadcast().

Rate limits

Example changing the rate limit for the myapp.mytask task to accept 200 tasks a minute on all servers:

>>> from celery.task.control import rate_limit
>>> rate_limit("myapp.mytask", "200/m")

Example changing the rate limit on a single host by specifying the destination hostname:

>>> rate_limit("myapp.mytask", "200/m",
...            destination=["worker1.example.com"])

Warning

This won’t affect workers with the CELERY_DISABLE_RATE_LIMITS setting on. To re-enable rate limits then you have to restart the worker.

Revoking tasks

All worker nodes keeps a memory of revoked task ids, either in-memory or persistent on disk (see Persistent revokes).

When a worker receives a revoke request it will skip executing the task, but it won’t terminate an already executing task unless the terminate option is set.

If terminate is set the worker child process processing the task will be terminated. The default signal sent is TERM, but you can specify this using the signal argument. Signal can be the uppercase name of any signal defined in the signal module in the Python Standard Library.

Terminating a task also revokes it.

Example

>>> from celery.task.control import revoke
>>> revoke("d9078da5-9915-40a0-bfa1-392c7bde42ed")

>>> revoke("d9078da5-9915-40a0-bfa1-392c7bde42ed",
...        terminate=True)

>>> revoke("d9078da5-9915-40a0-bfa1-392c7bde42ed",
...        terminate=True, signal="SIGKILL")

Remote shutdown

This command will gracefully shut down the worker remotely:

>>> broadcast("shutdown") # shutdown all workers
>>> broadcast("shutdown, destination="worker1.example.com")

Ping

This command requests a ping from alive workers. The workers reply with the string ‘pong’, and that’s just about it. It will use the default one second timeout for replies unless you specify a custom timeout:

>>> from celery.task.control import ping
>>> ping(timeout=0.5)
[{'worker1.example.com': 'pong'},
 {'worker2.example.com': 'pong'},
 {'worker3.example.com': 'pong'}]

ping() also supports the destination argument, so you can specify which workers to ping:

>>> ping(['worker2.example.com', 'worker3.example.com'])
[{'worker2.example.com': 'pong'},
 {'worker3.example.com': 'pong'}]

Enable/disable events

You can enable/disable events by using the enable_events, disable_events commands. This is useful to temporarily monitor a worker using celeryev/celerymon.

>>> broadcast("enable_events")
>>> broadcast("disable_events")

Writing your own remote control commands

Remote control commands are registered in the control panel and they take a single argument: the current ControlDispatch instance. From there you have access to the active Consumer if needed.

Here’s an example control command that restarts the broker connection:

from celery.worker.control import Panel

@Panel.register
def reset_connection(panel):
    panel.logger.critical("Connection reset by remote control.")
    panel.consumer.reset_connection()
    return {"ok": "connection reset"}

These can be added to task modules, or you can keep them in their own module then import them using the CELERY_IMPORTS setting:

CELERY_IMPORTS = ("myapp.worker.control", )

Inspecting workers

celery.task.control.inspect lets you inspect running workers. It uses remote control commands under the hood.

>>> from celery.task.control import inspect

# Inspect all nodes.
>>> i = inspect()

# Specify multiple nodes to inspect.
>>> i = inspect(["worker1.example.com", "worker2.example.com"])

# Specify a single node to inspect.
>>> i = inspect("worker1.example.com")

Dump of registered tasks

You can get a list of tasks registered in the worker using the registered_tasks():

>>> i.registered_tasks()
[{'worker1.example.com': ['celery.delete_expired_task_meta',
                          'celery.execute_remote',
                          'celery.map_async',
                          'celery.ping',
                          'celery.task.http.HttpDispatchTask',
                          'tasks.add',
                          'tasks.sleeptask']}]

Dump of currently executing tasks

You can get a list of active tasks using active():

>>> i.active()
[{'worker1.example.com':
    [{"name": "tasks.sleeptask",
      "id": "32666e9b-809c-41fa-8e93-5ae0c80afbbf",
      "args": "(8,)",
      "kwargs": "{}"}]}]

Dump of scheduled (ETA) tasks

You can get a list of tasks waiting to be scheduled by using scheduled():

>>> i.scheduled()
[{'worker1.example.com':
    [{"eta": "2010-06-07 09:07:52", "priority": 0,
      "request": {
        "name": "tasks.sleeptask",
        "id": "1a7980ea-8b19-413e-91d2-0b74f3844c4d",
        "args": "[1]",
        "kwargs": "{}"}},
     {"eta": "2010-06-07 09:07:53", "priority": 0,
      "request": {
        "name": "tasks.sleeptask",
        "id": "49661b9a-aa22-4120-94b7-9ee8031d219d",
        "args": "[2]",
        "kwargs": "{}"}}]}]

Note that these are tasks with an eta/countdown argument, not periodic tasks.

Dump of reserved tasks

Reserved tasks are tasks that has been received, but is still waiting to be executed.

You can get a list of these using reserved():

>>> i.reserved()
[{'worker1.example.com':
    [{"name": "tasks.sleeptask",
      "id": "32666e9b-809c-41fa-8e93-5ae0c80afbbf",
      "args": "(8,)",
      "kwargs": "{}"}]}]

Previous topic

Executing Tasks

Next topic

Periodic Tasks

This Page