This document describes the current stable version of Celery (5.6). For development docs, go here.

# “The Big Instance” Refactor

The app branch is a work-in-progress to remove the use of a global configuration in Celery.

Celery can now be instantiated and several instances of Celery may exist in the same process space. Also, large parts can be customized without resorting to monkey patching.

## Examples

Creating a Celery instance:

    >>> from celery import Celery
    >>> app = Celery()
    >>> app.config_from_object('celeryconfig')
    >>> #app.config_from_envvar('CELERY_CONFIG_MODULE')

Creating tasks:

    @app.task
    def add(x, y):
        return x + y

Creating custom Task subclasses:

    Task = celery.create_task_cls()

    class DebugTask(Task):

        def on_failure(self, *args, **kwargs):
            import pdb
            pdb.set_trace()

    @app.task(base=DebugTask)
    def add(x, y):
        return x + y

Starting a worker:

    worker = celery.Worker(loglevel='INFO')

Getting access to the configuration:

    celery.conf.task_always_eager = True
    celery.conf['task_always_eager'] = True

Controlling workers:

    >>> celery.control.inspect().active()
    >>> celery.control.rate_limit(add.name, '100/m')
    >>> celery.control.broadcast('shutdown')
    >>> celery.control.discard_all()

Other interesting attributes:

    # Establish broker connection.
    >>> celery.broker_connection()

    # AMQP Specific features.
    >>> celery.amqp
    >>> celery.amqp.Router
    >>> celery.amqp.get_queues()
    >>> celery.amqp.get_task_consumer()

    # Loader
    >>> celery.loader

    # Default backend
    >>> celery.backend

As you can probably see, this really opens up another dimension of customization abilities.

## Deprecated

- `celery.task.ping` `celery.task.PingTask`

  Inferior to the ping remote control command. Will be removed in Celery 2.3.

## Aliases (Pending deprecation)

- `celery.execute`

    - `.send_task` -> {`app.send_task`}

    - `.delay_task` -> no alternative

- `celery.log`

    - `.get_default_logger` -> {`app.log.get_default_logger`}

    - `.setup_logger` -> {`app.log.setup_logger`}

    - `.get_task_logger` -> {`app.log.get_task_logger`}

    - `.setup_task_logger` -> {`app.log.setup_task_logger`}

    - `.setup_logging_subsystem` -> {`app.log.setup_logging_subsystem`}

    - `.redirect_stdouts_to_logger` -> {`app.log.redirect_stdouts_to_logger`}

- `celery.messaging`

    - `.establish_connection` -> {`app.broker_connection`}

    - `.with_connection` -> {`app.with_connection`}

    - `.get_consumer_set` -> {`app.amqp.get_task_consumer`}

    - `.TaskPublisher` -> {`app.amqp.TaskPublisher`}

    - `.TaskConsumer` -> {`app.amqp.TaskConsumer`}

    - `.ConsumerSet` -> {`app.amqp.ConsumerSet`}

- `celery.conf.*` -> {`app.conf`}

  > NOTE: All configuration keys are now named the same as in the configuration. So the key `task_always_eager` is accessed as:

  >     >>> app.conf.task_always_eager

  > instead of:

  >     >>> from celery import conf
  >     >>> conf.always_eager

  > - `.get_queues` -> {`app.amqp.get_queues`}

- `celery.utils.info`

    - `.humanize_seconds` -> `celery.utils.time.humanize_seconds`

    - `.textindent` -> `celery.utils.textindent`

    - `.get_broker_info` -> {`app.amqp.get_broker_info`}

    - `.format_broker_info` -> {`app.amqp.format_broker_info`}

    - `.format_queues` -> {`app.amqp.format_queues`}

## Default App Usage

To be backward compatible, it must be possible to use all the classes/functions without passing an explicit app instance.

This is achieved by having all app-dependent objects use `default_app` if the app instance is missing.

    from celery.app import app_or_default

    class SomeClass:

        def __init__(self, app=None):
            self.app = app_or_default(app)

The problem with this approach is that there’s a chance that the app instance is lost along the way, and everything seems to be working normally. Testing app instance leaks is hard. The environment variable `CELERY_TRACE_APP` can be used, when this is enabled `celery.app.app_or_default()` will raise an exception whenever it has to go back to the default app instance.

### App Dependency Tree

- {`app`}

    - `celery.loaders.base.BaseLoader`

    - `celery.backends.base.BaseBackend`

    - {`app.TaskSet`}

        - `celery.task.sets.TaskSet` (`app.TaskSet`)

    - [`app.TaskSetResult`]

        - `celery.result.TaskSetResult` (`app.TaskSetResult`)

- {`app.AsyncResult`}

    - `celery.result.BaseAsyncResult` / `celery.result.AsyncResult`

- `celery.bin.worker.WorkerCommand`
    - `celery.apps.worker.Worker`
        - `celery.worker.WorkerController`
            - `celery.worker.consumer.Consumer`

                - `celery.worker.request.Request`

                - `celery.events.EventDispatcher`

                - `celery.worker.control.ControlDispatch`

                    - `celery.worker.control.registry.Panel`

                    - `celery.pidbox.BroadcastPublisher`

                - `celery.pidbox.BroadcastConsumer`

            - `celery.beat.EmbeddedService`

- `celery.bin.events.EvCommand`
    - `celery.events.snapshot.evcam`

        - `celery.events.snapshot.Polaroid`

        - `celery.events.EventReceiver`

    - `celery.events.cursesmon.evtop`

        - `celery.events.EventReceiver`

        - `celery.events.cursesmon.CursesMonitor`

    - `celery.events.dumper`

        - `celery.events.EventReceiver`

- `celery.bin.amqp.AMQPAdmin`

- `celery.bin.beat.BeatCommand`
    - `celery.apps.beat.Beat`
        - `celery.beat.Service`

            - `celery.beat.Scheduler`