celery/docs/whatsnew-4.0.rst

.. _whatsnew-4.0:

===========================================
 What's new in Celery 4.0 (0Today8)
===========================================
:Author: Ask Solem (``ask at celeryproject.org``)

.. sidebar:: Change history

    What's new documents describe the changes in major versions,
    we also have a :ref:`changelog` that lists the changes in bugfix
    releases (0.0.x), while older series are archived under the :ref:`history`
    section.

Celery is a simple, flexible, and reliable distributed system to
process vast amounts of messages, while providing operations with
the tools required to maintain such a system.

It's a task queue with focus on real-time processing, while also
supporting task scheduling.

Celery has a large and diverse community of users and contributors,
you should come join us :ref:`on IRC <irc-channel>`
or :ref:`our mailing-list <mailing-list>`.

To read more about Celery you should go read the :ref:`introduction <intro>`.

While this version is backward compatible with previous versions
it's important that you read the following section.

This version is officially supported on CPython 2.7, 3.4, and 3.5.
and also supported on PyPy.

.. _`website`: http://celeryproject.org/

.. topic:: Table of Contents

    Make sure you read the important notes before upgrading to this version.

.. contents::
    :local:
    :depth: 3

Preface
=======

XXX To be written


Wall of Contributors
--------------------

Aaron McMillin, Adam Chainz, Adam Renberg, Adriano Martins de Jesus,
Adrien Guinet, Ahmet Demir, Aitor Gómez-Goiri, Alan Justino,
Albert Wang, Alex Koshelev, Alex Rattray, Alex Williams, Alexander Koshelev,
Alexander Lebedev, Alexander Oblovatniy, Alexey Kotlyarov, Ali Bozorgkhan,
Alice Zoë Bevan–McGregor, Allard Hoeve, Alman One, Amir Rustamzadeh,
Andrea Rabbaglietti, Andrea Rosa, Andrei Fokau, Andrew Rodionoff,
Andrew Stewart, Andriy Yurchuk, Aneil Mallavarapu, Areski Belaid,
Armenak Baburyan, Arthur Vuillard, Artyom Koval, Asif Saifuddin Auvi,
Ask Solem, Balthazar Rouberol, Batiste Bieler, Berker Peksag,
Bert Vanderbauwhede, Brendan Smithyman, Brian Bouterse, Bryce Groff,
Cameron Will, ChangBo Guo, Chris Clark, Chris Duryee, Chris Erway,
Chris Harris, Chris Martin, Chillar Anand, Colin McIntosh, Conrad Kramer,
Corey Farwell, Craig Jellick, Cullen Rhodes, Dallas Marlow, Daniel Devine,
Daniel Wallace, Danilo Bargen, Davanum Srinivas, Dave Smith, David Baumgold,
David Harrigan, David Pravec, Dennis Brakhane, Derek Anderson,
Dmitry Dygalo, Dmitry Malinovsky, Dongweiming, Dudás Ádám,
Dustin J. Mitchell, Ed Morley, Edward Betts, Éloi Rivard, Emmanuel Cazenave,
Fahad Siddiqui, Fatih Sucu, Feanil Patel, Federico Ficarelli, Felix Schwarz,
Felix Yan, Fernando Rocha, Flavio Grossi, Frantisek Holop, Gao Jiangmiao,
George Whewell, Gerald Manipon, Gilles Dartiguelongue, Gino Ledesma, Greg Wilbur,
Guillaume Seguin, Hank John, Hogni Gylfason, Ilya Georgievsky,
Ionel Cristian Mărieș, Ivan Larin, James Pulec, Jared Lewis, Jason Veatch,
Jasper Bryant-Greene, Jeff Widman, Jeremy Tillman, Jeremy Zafran,
Jocelyn Delalande, Joe Jevnik, Joe Sanford, John Anderson, John Barham,
John Kirkham, John Whitlock, Jonathan Vanasco, Joshua Harlow, João Ricardo,
Juan Carlos Ferrer, Juan Rossi, Justin Patrin, Kai Groner, Kevin Harvey,
Kevin Richardson, Komu Wairagu, Konstantinos Koukopoulos, Kouhei Maeda,
Kracekumar Ramaraju, Krzysztof Bujniewicz, Latitia M. Haskins, Len Buckens,
Lev Berman, lidongming, Lorenzo Mancini, Lucas Wiman, Luke Pomfrey,
Luyun Xie, Maciej Obuchowski, Manuel Kaufmann, Marat Sharafutdinov,
Marc Sibson, Marcio Ribeiro, Marin Atanasov Nikolov, Mathieu Fenniak,
Mark Parncutt, Maxime Beauchemin, Maxime Vdb, Mher Movsisyan, Michael Aquilina,
Michael Duane Mooring, Michael Permana, Mickaël Penhard, Mike Attwood,
Mitchel Humpherys, Mohamed Abouelsaoud, Morris Tweed, Morton Fox,
Môshe van der Sterre, Nat Williams, Nathan Van Gheem, Nicolas Unravel,
Nik Nyby, Omer Katz, Omer Korner, Ori Hoch, Paul Pearce, Paulo Bu,
Pavlo Kapyshin, Philip Garnero, Pierre Fersing, Piotr Kilczuk,
Piotr Maślanka, Quentin Pradet, Radek Czajka, Raghuram Srinivasan,
Randy Barlow, Raphael Michel, Rémy Léone, Robert Coup, Robert Kolba,
Rockallite Wulf, Rodolfo Carvalho, Roger Hu, Romuald Brunet, Rongze Zhu,
Ross Deane, Ryan Luckie, Rémy Greinhofer, Samuel Giffard, Samuel Jaillet,
Sergey Azovskov, Sergey Tikhonov, Seungha Kim, Simon Peeters,
Spencer E. Olson, Srinivas Garlapati, Stephen Milner, Steve Peak, Stuart Axon,
Sukrit Khera, Tadej Janež, Taha Jahangir, Takeshi Kanemoto, Tayfun Sen,
Tewfik Sadaoui, Thomas French, Thomas Grainger, Tomas Machalek,
Tobias Schottdorf, Tocho Tochev, Valentyn Klindukh, Vic Kumar,
Vladimir Bolshakov, Vladimir Gorbunov, Wayne Chang, Wieland Hoffmann,
Wido den Hollander, Wil Langford, Will Thompson, William King, Yury Selivanov,
Vytis Banaitis, Zoran Pavlovic, Xin Li, 許邱翔, :github_user:`allenling`,
:github_user:`alzeih`, :github_user:`bastb`, :github_user:`bee-keeper`,
:github_user:`ffeast`, :github_user:`firefly4268`,
:github_user:`flyingfoxlee`, :github_user:`gdw2`, :github_user:`gitaarik`,
:github_user:`hankjin`, :github_user:`lvh`, :github_user:`m-vdb`,
:github_user:`kindule`, :github_user:`mdk`:, :github_user:`michael-k`,
:github_user:`mozillazg`, :github_user:`nokrik`, :github_user:`ocean1`,
:github_user:`orlo666`, :github_user:`raducc`, :github_user:`wanglei`,
:github_user:`worldexception`, :github_user:`xBeAsTx`.

Upgrading from Celery 3.1
=========================

Step 1: Upgrade to Celery 3.1.25
--------------------------------

If you haven't already, the first step is to upgrade to Celery 3.1.25.

This version adds foreward compatibility to the new message protocol,
so that you can incrementally upgrade from 3.1 to 4.0.

Deploy the workers first by upgrading to 3.1.25, this means these
workers can process messages sent by clients using both 3.1 and 4.0.

After the workers are upgraded you can upgrade the clients (e.g. web servers).

Step 2: Update your configuration with the new setting names
------------------------------------------------------------

This version radically changes the configuration setting names,
to be more consistent.

The changes are fully backwards compatible, so you have the option to wait
until the old setting names are deprecated, but to ease the transition
we have included a command-line utility that rewrites your settings
automatically.

See :ref:`v400-upgrade-settings` for more information.

Step 3: Read the important notes in this document
-------------------------------------------------

Make sure you are not affected by any of the important upgrade notes
mentioned in the following section.

An especially important note is that Celery now checks the arguments
you send to a task by matching it to the signature (:ref:`v400-typing`).

Step 4: Upgrade to Celery 4.0
-----------------------------

At this point you can upgrade your workers and clients with the new version.

.. _v400-important:

Important Notes
===============

Dropped support for Python 2.6
------------------------------

Celery now requires Python 2.7 or later,
and also drops support for Python 3.3 so supported versions are:

- CPython 2.7
- CPython 3.4
- CPython 3.5
- PyPy 5.4 (``pypy2``)
- PyPy 5.5-alpha (``pypy3``)

Last major version to support Python 2
--------------------------------------

Starting from Celery 5.0 only Python 3.5+ will be supported.

To make sure you're not affected by this change you should pin
the Celery version in your requirements file, either to a specific
version: ``celery==4.0.0``, or a range: ``celery>=4.0,<5.0``.

Dropping support for Python 2 will enable us to remove massive
amounts of compatibility code, and going with Python 3.5 allows
us to take advantage of typing, async/await, asyncio, and similar
concepts there's no alternative for in older versions.

Celery 4.x will continue to work on Python 2.7, 3.4, 3.5; just as Celery 3.x
still works on Python 2.6.

Django support
--------------

Celery 4.x requires Django 1.8 or later, but we really recommend
using at least Django 1.9 for the new ``transaction.on_commit`` feature.

A common problem when calling tasks from Django is when the task is related
to a model change, and you wish to cancel the task if the transaction is
rolled back, or ensure the task is only executed after the changes have been
written to the database.

``transaction.on_commit`` enables you to solve this problem by adding
the task as a callback to be called only when the transaction is committed.

Example usage:

.. code-block:: python

    from functools import partial
    from django.db import transaction

    from .models import Article, Log
    from .tasks import send_article_created_notification

    def create_article(request):
        with transaction.atomic():
            article = Article.objects.create(**request.POST)
            # send this task only if the rest of the transaction succeeds.
            transaction.on_commit(partial(
                send_article_created_notification.delay, article_id=article.pk)
            Log.objects.create(type=Log.ARTICLE_CREATED, object_pk=article.pk)

Removed features
----------------

- Microsoft Windows is no longer supported.

  The test suite is passing, and Celery seems to be working with Windows,
  but we make no guarantees as we are unable to diagnose issues on this
  platform.

- Jython is no longer supported.

Features removed for simplicity
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- Webhook task machinery (``celery.task.http``) has been removed.

    Nowadays it's easy to use the :pypi:`requests` module to write
    webhook tasks manually. We would love to use requests but we
    are simply unable to as there's a very vocal 'anti-dependency'
    mob in the Python community

    If you need backwards compatibility
    you can simply copy + paste the 3.1 version of the module and make sure
    it's imported by the worker:
    https://github.com/celery/celery/blob/3.1/celery/task/http.py

- Task no longer sends error emails.

    This also removes support for ``app.mail_admins``, and any functionality
    related to sending emails.

- ``celery.contrib.batches`` has been removed.

    This was an experimental feature, so not covered by our deprecation
    timeline guarantee.

    You can copy and pase the existing batches code for use within your projects:
    https://github.com/celery/celery/blob/3.1/celery/contrib/batches.py

Features removed for lack of funding
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We announced with the 3.1 release that some transports were
moved to experimental status, and that there'd be no official
support for the transports, citing a lack of resources.

As this subtle hint for the need of funding failed
we've removed them completely, breaking backwards compatibility.

- Using the Django ORM as a broker is no longer supported.

    You can still use the Django ORM as a result backend:
    see :ref:`django-celery-results` section for more information.

- Using SQLAlchemy as a broker is no longer supported.

    You can still use SQLAlchemy as a result backend.

- Using CouchDB as a broker is no longer supported.

    You can still use CouchDB as a result backend.

- Using IronMQ as a broker is no longer supported.

- Using Beanstalk as a broker is no longer supported.

In addition some features have been removed completely so that
attempting to use them will raise an exception:

- The ``--autoreload`` feature has been removed.

  This was an experimental feature, and not covered by our deprecation
  timeline guarantee. The flag is removed completely so the worker
  will crash at startup when present. Luckily this
  flag isn't used in production systems.

- The experimental ``threads`` pool is no longer supported and has been removed.

- The force_execv feature is no longer supported.

    The ``celery worker`` command now ignores the ``--no-execv``,
    ``--force-execv``, and the ``CELERYD_FORCE_EXECV`` setting.

    This flag will be removed completely in 5.0 and the worker
    will raise an error.

- The old legacy "amqp" result backend has been deprecated, and will
  be removed in Celery 5.0.

    Please use the ``rpc`` result backend for RPC-style calls, and a
    persistent result backend for multi-consumer results.

**Now to the good news**...

New Task Message Protocol
-------------------------
.. :sha:`e71652d384b1b5df2a4e6145df9f0efb456bc71c`

This version introduces a brand new task message protocol,
the first major change to the protocol since the beginning of the project.

The new protocol is enabled by default in this version and since the new
version isn't backwards compatible you have to be careful when upgrading.

The 3.1.25 version was released to add compatibility with the new protocol
so the easiest way to upgrade is to upgrade to that version first, then
upgrade to 4.0 in a second deployment.

If you wish to keep using the old protocol you may also configure
the protocol version number used:

.. code-block:: python

    app = Celery()
    app.conf.task_protocol = 1

Read more about the features available in the new protocol in the news
section found later in this document.

.. _v400-upgrade-settings:

Lowercase setting names
-----------------------

In the pursuit of beauty all settings are now renamed to be in all
lowercase and some setting names have been renamed for consistency.

This change is fully backwards compatible so you can still use the uppercase
setting names, but we would like you to upgrade as soon as possible and
you can this automatically using the :program:`celery upgrade settings`
command:

.. code-block:: console

    $ celery upgrade settings proj/settings.py

This command will modify your module in-place to use the new lower-case
names (if you want uppercase with a "``CELERY``" prefix see block below),
and save a backup in :file:`proj/settings.py.orig`.

.. admonition:: For Django users and others who want to keep uppercase names

    If you're loading Celery configuration from the Django settings module
    then you'll want to keep using the uppercase names.

    You also want to use a ``CELERY_`` prefix so that no Celery settings
    collide with Django settings used by other apps.

    To do this, you'll first need to convert your settings file
    to use the new consistent naming scheme, and add the prefix to all
    Celery related settings:

    .. code-block:: console

        $ celery upgrade settings --django proj/settings.py

    After upgrading the settings file, you need to set the prefix explicitly
    in your ``proj/celery.py`` module:

    .. code-block:: python

        app.config_from_object('django.conf:settings', namespace='CELERY')

    You can find the most up to date Django Celery integration example
    here: :ref:`django-first-steps`.

    .. note::

        This will also add a prefix to settings that didn't previously
        have one, for example ``BROKER_URL`` should be written
        ``CELERY_BROKER_URL`` with a namespace of ``CELERY``
        ``CELERY_BROKER_URL``.

    Luckily you don't have to manually change the files, as
    the :program:`celery upgrade settings --django` program should do the
    right thing.

The loader will try to detect if your configuration is using the new format,
and act accordingly, but this also means you're not allowed to mix and
match new and old setting names, that's unless you provide a value for both
alternatives.

The major difference between previous versions, apart from the lower case
names, are the renaming of some prefixes, like ``celerybeat_`` to ``beat_``,
``celeryd_`` to ``worker_``.

The ``celery_`` prefix has also been removed, and task related settings
from this name-space is now prefixed by ``task_``, worker related settings
with ``worker_``.

Apart from this most of the settings will be the same in lowercase, apart from
a few special ones:

=====================================  ==========================================================
**Setting name**                       **Replace with**
=====================================  ==========================================================
``CELERY_MAX_CACHED_RESULTS``          :setting:`result_cache_max`
``CELERY_MESSAGE_COMPRESSION``         :setting:`result_compression`/:setting:`task_compression`.
``CELERY_TASK_RESULT_EXPIRES``         :setting:`result_expires`
``CELERY_RESULT_DBURI``                :setting:`result_backend`
``CELERY_RESULT_ENGINE_OPTIONS``       :setting:`database_engine_options`
``-*-_DB_SHORT_LIVED_SESSIONS``        :setting:`database_short_lived_sessions`
``CELERY_RESULT_DB_TABLE_NAMES``       :setting:`database_db_names`
``CELERY_ACKS_LATE``                   :setting:`task_acks_late`
``CELERY_ALWAYS_EAGER``                :setting:`task_always_eager`
``CELERY_ANNOTATIONS``                 :setting:`task_annotations`
``CELERY_MESSAGE_COMPRESSION``         :setting:`task_compression`
``CELERY_CREATE_MISSING_QUEUES``       :setting:`task_create_missing_queues`
``CELERY_DEFAULT_DELIVERY_MODE``       :setting:`task_default_delivery_mode`
``CELERY_DEFAULT_EXCHANGE``            :setting:`task_default_exchange`
``CELERY_DEFAULT_EXCHANGE_TYPE``       :setting:`task_default_exchange_type`
``CELERY_DEFAULT_QUEUE``               :setting:`task_default_queue`
``CELERY_DEFAULT_RATE_LIMIT``          :setting:`task_default_rate_limit`
``CELERY_DEFAULT_ROUTING_KEY``         :setting:`task_default_routing_key`
``-"-_EAGER_PROPAGATES_EXCEPTIONS``    :setting:`task_eager_propagates`
``CELERY_IGNORE_RESULT``               :setting:`task_ignore_result`
``CELERY_TASK_PUBLISH_RETRY``          :setting:`task_publish_retry`
``CELERY_TASK_PUBLISH_RETRY_POLICY``   :setting:`task_publish_retry_policy`
``CELERY_QUEUES``                      :setting:`task_queues`
``CELERY_ROUTES``                      :setting:`task_routes`
``CELERY_SEND_TASK_SENT_EVENT``        :setting:`task_send_sent_event`
``CELERY_TASK_SERIALIZER``             :setting:`task_serializer`
``CELERYD_TASK_SOFT_TIME_LIMIT``       :setting:`task_soft_time_limit`
``CELERYD_TASK_TIME_LIMIT``            :setting:`task_time_limit`
``CELERY_TRACK_STARTED``               :setting:`task_track_started`
``CELERY_DISABLE_RATE_LIMITS``         :setting:`worker_disable_rate_limits`
``CELERY_ENABLE_REMOTE_CONTROL``       :setting:`worker_enable_remote_control`
``CELERYD_SEND_EVENTS``                :setting:`worker_send_task_events`
=====================================  ==========================================================

You can see a full table of the changes in :ref:`conf-old-settings-map`.

Json is now the default serializer
----------------------------------

The time has finally come to end the reign of :mod:`pickle` as the default
serialization mechanism, and json is the default serializer starting from this
version.

This change was :ref:`announced with the release of Celery 3.1
<last-version-to-enable-pickle>`.

If you're still depending on :mod:`pickle` being the default serializer,
then you have to configure your app before upgrading to 4.0:

.. code-block:: python

    task_serializer = 'pickle'
    result_serializer = 'pickle'
    accept_content = {'pickle'}


The Json serializer now also supports some additional types:

- :class:`~datetime.datetime`, :class:`~datetime.time`, :class:`~datetime.date`

    Converted to json text, in ISO-8601 format.

- :class:`~decimal.Decimal`

    Converted to json text.

- :class:`django.utils.functional.Promise`

    Django only: Lazy strings used for translation etc., are evaluated
    and conversion to a json type is attempted.

- :class:`uuid.UUID`

    Converted to json text.

You can also define a ``__json__`` method on your custom classes to support
JSON serialization (must return a json compatible type):

.. code-block:: python

    class Person:
        first_name = None
        last_name = None
        address = None

        def __json__(self):
            return {
                'first_name': self.first_name,
                'last_name': self.last_name,
                'address': self.address,
            }

The Task base class no longer automatically register tasks
----------------------------------------------------------

The :class:`~@Task` class is no longer using a special meta-class
that automatically registers the task in the task registry.

Instead this is now handled by the :class:`@task` decorators.

If you're still using class based tasks, then you need to register
these manually:

.. code-block:: python

    class CustomTask(Task):
        def run(self):
            print('running')
    app.tasks.register(CustomTask())

The best practice is to use custom task classes only for overriding
general behavior, and then using the task decorator to realize the task:

.. code-block:: python

    @app.task(bind=True, base=CustomTask)
    def custom(self):
        print('running')

This change also means that the ``abstract`` attribute of the task
no longer has any effect.

.. _v400-typing:

Task argument checking
----------------------

The arguments of the task are now verified when calling the task,
even asynchronously:

.. code-block:: pycon

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

    >>> add.delay(8, 8)
    <AsyncResult: f59d71ca-1549-43e0-be41-4e8821a83c0c>

    >>> add.delay(8)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "celery/app/task.py", line 376, in delay
        return self.apply_async(args, kwargs)
      File "celery/app/task.py", line 485, in apply_async
        check_arguments(*(args or ()), **(kwargs or {}))
    TypeError: add() takes exactly 2 arguments (1 given)

You can disable the argument checking for any task by setting its
:attr:`~@Task.typing` attribute to :const:`False`:

.. code-block:: pycon

    >>> @app.task(typing=False)
    ... def add(x, y):
    ...     return x + y

Or if you would like to disable this completely for all tasks
you can pass ``strict_typing=False`` when creating the app:

.. code-block::

    app = Celery(..., strict_typing=False)

Redis Events not backward compatible
------------------------------------

The Redis ``fanout_patterns`` and ``fanout_prefix`` transport
options are now enabled by default.

Workers/monitors without these flags enabled won't be able to
see workers with this flag disabled. They can still execute tasks,
but they cannot receive each others monitoring messages.

You can upgrade in a backward compatible manner by first configuring
your 3.1 workers and monitors to enable the settings, before the final
upgrade to 4.0:

.. code-block:: python

    BROKER_TRANSPORT_OPTIONS = {
        'fanout_patterns': True,
        'fanout_prefix': True,
    }

Redis Priorities Reversed
-------------------------

Priority 0 is now lowest, 9 is highest.

This change was made to make priority support consistent with how
it works in AMQP.

Contributed by **Alex Koshelev**.

Django: Auto-discover now supports Django app configurations
------------------------------------------------------------

The ``autodiscover_tasks()`` function can now be called without arguments,
and the Django handler will automatically find your installed apps:

.. code-block:: python

    app.autodiscover_tasks()

The Django integration :ref:`example in the documentation
<django-first-steps>` has been updated to use the argument-less call.

This also ensures compatibility with the new, ehm, ``AppConfig`` stuff
introduced in recent Django versions.

Worker direct queues no longer use auto-delete
----------------------------------------------

Workers/clients running 4.0 will no longer be able to send
worker direct messages to workers running older versions, and vice versa.

If you're relying on worker direct messages you should upgrade
your 3.x workers and clients to use the new routing settings first,
by replacing :func:`celery.utils.worker_direct` with this implementation:

.. code-block:: python

    from kombu import Exchange, Queue

    worker_direct_exchange = Exchange('C.dq2')

    def worker_direct(hostname):
        return Queue(
            '{hostname}.dq2'.format(hostname),
            exchange=worker_direct_exchange,
            routing_key=hostname,
        )

This feature closed Issue #2492.


Old command-line programs removed
---------------------------------

Installing Celery will no longer install the ``celeryd``,
``celerybeat`` and ``celeryd-multi`` programs.

This was announced with the release of Celery 3.1, but you may still
have scripts pointing to the old names, so make sure you update these
to use the new umbrella command:

+-------------------+--------------+-------------------------------------+
| Program           | New Status   | Replacement                         |
+===================+==============+=====================================+
| ``celeryd``       | **REMOVED**  | :program:`celery worker`            |
+-------------------+--------------+-------------------------------------+
| ``celerybeat``    | **REMOVED**  | :program:`celery beat`              |
+-------------------+--------------+-------------------------------------+
| ``celeryd-multi`` | **REMOVED**  | :program:`celery multi`             |
+-------------------+--------------+-------------------------------------+

.. _v400-news:

News
====

New protocol highlights
-----------------------

The new protocol fixes many problems with the old one, and enables
some long-requested features:

- Most of the data are now sent as message headers, instead of being
  serialized with the message body.

    In version 1 of the protocol the worker always had to deserialize
    the message to be able to read task meta-data like the task id,
    name, etc. This also meant that the worker was forced to double-decode
    the data, first deserializing the message on receipt, serializing
    the message again to send to child process, then finally the child process
    deserializes the message again.

    Keeping the meta-data fields in the message headers means the worker
    doesn't actually have to decode the payload before delivering
    the task to the child process, and also that it's now possible
    for the worker to reroute a task written in a language different
    from Python to a different worker.

- A new ``lang`` message header can be used to specify the programming
  language the task is written in.

- Worker stores results for internal errors like ``ContentDisallowed``,
  and other deserialization errors.

- Worker stores results and sends monitoring events for unregistered
  task errors.

- Worker calls callbacks/errbacks even when the result is sent by the
  parent process (e.g., :exc:`WorkerLostError` when a child process
  terminates, deserialization errors, unregistered tasks).

- A new ``origin`` header contains information about the process sending
  the task (worker node-name, or PID and host-name information).

- A new ``shadow`` header allows you to modify the task name used in logs.

    This is useful for dispatch like patterns, like a task that calls
    any function using pickle (don't do this at home):

    .. code-block:: python

        from celery import Task
        from celery.utils.imports import qualname

        class call_as_task(Task):

            def shadow_name(self, args, kwargs, options):
                return 'call_as_task:{0}'.format(qualname(args[0]))

            def run(self, fun, *args, **kwargs):
                return fun(*args, **kwargs)
        call_as_task = app.tasks.register(call_as_task())

- New ``argsrepr`` and ``kwargsrepr`` fields contain textual representations
  of the task arguments (possibly truncated) for use in logs, monitors, etc.

    This means the worker doesn't have to deserialize the message payload
    to display the task arguments for informational purposes.

- Chains now use a dedicated ``chain`` field enabling support for chains
  of thousands and more tasks.

- New ``parent_id`` and ``root_id`` headers adds information about
  a tasks relationship with other tasks.

    - ``parent_id`` is the task id of the task that called this task
    - ``root_id`` is the first task in the work-flow.

    These fields can be used to improve monitors like flower to group
    related messages together (like chains, groups, chords, complete
    work-flows, etc).

- ``app.TaskProducer`` replaced by :meth:`@amqp.create_task_message` and
  :meth:`@amqp.send_task_message`.

    Dividing the responsibilities into creating and sending means that
    people who want to send messages using a Python AMQP client directly,
    doesn't have to implement the protocol.

    The :meth:`@amqp.create_task_message` method calls either
    :meth:`@amqp.as_task_v2`, or :meth:`@amqp.as_task_v1` depending
    on the configured task protocol, and returns a special
    :class:`~celery.app.amqp.task_message` tuple containing the
    headers, properties and body of the task message.

.. seealso::

    The new task protocol is documented in full here:
    :ref:`message-protocol-task-v2`.

Prefork Pool Improvements
-------------------------

Tasks now log from the child process
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Logging of task success/failure now happens from the child process
executing the task.  As a result logging utilities,
like Sentry can get full information about tasks, including
variables in the traceback stack.

``-Ofair`` is now the default scheduling strategy
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To re-enable the default behavior in 3.1 use the ``-Ofast`` command-line
option.

There's been lots of confusion about what the ``-Ofair`` command-line option
does, and using the term "prefetch" in explanations have probably not helped
given how confusing this terminology is in AMQP.

When a Celery worker using the prefork pool receives a task, it needs to
delegate that task to a child process for execution.

The prefork pool has a configurable number of child processes
(``--concurrency``) that can be used to execute tasks, and each child process
uses pipes/sockets to communicate with the parent process:

- inqueue (pipe/socket): parent sends task to the child process
- outqueue (pipe/socket): child sends result/return value to the parent.

In Celery 3.1 the default scheduling mechanism was simply to send
the task to the first ``inqueue`` that was writable, with some heuristics
to make sure we round-robin between them to ensure each child process
would receive the same amount of tasks.

This means that in the default scheduling strategy, a worker may send
tasks to the same child process that is already executing a task.  If that
task is long running, it may block the waiting task for a long time.  Even
worse, hundreds of short-running tasks may be stuck behind a long running task
even when there are child processes free to do work.

The ``-Ofair`` scheduling strategy was added to avoid this situation,
and when enabled it adds the rule that no task should be sent to the a child
process that is already executing a task.

The fair scheduling strategy may perform slightly worse if you have only
short running tasks.

Limit child process resident memory size
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. :sha:`5cae0e754128750a893524dcba4ae030c414de33`

You can now limit the maximum amount of memory allocated per prefork
pool child process by setting the worker
:option:`--max-memory-per-child <celery worker --max-memory-per-child>` option,
or the :setting:`worker_max_memory_per_child` setting.

The limit is for RSS/resident memory size and is specified in kilobytes.

A child process having exceeded the limit will be terminated and replaced
with a new process after the currently executing task returns.

See :ref:`worker-max-memory-per-child` for more information.

Contributed by **Dave Smith**.

One log-file per child process
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Init-scrips and :program:`celery multi` now uses the `%I` log file format
option (e.g., :file:`/var/log/celery/%n%I.log`).

This change was necessary to ensure each child
process has a separate log file after moving task logging
to the child process, as multiple processes writing to the same
log file can cause corruption.

You're encouraged to upgrade your init-scripts and
:program:`celery multi` arguments to use this new option.

Transports
----------

RabbitMQ priority queue support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See :ref:`routing-options-rabbitmq-priorities` for more information.

Contributed by **Gerald Manipon**.

Configure broker URL for read/write separately
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

New :setting:`broker_read_url` and :setting:`broker_write_url` settings
have been added so that separate broker URLs can be provided
for connections used for consuming/publishing.

In addition to the configuration options, two new methods have been
added the app API:

    - ``app.connection_for_read()``
    - ``app.connection_for_write()``

These should now be used in place of ``app.connection()`` to specify
the intent of the required connection.

.. note::

    Two connection pools are available: ``app.pool`` (read), and
    ``app.producer_pool`` (write). The latter doesn't actually give connections
    but full :class:`kombu.Producer` instances.

    .. code-block:: python

        def publish_some_message(app, producer=None):
            with app.producer_or_acquire(producer) as producer:
                ...

        def consume_messages(app, connection=None):
            with app.connection_or_acquire(connection) as connection:
                ...

RabbitMQ queue extensions support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Queue declarations can now set a message TTL and queue expiry time directly,
by using the ``message_ttl`` and ``expires`` arguments

New arguments have been added to :class:`~kombu.Queue` that lets
you directly and conveniently configure RabbitMQ queue extensions
in queue declarations:

- ``Queue(expires=20.0)``

    Set queue expiry time in float seconds.

    See :attr:`kombu.Queue.expires`.

- ``Queue(message_ttl=30.0)``

    Set queue message time-to-live float seconds.

    See :attr:`kombu.Queue.message_ttl`.

- ``Queue(max_length=1000)``

    Set queue max length (number of messages) as int.

    See :attr:`kombu.Queue.max_length`.

- ``Queue(max_length_bytes=1000)``

    Set queue max length (message size total in bytes) as int.

    See :attr:`kombu.Queue.max_length_bytes`.

- ``Queue(max_priority=10)``

    Declare queue to be a priority queue that routes messages
    based on the ``priority`` field of the message.

    See :attr:`kombu.Queue.max_priority`.

Amazon SQS transport now officially supported
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The SQS broker transport has been rewritten to use async I/O and as such
joins RabbitMQ, Redis and QPid as officially supported transports.

The new implementation also takes advantage of long polling,
and closes several issues related to using SQS as a broker.

This work was sponsored by Nextdoor.

Apache QPid transport now officially supported
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Contributed by **Brian Bouterse**.

Redis: Support for Sentinel
---------------------------

You can point the connection to a list of sentinel URLs like:

.. code-block:: text

    sentinel://0.0.0.0:26379;sentinel://0.0.0.0:26380/...

where each sentinel is separated by a `;`. Multiple sentinels are handled
by :class:`kombu.Connection` constructor, and placed in the alternative
list of servers to connect to in case of connection failure.

Contributed by **Sergey Azovskov**, and **Lorenzo Mancini**.

Tasks
-----

Task Auto-retry Decorator
~~~~~~~~~~~~~~~~~~~~~~~~~

Writing custom retry handling for exception events is so common
that we now have built-in support for it.

For this a new ``autoretry_for`` argument is now supported by
the task decorators, where you can specify a tuple of exceptions
to automatically retry for:

.. code-block:: python

    from twitter.exceptions import FailWhaleError

    @app.task(autoretry_for=(FailWhaleError,))
    def refresh_timeline(user):
        return twitter.refresh_timeline(user)

See :ref:`task-autoretry` for more information.

Contributed by **Dmitry Malinovsky**.

.. :sha:`75246714dd11e6c463b9dc67f4311690643bff24`

``Task.replace`` Improvements
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- ``self.replace(signature)`` can now replace any task, chord or group,
  and the signature to replace with can be a chord, group or any other
  type of signature.

- No longer inherits the callbacks and errbacks of the existing task.

    If you replace a node in a tree, then you wouldn't expect the new node to
    inherit the children of the old node.

- ``Task.replace_in_chord`` has been removed, use ``.replace`` instead.

- If the replacement is a group, that group will be automatically converted
  to a chord, where the callback "accumulates" the results of the group tasks.

    A new built-in task (`celery.accumulate` was added for this purpose)

Contributed by **Steeve Morin**, and **Ask Solem**.

Remote Task Tracebacks
~~~~~~~~~~~~~~~~~~~~~~

The new :setting:`task_remote_tracebacks` will make task tracebacks more
useful by injecting the stack of the remote worker.

This feature requires the additional :pypi:`tblib` library.

Contributed by **Ionel Cristian Mărieș**.

Handling task connection errors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Connection related errors occuring while sending a task is now re-raised
as a :exc:`kombu.exceptions.OperationalError` error:

.. code-block:: pycon

    >>> try:
    ...     add.delay(2, 2)
    ... except add.OperationalError as exc:
    ...     print('Could not send task %r: %r' % (add, exc))

See :ref:`calling-connection-errors` for more information.

Gevent/Eventlet: Dedicated thread for consuming results
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When using :pypi:`gevent`, or :pypi:`eventlet` there is now a single
thread responsible for consuming events.

This means that if you have many calls retrieving results, there will be
a dedicated thread for consuming them:

.. code-block:: python


    result = add.delay(2, 2)

    # this call will delegate to the result consumer thread:
    #   once the consumer thread has received the result this greenlet can
    # continue.
    value = result.get(timeout=3)

This makes performing RPC calls when using gevent/eventlet perform much
better.

``AsyncResult.then(on_success, on_error)``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The AsyncResult API has been extended to support the :class:`~vine.promise` protocol.

This currently only works with the RPC (amqp) and Redis result backends, but
lets you attach callbacks to when tasks finish:

.. code-block:: python

    import gevent.monkey
    monkey.patch_all()

    import time
    from celery import Celery

    app = Celery(broker='amqp://', backend='rpc')

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

    def on_result_ready(result):
        print('Received result for id %r: %r' % (result.id, result.result,))

    add.delay(2, 2).then(on_result_ready)

    time.sleep(3)  # run gevent event loop for a while.

Demonstrated using gevent here, but really this is an API that's more useful
in callback-based event loops like :pypi:`twisted`, or :pypi:`tornado`.

New Task Router API
~~~~~~~~~~~~~~~~~~~

The :setting:`task_routes` setting can now hold functions, and map routes
now support glob patterns and regexes.

Instead of using router classes you can now simply define a function:

.. code-block:: python

    def route_for_task(name, args, kwargs, options, task=None, **kwargs):
        from proj import tasks

        if name == tasks.add.name:
            return {'queue': 'hipri'}

If you don't need the arguments you can use start arguments, just make
sure you always also accept star arguments so that we have the ability
to add more features in the future:

.. code-block:: python

    def route_for_task(name, *args, **kwargs):
        from proj import tasks
        if name == tasks.add.name:
            return {'queue': 'hipri', 'priority': 9}

Both the ``options`` argument and the new ``task`` keyword argument
are new to the function-style routers, and will make it easier to write
routers based on execution options, or properties of the task.

The optional ``task`` keyword argument won't be set if a task is called
by name using :meth:`@send_task`.

For more examples, including using glob/regexes in routers please see
:setting:`task_routes` and :ref:`routing-automatic`.

Canvas Refactor
~~~~~~~~~~~~~~~

The canvas/work-flow implementation have been heavily refactored
to fix some long outstanding issues.

.. :sha:`d79dcd8e82c5e41f39abd07ffed81ca58052bcd2`
.. :sha:`1e9dd26592eb2b93f1cb16deb771cfc65ab79612`
.. :sha:`e442df61b2ff1fe855881c1e2ff9acc970090f54`
.. :sha:`0673da5c09ac22bdd49ba811c470b73a036ee776`

- Error callbacks can now take real exception and traceback instances
  (Issue #2538).

    .. code-block:: pycon

        >>> add.s(2, 2).on_error(log_error.s()).delay()

    Where ``log_error`` could be defined as:

    .. code-block:: python

        @app.task
        def log_error(request, exc, traceback):
            with open(os.path.join('/var/errors', request.id), 'a') as fh:
                print('--\n\n{0} {1} {2}'.format(
                    task_id, exc, traceback), file=fh)

    See :ref:`guide-canvas` for more examples.

- ``chain(a, b, c)`` now works the same as ``a | b | c``.

    This means chain may no longer return an instance of ``chain``,
    instead it may optimize the workflow so that e.g. two groups
    chained together becomes one group.

- Now unrolls groups within groups into a single group (Issue #1509).
- chunks/map/starmap tasks now routes based on the target task
- chords and chains can now be immutable.
- Fixed bug where serialized signatures weren't converted back into
  signatures (Issue #2078)

    Fix contributed by **Ross Deane**.

- Fixed problem where chains and groups didn't work when using JSON
  serialization (Issue #2076).

    Fix contributed by **Ross Deane**.

- Creating a chord no longer results in multiple values for keyword
  argument 'task_id' (Issue #2225).

    Fix contributed by **Aneil Mallavarapu**.

- Fixed issue where the wrong result is returned when a chain
  contains a chord as the penultimate task.

    Fix contributed by **Aneil Mallavarapu**.

- Special case of ``group(A.s() | group(B.s() | C.s()))`` now works.

- Chain: Fixed bug with incorrect id set when a subtask is also a chain.

- ``group | group`` is now flattened into a single group (Issue #2573).

- Fixed issue where ``group | task`` wasn't upgrading correctly
  to chord (Issue #2922).

- Chords now properly sets ``result.parent`` links.

- ``chunks``/``map``/``starmap`` are now routed based on the target task.

- ``Signature.link`` now works when argument is scalar (not a list)
    (Issue #2019).

- ``group()`` now properly forwards keyword arguments (Issue #3426).

    Fix contributed by **Samuel Giffard**.

- A ``chord`` where the header group only consists of a single task
  is now turned into a simple chain.

- Passing a ``link`` argument to ``group.apply_async()`` now raises an error
  (Issue #3508).

- ``chord | sig`` now attaches to the chord callback (Issue #3356).

Periodic Tasks
--------------

New API for configuring periodic tasks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This new API enables you to use signatures when defining periodic tasks,
removing the chance of mistyping task names.

An example of the new API is :ref:`here <beat-entries>`.

.. :sha:`bc18d0859c1570f5eb59f5a969d1d32c63af764b`
.. :sha:`132d8d94d38f4050db876f56a841d5a5e487b25b`

Optimized Beat implementation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The :program:`celery beat` implementation has been optimized
for millions of periodic tasks by using a heap to schedule entries.

Contributed by **Ask Solem** and **Alexander Koshelev**.

Schedule tasks based on sunrise, sunset, dawn and dusk
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See :ref:`beat-solar` for more information.

Contributed by **Mark Parncutt**.

Result Backends
---------------

RPC Result Backend matured
~~~~~~~~~~~~~~~~~~~~~~~~~~

Lots of bugs in the previously experimental RPC result backend have been fixed
and can now be considered to production use.

Contributed by **Ask Solem**, **Morris Tweed**.

Redis: Result backend optimizations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``result.get()`` is now using pub/sub for streaming task results
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Calling ``result.get()`` when using the Redis result backend
used to be extremely expensive as it was using polling to wait
for the result to become available. A default polling
interval of 0.5 seconds didn't help performance, but was
necessary to avoid a spin loop.

The new implementation is using Redis Pub/Sub mechanisms to
publish and retrieve results immediately, greatly improving
task round-trip times.

Contributed by **Yaroslav Zhavoronkov** and **Ask Solem**.

New optimized chord join implementation
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This was an experimental feature introduced in Celery 3.1,
that could only be enabled by adding ``?new_join=1`` to the
result backend URL configuration.

We feel that the implementation has been tested thoroughly enough
to be considered stable and enabled by default.

The new implementation greatly reduces the overhead of chords,
and especially with larger chords the performance benefit can be massive.

New Riak result backend introduced
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See :ref:`conf-riak-result-backend` for more information.

Contributed by **Gilles Dartiguelongue**, **Alman One** and **NoKriK**.

New CouchDB result backend introduced
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See :ref:`conf-couchdb-result-backend` for more information.

Contributed by **Nathan Van Gheem**.

New Consul result backend introduced
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add support for Consul as a backend using the Key/Value store of Consul.

Consul has an HTTP API where through you can store keys with their values.

The backend extends KeyValueStoreBackend and implements most of the methods.

Mainly to set, get and remove objects.

This allows Celery to store Task results in the K/V store of Consul.

Consul also allows to set a TTL on keys using the Sessions from Consul. This way
the backend supports auto expiry of Task results.

For more information on Consul visit http://consul.io/

The backend uses :pypi:`python-consul` for talking to the HTTP API.
This package is fully Python 3 compliant just as this backend is:

.. code-block:: console

    $ pip install python-consul

That installs the required package to talk to Consul's HTTP API from Python.

You can also specify consul as an extension in your dependency on Celery:

.. code-block:: console

    $ pip install celery[consul]

See :ref:`bundles` for more information.


Contributed by **Wido den Hollander**.

Brand new Cassandra result backend
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A brand new Cassandra backend utilizing the new :pypi:`cassandra-driver`
library is replacing the old result backend using the older
:pypi:`pycassa` library.

See :ref:`conf-cassandra-result-backend` for more information.

To depend on Celery with Cassandra as the result backend use:

.. code-block:: console

    $ pip install celery[cassandra]

You can also combine multiple extension requirements,
please see :ref:`bundles` for more information.

.. # XXX What changed?

New Elasticsearch result backend introduced
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See :ref:`conf-elasticsearch-result-backend` for more information.

To depend on Celery with Elasticsearch as the result bakend use:

.. code-block:: console

    $ pip install celery[elasticsearch]

You can also combine multiple extension requirements,
please see :ref:`bundles` for more information.

Contributed by **Ahmet Demir**.

New File-system result backend introduced
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See :ref:`conf-filesystem-result-backend` for more information.

Contributed by **Môshe van der Sterre**.

Event Batching
--------------

Events are now buffered in the worker and sent as a list, reducing
the overhead required to send monitoring events.

For authors of custom event monitors there will be no action
required as long as you're using the Python Celery
helpers (:class:`~@events.Receiver`) to implement your monitor.

However, if you're parsing raw event messages you must now account
for batched event messages,  as they differ from normal event messages
in the following way:

- The routing key for a batch of event messages will be set to
  ``<event-group>.multi`` where the only batched event group
  is currently ``task`` (giving a routing key of ``task.multi``).

- The message body will be a serialized list-of-dictionaries instead
  of a dictionary. Each item in the list can be regarded
  as a normal event message body.

.. :sha:`03399b4d7c26fb593e61acf34f111b66b340ba4e`

In Other News...
----------------

Requirements
~~~~~~~~~~~~

- Now depends on :ref:`Kombu 4.0 <kombu:version-4.0>`.

- Now depends on :pypi:`billiard` version 3.5.

- No longer depends on :pypi:`anyjson`. Good-bye old friend :(


Tasks
~~~~~

- The "anon-exchange" is now used for simple name-name direct routing.

  This increases performance as it completely bypasses the routing table,
  in addition it also improves reliability for the Redis broker transport.

- An empty ResultSet now evaluates to True.

    Fix contributed by **Colin McIntosh**.

- The default routing key and exchange name is now taken from the
  :setting:`task_default_queue` setting.

    This means that to change the name of the default queue, you now
    only have to set a single setting.

- New :setting:`task_reject_on_worker_lost` setting, and
  :attr:`~@Task.reject_on_worker_lost` task attribute decides what happens
  when the child worker process executing a late ack task is terminated.

    Contributed by **Michael Permana**.

- ``Task.subtask`` renamed to ``Task.signature`` with alias.

- ``Task.subtask_from_request`` renamed to
  ``Task.signature_from_request`` with alias.

- The ``delivery_mode`` attribute for :class:`kombu.Queue` is now
  respected (Issue #1953).

- Routes in :setting:`task-routes` can now specify a
  :class:`~kombu.Queue` instance directly.

    Example:

    .. code-block:: python

        task_routes = {'proj.tasks.add': {'queue': Queue('add')}}

- ``AsyncResult`` now raises :exc:`ValueError` if task_id is None.
  (Issue #1996).

- Retried tasks didn't forward expires setting (Issue #3297).

- ``result.get()`` now supports an ``on_message`` argument to set a
  callback to be called for every message received.

- New abstract classes added:

    - :class:`~celery.utils.abstract.CallableTask`

        Looks like a task.

    - :class:`~celery.utils.abstract.CallableSignature`

        Looks like a task signature.

- ``Task.replace`` now properly forwards callbacks (Issue #2722).

    Fix contributed by **Nicolas Unravel**.

- ``Task.replace``: Append to chain/chord (Closes #3232)

    Fixed issue #3232, adding the signature to the chain (if there's any).
    Fixed the chord suppress if the given signature contains one.

    Fix contributed by :github_user:`honux`.

- Task retry now also throws in eager mode.

    Fix contributed by **Feanil Patel**.


Beat
~~~~

- Fixed crontab infinite loop with invalid date.

    When occurrence can never be reached (example, April, 31th), trying
    to reach the next occurrence would trigger an infinite loop.

    Try fixing that by raising a RuntimeError after 2,000 iterations

    (Also added a test for crontab leap years in the process)

    Fix contributed by **Romuald Brunet**.

- Now ensures the program exits with a non-zero exit code when an
  exception terminates the service.

    Fix contributed by **Simon Peeters**.

App
~~~

- Dates are now always timezone aware even if
  :setting:`enable_utc` is disabled (Issue #943).

    Fix contributed by **Omer Katz**.

- **Config**: App preconfiguration is now also pickled with the configuration.

    Fix contributed by **Jeremy Zafran**.

- The application can now change how task names are generated using
    the :meth:`~@gen_task_name` method.

    Contributed by **Dmitry Malinovsky**.

- App has new ``app.current_worker_task`` property that
  returns the task that's currently being worked on (or :const:`None`).
  (Issue #2100).

Logging
~~~~~~~

- :func:`~celery.utils.log.get_task_logger` now raises an exception
  if trying to use the name "celery" or "celery.task" (Issue #3475).

Execution Pools
~~~~~~~~~~~~~~~

- **Eventlet/Gevent**: now enables AMQP heartbeat (Issue #3338).

- **Eventlet/Gevent**: Fixed race condition leading to "simultaneous read"
  errors (Issue #2812).

- **Prefork**: Prefork pool now uses ``poll`` instead of ``select`` where
  available (Issue #2373).

- **Prefork**: Fixed bug where the pool would refuse to shut down the
  worker (Issue #2606).

- **Eventlet**: Now returns pool size in :program:`celery inspect stats`
  command.

    Contributed by **Alexander Oblovatniy**.

Testing
-------

- Celery is now a :pypi:`pytest` plugin, including fixtures
  useful for unit and integration testing.

    See the :ref:`testing user guide <testing>` for more information.

Transports
~~~~~~~~~~

- ``amqps://`` can now be specified to require SSL.

- **Redis Transport**: The Redis transport now supports the
  :setting:`broker_use_ssl` option.

    Contributed by **Robert Kolba**.

- JSON serializer now calls ``obj.__json__`` for unsupported types.

    This means you can now define a ``__json__`` method for custom
    types that can be reduced down to a built-in json type.

    Example:

    .. code-block:: python

        class Person:
            first_name = None
            last_name = None
            address = None

            def __json__(self):
                return {
                    'first_name': self.first_name,
                    'last_name': self.last_name,
                    'address': self.address,
                }

- JSON serializer now handles datetimes, Django promise, UUID and Decimal.

- New ``Queue.consumer_arguments`` can be used for the ability to
  set consumer priority via ``x-priority``.

  See https://www.rabbitmq.com/consumer-priority.html

  Example:

  .. code-block:: python

        consumer = Consumer(channel, consumer_arguments={'x-priority': 3})

- Queue/Exchange: ``no_declare`` option added (also enabled for
  internal amq. exchanges).

Programs
~~~~~~~~

- Celery is now using :mod:`argparse`, instead of :mod:`optparse`.

- All programs now disable colors if the controlling terminal is not a TTY.

- :program:`celery worker`: The ``-q`` argument now disables the startup
  banner.

- :program:`celery worker`: The "worker ready" message is now logged
  using severity info, instead of warn.

- :program:`celery multi`: ``%n`` format for is now synonym with
  ``%N`` to be consistent with :program:`celery worker`.

- :program:`celery inspect`/:program:`celery control`: now supports a new
  :option:`--json <celery inspect --json>` option to give output in json format.

- :program:`celery inspect registered`: now ignores built-in tasks.

- :program:`celery purge` now takes ``-Q`` and ``-X`` options
  used to specify what queues to include and exclude from the purge.

- New :program:`celery logtool`: Utility for filtering and parsing
  celery worker log-files

- :program:`celery multi`: now passes through `%i` and `%I` log
  file formats.

- General: ``%p`` can now be used to expand to the full worker node-name
  in log-file/pid-file arguments.

- A new command line option
   :option:`--executable <celery worker --executable>` is now
   available for daemonizing programs (:program:`celery worker` and
   :program:`celery beat`).

    Contributed by **Bert Vanderbauwhede**.

- :program:`celery worker`: supports new
  :option:`--prefetch-multiplier <celery worker --prefetch-multiplier>` option.

    Contributed by **Mickaël Penhard**.

- The ``--loader`` argument is now always effective even if an app argument is
  set (Issue #3405).

- inspect/control now takes commands from registry

    This means user remote-control commands can also be used from the
    command-line.

    Note that you need to specify the arguments/and type of arguments
    for the arguments to be correctly passed on the command-line.

    There are now two decorators, which use depends on the type of
    command: `@inspect_command` + `@control_command`:

    .. code-block:: python

        from celery.worker.control import control_command

        @control_command(
            args=[('n', int)]
            signature='[N=1]',
        )
        def something(state, n=1, **kwargs):
            ...

    Here ``args`` is a list of args supported by the command.
    The list must contain tuples of ``(argument_name, type)``.

    ``signature`` is just the command-line help used in e.g.
    ``celery -A proj control --help``.

    Commands also support `variadic` arguments, which means that any
    arguments left over will be added to a single variable.  Here demonstrated
    by the ``terminate`` command which takes a signal argument and a variable
    number of task_ids:

    .. code-block:: python

        from celery.worker.control import control_command

        @control_command(
            args=[('signal', str)],
            signature='<signal> [id1, [id2, [..., [idN]]]]',
            variadic='ids',
        )
        def terminate(state, signal, ids, **kwargs):
            ...

    This command can now be called using:

    .. code-block:: console

        $ celery -A proj control terminate SIGKILL id1 id2 id3`

    See :ref:`worker-custom-control-commands` for more information.

Worker
~~~~~~

- Improvements and fixes for :class:`~celery.utils.collections.LimitedSet`.

    Getting rid of leaking memory + adding ``minlen`` size of the set:
    the minimal residual size of the set after operating for some time.
    ``minlen`` items are kept, even if they should've been expired.

    Problems with older and even more old code:

    #. Heap would tend to grow in some scenarios
       (like adding an item multiple times).

    #. Adding many items fast wouldn't clean them soon enough (if ever).

    #. When talking to other workers, revoked._data was sent, but
       it was processed on the other side as iterable.
       That means giving those keys new (current)
       time-stamp. By doing this workers could recycle
       items forever. Combined with 1) and 2), this means that in
       large set of workers, you're getting out of memory soon.

    All those problems should be fixed now.

    This should fix issues #3095, #3086.

    Contributed by **David Pravec**.

- New settings to control remote control command queues.

    - :setting:`control_queue_expires`

        Set queue expiry time for both remote control command queues,
        and remote control reply queues.

    - :setting:`control_queue_ttl`

        Set message time-to-live for both remote control command queues,
        and remote control reply queues.

    Contributed by **Alan Justino**.

- The :signal:`worker_shutdown` signal is now always called during shutdown.

    Previously it would not be called if the worker instance was collected
    by gc first.

- Worker now only starts the remote control command consumer if the
  broker transport used actually supports them.

- Gossip now sets ``x-message-ttl`` for event queue to heartbeat_interval s.
  (Issue #2005).

- Now preserves exit code (Issue #2024).

- Now rejects messages with an invalid ETA value (instead of ack, which means
  they will be sent to the dead-letter exchange if one is configured).

- Fixed crash when the ``-purge`` argument was used.

- Log--level for unrecoverable errors changed from ``error`` to
  ``critical``.

- Improved rate limiting accuracy.

- Account for missing timezone information in task expires field.

    Fix contributed by **Albert Wang**.

- The worker no longer has a ``Queues`` bootsteps, as it is now
    superfluous.

- Now emits the "Received task" line even for revoked tasks.
  (Issue #3155).

- Now respects :setting:`broker_connection_retry` setting.

    Fix contributed by **Nat Williams**.

- New :setting:`control_queue_ttl` and :setting:`control_queue_expires`
  settings now enables you to configure remote control command
  message TTLs, and queue expiry time.

    Contributed by **Alan Justino**.

- New :data:`celery.worker.state.requests` enables O(1) loookup
  of active/reserved tasks by id.

- Auto-scale didn't always update keep-alive when scaling down.

    Fix contributed by **Philip Garnero**.

- Fixed typo ``options_list`` -> ``option_list``.

    Fix contributed by **Greg Wilbur**.

- Some worker command-line arguments and ``Worker()`` class arguments have
  been renamed for consistency.

    All of these have aliases for backward compatibility.

    - ``--send-events`` -> ``--task-events``

    - ``--schedule`` -> ``--schedule-filename``

    - ``--maxtasksperchild`` -> ``--max-tasks-per-child``

    - ``Beat(scheduler_cls=)`` -> ``Beat(scheduler=)``

    - ``Worker(send_events=True)`` -> ``Worker(task_events=True)``

    - ``Worker(task_time_limit=)`` -> ``Worker(time_limit=``)

    - ``Worker(task_soft_time_limit=)`` -> ``Worker(soft_time_limit=)``

    - ``Worker(state_db=)`` -> ``Worker(statedb=)``

    - ``Worker(working_directory=)`` -> ``Worker(workdir=)``


Debugging Utilities
~~~~~~~~~~~~~~~~~~~

- :mod:`celery.contrib.rdb`: Changed remote debugger banner so that you can copy and paste
  the address easily (no longer has a period in the address).

    Contributed by **Jonathan Vanasco**.

- Fixed compatibility with recent :pypi:`psutil` versions (Issue #3262).


Signals
~~~~~~~

- **App**: New signals for app configuration/finalization:

    - :data:`app.on_configure <@on_configure>`
    - :data:`app.on_after_configure <@on_after_configure>`
    - :data:`app.on_after_finalize <@on_after_finalize>`

- **Task**: New task signals for rejected task messages:

    - :data:`celery.signals.task_rejected`.
    - :data:`celery.signals.task_unknown`.

- **Worker**: New signal for when a heartbeat event is sent.

    - :data:`celery.signals.heartbeat_sent`

        Contributed by **Kevin Richardson**.

Events
~~~~~~

- Event messages now uses the RabbitMQ ``x-message-ttl`` option
  to ensure older event messages are discarded.

    The default is 5 seconds, but can be changed using the
    :setting:`event_queue_ttl` setting.

- ``Task.send_event`` now automatically retries sending the event
  on connection failure, according to the task publish retry settings.

- Event monitors now sets the :setting:`event_queue_expires`
  setting by default.

    The queues will now expire after 60 seconds after the monitor stops
    consuming from it.

- Fixed a bug where a None value wasn't handled properly.

    Fix contributed by **Dongweiming**.

- New :setting:`event_queue_prefix` setting can now be used
  to change the default ``celeryev`` queue prefix for event receiver queues.

    Contributed by **Takeshi Kanemoto**.

- ``State.tasks_by_type`` and ``State.tasks_by_worker`` can now be
  used as a mapping for fast access to this information.

Deployment
~~~~~~~~~~

- Generic init-scripts now support
  :envvar:`CELERY_SU` and :envvar:`CELERYD_SU_ARGS` environment variables
  to set the path and arguments for :command:`su` (:manpage:`su(1)`).

- Generic init-scripts now better support FreBSD and other BSD
  systems by searching :file:`/usr/local/etc/` for the configuration file.

    Contributed by **Taha Jahangir**.

- Generic init-script: Fixed strange bug for ``celerybeat`` where
  restart didn't always work (Issue #3018).

- The systemd init script now uses a shell when executing
  services.

    Contributed by **Tomas Machalek**.

Result Backends
~~~~~~~~~~~~~~~

- Redis: Now has a default socket timeout of 120 seconds.

    The default can be changed using the new :setting:`redis_socket_timeout`
    setting.

    Contributed by **Raghuram Srinivasan**.

- RPC Backend result queues are now auto delete by default (Issue #2001).

- RPC Backend: Fixed problem where exception
  wasn't deserialized properly with the json serializer (Issue #2518).

    Fix contributed by **Allard Hoeve**.

- CouchDB: The backend used to double-json encode results.

    Fix contributed by **Andrew Stewart**.

- CouchDB: Fixed typo causing the backend to not be found
  (Issue #3287).

    Fix contributed by **Andrew Stewart**.

- MongoDB: Now supports setting the :setting:`result_serialzier` setting
  to ``bson`` to use the MongoDB libraries own serializer.

    Contributed by **Davide Quarta**.

- MongoDB: URI handling has been improved to use
    database name, user and password from the URI if provided.

    Contributed by **Samuel Jaillet**.

- SQLAlchemy result backend: Now ignores all result
  engine options when using NullPool (Issue #1930).

- SQLAlchemy result backend: Now sets max char size to 155 to deal
  with brain damaged MySQL unicode implementation (Issue #1748).

- **General**: All Celery exceptions/warnings now inherit from common
  :class:`~celery.exceptions.CeleryError`/:class:`~celery.exceptions.CeleryWarning`.
  (Issue #2643).

Documentation Improvements
~~~~~~~~~~~~~~~~~~~~~~~~~~

Contributed by:

- Adam Chainz
- Amir Rustamzadeh
- Arthur Vuillard
- Batiste Bieler
- Berker Peksag
- Bryce Groff
- Daniel Devine
- Edward Betts
- Jason Veatch
- Jeff Widman
- Maciej Obuchowski
- Manuel Kaufmann
- Maxime Beauchemin
- Mitchel Humpherys
- Pavlo Kapyshin
- Pierre Fersing
- Rik
- Tayfun Sen
- Wieland Hoffmann

Reorganization, Deprecations, and Removals
==========================================

Incompatible changes
--------------------

- Prefork: Calling ``result.get()`` or joining any result from within a task
  now raises :exc:`RuntimeError`.

    In previous versions this would emit a warning.

- :mod:`celery.worker.consumer` is now a package, not a module.

- Module ``celery.worker.job`` renamed to :mod:`celery.worker.request`.

- Beat: ``Scheduler.Publisher``/``.publisher`` renamed to
  ``.Producer``/``.producer``.

- Result: The task_name argument/attribute of :class:`@AsyncResult` was
  removed.

    This was historically a field used for :mod:`pickle` compatibility,
    but is no longer needed.

- Backends: Arguments named ``status`` renamed to ``state``.

- Backends: ``backend.get_status()`` renamed to ``backend.get_state()``.

- Backends: ``backend.maybe_reraise()`` renamed to ``.maybe_throw()``

    The promise API uses .throw(), so this change was made to make it more
    consistent.

    There's an alias available, so you can still use maybe_reraise until
    Celery 5.0.

.. _v400-unscheduled-removals:

Unscheduled Removals
--------------------

- The experimental :mod:`celery.contrib.methods` feature has been removed,
  as there were far many bugs in the implementation to be useful.

- The CentOS init-scripts have been removed.

    These didn't really add any features over the generic init-scripts,
    so you're encouraged to use them instead, or something like
    :pypi:`supervisor`.


.. _v400-deprecations-reorg:

Reorganization Deprecations
---------------------------

These symbols have been renamed, and while there's an alias available in this
version for backward compatibility, they will be removed in Celery 5.0, so
make sure you rename these ASAP to make sure it won't break for that release.

Chances are that you'll only use the first in this list, but you never
know:

- ``celery.utils.worker_direct`` ->
  :meth:`celery.utils.nodenames.worker_direct`.

- ``celery.utils.nodename`` -> :meth:`celery.utils.nodenames.nodename`.

- ``celery.utils.anon_nodename`` ->
  :meth:`celery.utils.nodenames.anon_nodename`.

- ``celery.utils.nodesplit`` -> :meth:`celery.utils.nodenames.nodesplit`.

- ``celery.utils.default_nodename`` ->
  :meth:`celery.utils.nodenames.default_nodename`.

- ``celery.utils.node_format`` -> :meth:`celery.utils.nodenames.node_format`.

- ``celery.utils.host_format`` -> :meth:`celery.utils.nodenames.host_format`.

.. _v400-removals:

Scheduled Removals
------------------

Modules
~~~~~~~

- Module ``celery.worker.job`` has been renamed to :mod:`celery.worker.request`.

    This was an internal module so shouldn't have any effect.
    It's now part of the public API so must not change again.

- Module ``celery.task.trace`` has been renamed to ``celery.app.trace``
  as the ``celery.task`` package is being phased out. The module
  will be removed in version 5.0 so please change any import from::

    from celery.task.trace import X

  to::

    from celery.app.trace import X

- Old compatibility aliases in the :mod:`celery.loaders` module
  has been removed.

    - Removed ``celery.loaders.current_loader()``, use: ``current_app.loader``

    - Removed ``celery.loaders.load_settings()``, use: ``current_app.conf``

Result
~~~~~~

- ``AsyncResult.serializable()`` and ``celery.result.from_serializable``
    has been removed:

    Use instead:

    .. code-block:: pycon

        >>> tup = result.as_tuple()
        >>> from celery.result import result_from_tuple
        >>> result = result_from_tuple(tup)

- Removed ``BaseAsyncResult``, use ``AsyncResult`` for instance checks
  instead.

- Removed ``TaskSetResult``, use ``GroupResult`` instead.

    - ``TaskSetResult.total`` -> ``len(GroupResult)``

    - ``TaskSetResult.taskset_id`` -> ``GroupResult.id``

- Removed ``ResultSet.subtasks``, use ``ResultSet.results`` instead.


TaskSet
~~~~~~~

TaskSet has been removed, as it was replaced by the ``group`` construct in
Celery 3.0.

If you have code like this:

.. code-block:: pycon

    >>> from celery.task import TaskSet

    >>> TaskSet(add.subtask((i, i)) for i in xrange(10)).apply_async()

You need to replace that with:

.. code-block:: pycon

    >>> from celery import group
    >>> group(add.s(i, i) for i in xrange(10))()

Events
~~~~~~

- Removals for class :class:`celery.events.state.Worker`:

    - ``Worker._defaults`` attribute.

        Use ``{k: getattr(worker, k) for k in worker._fields}``.

    - ``Worker.update_heartbeat``

        Use ``Worker.event(None, timestamp, received)``

    - ``Worker.on_online``

        Use ``Worker.event('online', timestamp, received, fields)``

    - ``Worker.on_offline``

        Use ``Worker.event('offline', timestamp, received, fields)``

    - ``Worker.on_heartbeat``

        Use ``Worker.event('heartbeat', timestamp, received, fields)``

- Removals for class :class:`celery.events.state.Task`:

    - ``Task._defaults`` attribute.

        Use ``{k: getattr(task, k) for k in task._fields}``.

    - ``Task.on_sent``

        Use ``Worker.event('sent', timestamp, received, fields)``

    - ``Task.on_received``

        Use ``Task.event('received', timestamp, received, fields)``

    - ``Task.on_started``

        Use ``Task.event('started', timestamp, received, fields)``

    - ``Task.on_failed``

        Use ``Task.event('failed', timestamp, received, fields)``

    - ``Task.on_retried``

        Use ``Task.event('retried', timestamp, received, fields)``

    - ``Task.on_succeeded``

        Use ``Task.event('succeeded', timestamp, received, fields)``

    - ``Task.on_revoked``

        Use ``Task.event('revoked', timestamp, received, fields)``

    - ``Task.on_unknown_event``

        Use ``Task.event(short_type, timestamp, received, fields)``

    - ``Task.update``

        Use ``Task.event(short_type, timestamp, received, fields)``

    - ``Task.merge``

        Contact us if you need this.

Magic keyword arguments
~~~~~~~~~~~~~~~~~~~~~~~

Support for the very old magic keyword arguments accepted by tasks is
finally removed in this version.

If you're still using these you have to rewrite any task still
using the old ``celery.decorators`` module and depending
on keyword arguments being passed to the task,
for example::

    from celery.decorators import task

    @task()
    def add(x, y, task_id=None):
        print('My task id is %r' % (task_id,))

should be rewritten into::

    from celery import task

    @task(bind=True)
    def add(self, x, y):
        print('My task id is {0.request.id}'.format(self))

Removed Settings
----------------

The following settings have been removed, and is no longer supported:

Logging Settings
~~~~~~~~~~~~~~~~

=====================================  =====================================
**Setting name**                       **Replace with**
=====================================  =====================================
``CELERYD_LOG_LEVEL``                  :option:`celery worker --loglevel`
``CELERYD_LOG_FILE``                   :option:`celery worker --logfile`
``CELERYBEAT_LOG_LEVEL``               :option:`celery beat --loglevel`
``CELERYBEAT_LOG_FILE``                :option:`celery beat --loglevel`
``CELERYMON_LOG_LEVEL``                celerymon is deprecated, use flower
``CELERYMON_LOG_FILE``                 celerymon is deprecated, use flower
``CELERYMON_LOG_FORMAT``               celerymon is deprecated, use flower
=====================================  =====================================

Task Settings
~~~~~~~~~~~~~~

=====================================  =====================================
**Setting name**                       **Replace with**
=====================================  =====================================
``CELERY_CHORD_PROPAGATES``            N/A
=====================================  =====================================

Changes to internal API
-----------------------

- Module ``celery.datastructures`` renamed to :mod:`celery.utils.collections`.

- Module ``celery.utils.timeutils`` renamed to :mod:`celery.utils.time`.

- ``celery.utils.datastructures.DependencyGraph`` moved to
  :mod:`celery.utils.graph`.

- ``celery.utils.jsonify`` is now :func:`celery.utils.serialization.jsonify`.

- ``celery.utils.strtobool`` is now
  :func:`celery.utils.serialization.strtobool`.

- ``celery.utils.is_iterable`` has been removed.

    Instead use:

    .. code-block:: python

        isinstance(x, collections.Iterable)

- ``celery.utils.lpmerge`` is now :func:`celery.utils.collections.lpmerge`.

- ``celery.utils.cry`` is now :func:`celery.utils.debug.cry`.

- ``celery.utils.isatty`` is now :func:`celery.platforms.isatty`.

- ``celery.utils.gen_task_name`` is now
  :func:`celery.utils.imports.gen_task_name`.

- ``celery.utils.deprecated`` is now :func:`celery.utils.deprecated.Callable`

- ``celery.utils.deprecated_property`` is now
  :func:`celery.utils.deprecated.Property`.

- ``celery.utils.warn_deprecated`` is now :func:`celery.utils.deprecated.warn`


.. _v400-deprecations:

Deprecation Time-line Changes
=============================

See the :ref:`deprecation-timeline`.