.. _broker-rabbitmq:

================
 Using RabbitMQ
================

.. contents::
    :local:

Installation & Configuration
============================

RabbitMQ is the default broker so it doesn't require any additional
dependencies or initial configuration, other than the URL location of
the broker instance you want to use:

.. code-block:: python

    broker_url = 'amqp://myuser:mypassword@localhost:5672/myvhost'

For a description of broker URLs and a full list of the
various broker configuration options available to Celery,
see :ref:`conf-broker-settings`, and see below for setting up the
username, password and vhost.

.. _installing-rabbitmq:

Installing the RabbitMQ Server
==============================

See `Downloading and Installing RabbitMQ`_ over at RabbitMQ's website. For macOS
see `Installing RabbitMQ on macOS`_.

.. _`Downloading and Installing RabbitMQ`: https://www.rabbitmq.com/download.html

.. note::

    If you're getting `nodedown` errors after installing and using
    :command:`rabbitmqctl` then this blog post can help you identify
    the source of the problem:

        http://www.somic.org/2009/02/19/on-rabbitmqctl-and-badrpcnodedown/

.. _rabbitmq-configuration:

Setting up RabbitMQ
-------------------

To use Celery we need to create a RabbitMQ user, a virtual host and
allow that user access to that virtual host:

.. code-block:: console

    $ sudo rabbitmqctl add_user myuser mypassword

.. code-block:: console

    $ sudo rabbitmqctl add_vhost myvhost

.. code-block:: console

    $ sudo rabbitmqctl set_user_tags myuser mytag

.. code-block:: console

    $ sudo rabbitmqctl set_permissions -p myvhost myuser ".*" ".*" ".*"

Substitute in appropriate values for ``myuser``, ``mypassword`` and ``myvhost`` above.

See the RabbitMQ `Admin Guide`_ for more information about `access control`_.

.. _`Admin Guide`: https://www.rabbitmq.com/admin-guide.html

.. _`access control`: https://www.rabbitmq.com/access-control.html

.. _rabbitmq-macOS-installation:

Installing RabbitMQ on macOS
----------------------------

The easiest way to install RabbitMQ on macOS is using `Homebrew`_ the new and
shiny package management system for macOS.

First, install Homebrew using the one-line command provided by the `Homebrew
documentation`_:

.. code-block:: console

    /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

Finally, we can install RabbitMQ using :command:`brew`:

.. code-block:: console

    $ brew install rabbitmq

.. _`Homebrew`: https://github.com/mxcl/homebrew/
.. _`Homebrew documentation`: https://github.com/Homebrew/homebrew/wiki/Installation

.. _rabbitmq-macOS-system-hostname:

After you've installed RabbitMQ with :command:`brew` you need to add the following to
your path to be able to start and stop the broker: add it to the start-up file for your
shell (e.g., :file:`.bash_profile` or :file:`.profile`).

.. code-block:: bash

    PATH=$PATH:/usr/local/sbin

Configuring the system host name
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you're using a DHCP server that's giving you a random host name, you need
to permanently configure the host name. This is because RabbitMQ uses the host name
to communicate with nodes.

Use the :command:`scutil` command to permanently set your host name:

.. code-block:: console

    $ sudo scutil --set HostName myhost.local

Then add that host name to :file:`/etc/hosts` so it's possible to resolve it
back into an IP address::

    127.0.0.1       localhost myhost myhost.local

If you start the :command:`rabbitmq-server`, your rabbit node should now
be `rabbit@myhost`, as verified by :command:`rabbitmqctl`:

.. code-block:: console

    $ sudo rabbitmqctl status
    Status of node rabbit@myhost ...
    [{running_applications,[{rabbit,"RabbitMQ","1.7.1"},
                        {mnesia,"MNESIA  CXC 138 12","4.4.12"},
                        {os_mon,"CPO  CXC 138 46","2.2.4"},
                        {sasl,"SASL  CXC 138 11","2.1.8"},
                        {stdlib,"ERTS  CXC 138 10","1.16.4"},
                        {kernel,"ERTS  CXC 138 10","2.13.4"}]},
    {nodes,[rabbit@myhost]},
    {running_nodes,[rabbit@myhost]}]
    ...done.

This is especially important if your DHCP server gives you a host name
starting with an IP address, (e.g., `23.10.112.31.comcast.net`).  In this
case RabbitMQ will try to use `rabbit@23`: an illegal host name.

.. _rabbitmq-macOS-start-stop:

Starting/Stopping the RabbitMQ server
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To start the server:

.. code-block:: console

    $ sudo rabbitmq-server

you can also run it in the background by adding the ``-detached`` option
(note: only one dash):

.. code-block:: console

    $ sudo rabbitmq-server -detached

Never use :command:`kill` (:manpage:`kill(1)`) to stop the RabbitMQ server,
but rather use the :command:`rabbitmqctl` command:

.. code-block:: console

    $ sudo rabbitmqctl stop

When the server is running, you can continue reading `Setting up RabbitMQ`_.

.. _using-quorum-queues:

Using Quorum Queues
===================

.. versionadded:: 5.5

.. warning::

    Quorum Queues require disabling global QoS which means some features won't work as expected.
    See `limitations`_ for details.

Celery supports `Quorum Queues`_ by setting the ``x-queue-type`` header to ``quorum`` like so:

.. code-block:: python

    from kombu import Queue

    task_queues = [Queue('my-queue', queue_arguments={'x-queue-type': 'quorum'})]
    broker_transport_options = {"confirm_publish": True}

If you'd like to change the type of the default queue, set the :setting:`task_default_queue_type` setting to ``quorum``.

Another way to configure `Quorum Queues`_ is by relying on default settings and using ``task_routes``:

.. code-block:: python

    task_default_queue_type = "quorum"
    task_default_exchange_type = "topic"
    task_default_queue = "my-queue"
    broker_transport_options = {"confirm_publish": True}

    task_routes = {
        "*": {
            "routing_key": "my-queue",
        },
    }

Celery automatically detects if quorum queues are used using the :setting:`worker_detect_quorum_queues` setting.
We recommend to keep the default behavior turned on.

To migrate from classic mirrored queues to quorum queues, please refer to RabbitMQ's `documentation <https://www.rabbitmq.com/blog/2023/03/02/quorum-queues-migration>`_ on the subject.

.. _`Quorum Queues`: https://www.rabbitmq.com/docs/quorum-queues

.. _limitations:

Limitations
-----------

Disabling global QoS means that the per-channel QoS is now static.
This means that some Celery features won't work when using Quorum Queues.

Autoscaling relies on increasing and decreasing the prefetch count whenever a new process is instantiated
or terminated so it won't work when Quorum Queues are detected.

Similarly, the :setting:`worker_enable_prefetch_count_reduction` setting will be a no-op even when set to ``True``
when Quorum Queues are detected.

In addition, :ref:`ETA/Countdown <calling-eta>` will block the worker when received until the ETA arrives since
we can no longer increase the prefetch count and fetch another task from the queue.

In order to properly schedule ETA/Countdown tasks we automatically detect if quorum queues are used
and in case they are, Celery automatically enables :ref:`Native Delayed Delivery <native-delayed-delivery>`.

.. _native-delayed-delivery:

Native Delayed Delivery
-----------------------

Since tasks with ETA/Countdown will block the worker until they are scheduled for execution,
we need to use RabbitMQ's native capabilities to schedule the execution of tasks.

The design is borrowed from NServiceBus. If you are interested in the implementation details, refer to their `documentation`_.

.. _documentation: https://docs.particular.net/transports/rabbitmq/delayed-delivery

Native Delayed Delivery is automatically enabled when quorum queues are detected.

By default the Native Delayed Delivery queues are quorum queues.
If you'd like to change them to classic queues you can set the :setting:`broker_native_delayed_delivery_queue_type`
to classic.
