FAQ

============================
 Frequently Asked Questions
============================

Questions
=========

MySQL is throwing deadlock errors, what can I do?
-------------------------------------------------

**Answer:** MySQL has default isolation level set to ``REPEATABLE-READ``,
if you don't really need that, set it to ``READ-COMMITTED``.
You can do that by adding the following to your ``my.cnf``::

    [mysqld]
    transaction-isolation = READ-COMMITTED

For more information about InnoDBs transaction model see `MySQL - The InnoDB
Transaction Model and Locking`_ in the MySQL user manual.

(Thanks to Honza Kral and Anton Tsigularov for this solution)

.. _`MySQL - The InnoDB Transaction Model and Locking`: http://dev.mysql.com/doc/refman/5.1/en/innodb-transaction-model.html

celeryd is not doing anything, just hanging
--------------------------------------------

**Answer:** See `MySQL is throwing deadlock errors, what can I do?`_.

I'm having ``IntegrityError: Duplicate Key`` errors. Why?
----------------------------------------------------------

**Answer:** See `MySQL is throwing deadlock errors, what can I do?`_.
Thanks to howsthedotcom.

Why won't my Task run?
----------------------

**Answer:** Did you register the task in the applications ``tasks.py`` module?
(or in some other module Django loads by default, like ``models.py``?).
Also there might be syntax errors preventing the tasks module being imported.

You can find out if the celery daemon is able to run the task by executing the
task manually:

    >>> from myapp.tasks import MyPeriodicTask
    >>> MyPeriodicTask.delay()

Watch celery daemons logfile (or output if not running as a daemon), to see
if it's able to find the task, or if some other error is happening.

Why won't my Periodic Task run?
-------------------------------

See `Why won't my Task run?`_.

Can I send some tasks to only some servers?
--------------------------------------------

As of now there is only one use-case that works like this, and that is
tasks of type ``A`` can be sent to servers ``x`` and ``y``, while tasks
of type ``B`` can be sent to server ``z``. One server can't handle more than
one routing_key, but this is coming in a later release.

Say you have two servers, ``x``, and ``y`` that handles regular tasks,
and one server ``z``, that only handles feed related tasks, you can use this
configuration:

    * Servers ``x`` and ``y``: settings.py:

    .. code-block:: python

        AMQP_SERVER = "rabbit"
        AMQP_PORT = 5678
        AMQP_USER = "myapp"
        AMQP_PASSWORD = "secret"
        AMQP_VHOST = "myapp"

        CELERY_AMQP_CONSUMER_QUEUE = "regular_tasks"
        CELERY_AMQP_EXCHANGE = "tasks"
        CELERY_AMQP_PUBLISHER_ROUTING_KEY = "task.regular"
        CELERY_AMQP_CONSUMER_ROUTING_KEY = "task.#"
        CELERY_AMQP_EXCHANGE_TYPE = "topic"

    * Server ``z``: settings.py:

    .. code-block:: python

        AMQP_SERVER = "rabbit"
        AMQP_PORT = 5678
        AMQP_USER = "myapp"
        AMQP_PASSWORD = "secret"
        AMQP_VHOST = "myapp"
        
        CELERY_AMQP_EXCHANGE = "tasks"
        CELERY_AMQP_PUBLISHER_ROUTING_KEY = "task.regular"
        CELERY_AMQP_EXCHANGE_TYPE = "topic"
        # This is the settings different for this server:
        CELERY_AMQP_CONSUMER_QUEUE = "feed_tasks"
        CELERY_AMQP_CONSUMER_ROUTING_KEY = "feed.#"

Now to make a Task run on the ``z`` server you need to set its
``routing_key`` attribute so it starts with the words ``"task.feed."``:

.. code-block:: python

    from feedaggregator.models import Feed
    from celery.task import Task

    class FeedImportTask(Task):
        name = "import_feed"
        routing_key = "feed.importer"

        def run(self, feed_url):
            # something importing the feed
            Feed.objects.import_feed(feed_url)


You can also override this using the ``routing_key`` argument to
:func:`celery.task.apply_async`:

    >>> from celery.task import apply_async
    >>> from myapp.tasks import RefreshFeedTask
    >>> apply_async(RefreshFeedTask, args=["http://cnn.com/rss"],
    ...             routing_key="feed.importer")