============================ Frequently Asked Questions ============================ Misconceptions ============== Is celery dependent on pickle? ------------------------------ **Answer:** No. Celery can support any serialization scheme and has support for JSON/YAML and Pickle by default. You can even send one task using pickle, and another one with JSON seamlessly, this is because every task is associated with a content-type. The default serialization scheme is pickle because it's the most used, and it has support for sending complex objects as task arguments. You can set a global default serializer, the default serializer for a particular Task, and even what serializer to use when sending a single task instance. Is celery for Django only? -------------------------- **Answer:** No. While django itself is a dependency, you can still use all of celerys features outside of a django project. Do I have to use AMQP/RabbitMQ? ------------------------------- **Answer**: No. You can also use Redis or an SQL database, for instructions see `Using other queues`_. .. _`Using other queues`: http://ask.github.com/celery/tutorials/otherqueues.html Redis or a database won't meet up to the standards of an AMQP broker. If you have strict reliability requirements you are encouraged to use RabbitMQ or another AMQP broker. Redis/database also uses pulling, so they are likely to consume more resources. However, if you for some reason is not able to use AMQP, feel free to use these alternatives. They will probably work fine for most use cases, and note that the above points are not specific to celery; If using Redis/database as a queue worked fine for you before, it probably will now. And you can always upgrade later. Is celery multi-lingual? ------------------------ **Answer:** Yes. celeryd is an implementation of celery in python. If the language has an AMQP client, there shouldn't be much work to create a worker in your language. A celery worker is just a program connecting to the broker to consume messages. There's no other communication involved. Also, there's another way to be language indepedent, and that is to use REST tasks, instead of your tasks being functions, they're URLs. With this information you can even create simple web servers that enable preloading of code. For more information about REST tasks see: `User Guide: Remote Tasks`_. .. _`User Guide: Remote Tasks`: http://ask.github.com/celery/userguide/remote-tasks.html Troubleshooting =============== 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?`_. or `Why is Task.delay/apply\* just hanging?`. Why is Task.delay/apply\*/celeryd just hanging? ----------------------------------------------- **Answer:** There is a bug in some AMQP clients that will make it hang if it's not able to authenticate the current user, the password doesn't match or the user does not have access to the virtual host specified. Be sure to check your broker logs (for RabbitMQ that is ``/var/log/rabbitmq/rabbit.log`` on most systems), it usually contains a message describing the reason. Why won't celeryd run on FreeBSD? --------------------------------- **Answer:** multiprocessing.Pool requires a working POSIX semaphore implementation which isn't enabled in FreeBSD by default. You have to enable POSIX semaphores in the kernel and manually recompile multiprocessing. Luckily, Viktor Petersson has written a tutorial to get you started with Celery on FreeBSD here: http://www.playingwithwire.com/2009/10/how-to-get-celeryd-to-work-on-freebsd/ I'm having ``IntegrityError: Duplicate Key`` errors. Why? ---------------------------------------------------------- **Answer:** See `MySQL is throwing deadlock errors, what can I do?`_. Thanks to howsthedotcom. Why isn't my tasks processed? ----------------------------- **Answer:** With RabbitMQ you can see how many consumers are currently receiving tasks by running the following command:: $ rabbitmqctl list_queues -p name messages consumers Listing queues ... celery 2891 2 This shows that there's 2891 messages waiting to be processed in the task queue, and there are two consumers processing them. One reason that the queue is never emptied could be that you have a stale celery process taking the messages hostage. This could happen if celeryd wasn't properly shut down. When a message is recieved by a worker the broker waits for it to be acknowledged before marking the message as processed. The broker will not re-send that message to another consumer until the consumer is shutdown properly. If you hit this problem you have to kill all workers manually and restart them:: ps auxww | grep celeryd | awk '{print $2}' | xargs kill You might have to wait a while until all workers has finished the work they're doing, if it's still hanging after a long time you can kill them by force with:: ps auxww | grep celeryd | awk '{print $2}' | xargs kill -9 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 celery is able to run the task by executing the task manually: >>> from myapp.tasks import MyPeriodicTask >>> MyPeriodicTask.delay() Watch celeryds logfile to see if it's able to find the task, or if some other error is happening. Why won't my Periodic Task run? ------------------------------- **Answer:** See `Why won't my Task run?`_. How do I discard all waiting tasks? ------------------------------------ **Answer:** Use ``celery.task.discard_all()``, like this: >>> from celery.task import discard_all >>> discard_all() 1753 The number ``1753`` is the number of messages deleted. You can also start celeryd with the ``--discard`` argument which will accomplish the same thing. I've discarded messages, but there are still messages left in the queue? ------------------------------------------------------------------------ **Answer:** Tasks are acknowledged (removed from the queue) as soon as they are actually executed. After the worker has received a task, it will take some time until it is actually executed, especially if there are a lot of tasks already waiting for execution. Messages that are not acknowledged are hold on to by the worker until it closes the connection to the broker (AMQP server). When that connection is closed (e.g because the worker was stopped) the tasks will be re-sent by the broker to the next available worker (or the same worker when it has been restarted), so to properly purge the queue of waiting tasks you have to stop all the workers, and then discard the tasks using ``discard_all``. Brokers ======= Can I use celery with ActiveMQ/STOMP? ------------------------------------- **Answer**: Yes. But this is somewhat experimental for now. It is certainly working ok for me in a test configuration, but it has not been tested in production like RabbitMQ. If you have any problems with using STOMP and celery, please report the bugs to the issue tracker: http://github.com/ask/celery/issues/ First you have to use the ``master`` branch of ``celery``:: $ git clone git://github.com/ask/celery.git $ cd celery $ sudo python setup.py install $ cd .. Then you need to install the ``stompbackend`` branch of ``carrot``:: $ git clone git://github.com/ask/carrot.git $ cd carrot $ git checkout stompbackend $ sudo python setup.py install $ cd .. And my fork of ``python-stomp`` which adds non-blocking support:: $ hg clone http://bitbucket.org/asksol/python-stomp/ $ cd python-stomp $ sudo python setup.py install $ cd .. In this example we will use a queue called ``celery`` which we created in the ActiveMQ web admin interface. **Note**: For ActiveMQ the queue name has to have ``"/queue/"`` prepended to it. i.e. the queue ``celery`` becomes ``/queue/celery``. Since a STOMP queue is a single named entity and it doesn't have the routing capabilities of AMQP you need to set both the ``queue``, and ``exchange`` settings to your queue name. This is a minor inconvenience since carrot needs to maintain the same interface for both AMQP and STOMP (obviously the one with the most capabilities won). Use the following specific settings in your ``settings.py``: .. code-block:: python # Makes python-stomp the default backend for carrot. CARROT_BACKEND = "stomp" # STOMP hostname and port settings. BROKER_HOST = "localhost" BROKER_PORT = 61613 # The queue name to use (both queue and exchange must be set to the # same queue name when using STOMP) CELERY_DEFAULT_QUEUE = "/queue/celery" CELERY_DEFAULT_EXCHANGE = "/queue/celery" CELERY_QUEUES = { "/queue/celery": {"exchange": "/queue/celery"} } Now you can go on reading the tutorial in the README, ignoring any AMQP specific options. Which features are not supported when using STOMP? -------------------------------------------------- This is a (possible incomplete) list of features not available when using the STOMP backend: * routing keys * exchange types (direct, topic, headers, etc) * immediate * mandatory Features ======== Can I send some tasks to only some servers? -------------------------------------------- **Answer:** Yes. You can route tasks to an arbitrary server using AMQP, and a worker can bind to as many queues as it wants. 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 CELERY_DEFAULT_QUEUE = "regular_tasks" CELERY_QUEUES = { "regular_tasks": { "binding_key": "task.#", }, } CELERY_DEFAULT_EXCHANGE = "tasks" CELERY_DEFAULT_EXCHANGE_TYPE = "topic" CELERY_DEFAULT_ROUTING_KEY = "task.regular" * Server ``z``: settings.py: .. code-block:: python CELERY_DEFAULT_QUEUE = "feed_tasks" CELERY_QUEUES = { "feed_tasks": { "binding_key": "feed.#", }, } CELERY_DEFAULT_EXCHANGE = "tasks" CELERY_DEFAULT_ROUTING_KEY = "task.regular" CELERY_DEFAULT_EXCHANGE_TYPE = "topic" ``CELERY_QUEUES`` is a map of queue names and their exchange/type/binding_key, if you don't set exchange or exchange type, they will be taken from the ``CELERY_DEFAULT_EXCHANGE``/``CELERY_DEFAULT_EXCHANGE_TYPE`` settings. 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.decorators import task @task(routing_key="feed.importer") def import_feed(feed_url): Feed.objects.import_feed(feed_url) or if subclassing the ``Task`` class directly: .. code-block:: python class FeedImportTask(Task): routing_key = "feed.importer" def run(self, feed_url): Feed.objects.import_feed(feed_url) You can also override this using the ``routing_key`` argument to :func:`celery.task.apply_async`: >>> from myapp.tasks import RefreshFeedTask >>> RefreshFeedTask.apply_async(args=["http://cnn.com/rss"], ... routing_key="feed.importer") If you want, you can even have your feed processing worker handle regular tasks as well, maybe in times when there's a lot of work to do. Just add a new queue to server ``z``'s ``CELERY_QUEUES``: .. code-block:: python CELERY_QUEUES = { "feed_tasks": { "binding_key": "feed.#", }, "regular_tasks": { "binding_key": "task.#", }, } Since the default exchange is ``tasks``, they will both use the same exchange. If you have another queue but on another exchange you want to add, just specify a custom exchange and exchange type: .. code-block:: python CELERY_QUEUES = { "feed_tasks": { "binding_key": "feed.#", }, "regular_tasks": { "binding_key": "task.#", } "image_tasks": { "binding_key": "image.compress", "exchange": "mediatasks", "exchange_type": "direct", }, } Easy? No? If you're confused about these terms, you should read up on AMQP and RabbitMQ. It might be hard to grok the concepts of queues, exchanges and routing/binding keys at first, but it's all very simple, I assure you. Can I use celery without Django? -------------------------------- **Answer:** Yes. Celery uses something called loaders to read/setup configuration, import modules that registers tasks and to decide what happens when a task is executed. Currently there are two loaders, the default loader and the Django loader. If you want to use celery without a Django project, you either have to use the default loader, or write a loader of your own. The rest of this answer describes how to use the default loader. First of all, installation. You need to get the development version of celery from github:: $ git clone git://github.com/ask/celery.git $ cd celery # python setup.py install # as root While it is possible to use celery from outside of Django, we still need Django itself to run, this is to use the ORM and cache-framework, etc. Duplicating these features would be time consuming and mostly pointless, so we decided that having a dependency on Django itself was a good thing. Install Django using your favorite install tool, ``easy_install``, ``pip``, or whatever:: # easy_install django # as root You need a configuration file named ``celeryconfig.py``, either in the directory you run ``celeryd`` in, or in a Python library path where it is able to find it. The configuration file can contain any of the settings described in :mod:`celery.conf`, and in additional if you're using the database backend you have to configure the database. Here is an example configuration using the database backend with MySQL: .. code-block:: python # Broker configuration BROKER_HOST = "localhost" BROKER_PORT = "5672" BROKER_VHOST = "celery" BROKER_USER = "celery" BROKER_PASSWORD = "celerysecret" CARROT_BACKEND="amqp" # Using the database backend. CELERY_BACKEND = "database" DATABASE_ENGINE = "mysql" # see Django docs for a description of these. DATABASE_NAME = "mydb" DATABASE_HOST = "mydb.example.org" DATABASE_USER = "myuser" DATABASE_PASSWORD = "mysecret" # Number of processes that processes tasks simultaneously. CELERYD_CONCURRENCY = 8 # Modules to import when celeryd starts. # This must import every module where you register tasks so celeryd # is able to find and run them. CELERY_IMPORTS = ("mytaskmodule1", "mytaskmodule2") Now with this configuration file in the current directory you have to run ``celeryinit`` to create the database tables:: $ celeryinit Then you should be able to successfully run ``celeryd``:: $ celeryd --loglevel=INFO and send a task from a python shell (note that it must be able to import ``celeryconfig.py``): >>> from celery.task.builtins import PingTask >>> result = PingTask.apply_async() >>> result.get() 'pong' The celery test-suite is failing -------------------------------- **Answer**: You're running tests from your own Django applicaiton, and celerys tests are failing and celerys tests are failing in that context? If so, read on for a trick, if not please report the test failure to our issue tracker at GitHub. http://github.com/ask/celery/issues/ That Django is running tests for all applications in ``INSTALLED_APPS`` is a pet peeve of mine. You should use a test runner that either 1) Explicitly lists the apps you want to run tests for, or 2) make a test runner that skips tests for apps you don't want to run. For example this test runner that celery is using: http://bit.ly/NVKep To use this add the following to your settings.py: .. code-block:: python TEST_RUNNER = "celery.tests.runners.run_tests" TEST_APPS = ( "app1", "app2", "app3", "app4", ) If you just want to skip celery you could use: .. code-block:: python INSTALLED_APPS = (.....) TEST_RUNNER = "celery.tests.runners.run_tests" TEST_APPS = filter(lambda k: k != "celery", INSTALLED_APPS) Can I change the interval of a periodic task at runtime? -------------------------------------------------------- **Answer**: Yes. You can override ``PeriodicTask.is_due`` or turn ``PeriodicTask.run_every`` into a property: .. code-block:: python class MyPeriodic(PeriodicTask): def run(self): # ... @property def run_every(self): return get_interval_from_database(...) Does celery support task priorities? ------------------------------------ **Answer**: No, or theoretically as AMQP supports priorities but RabbitMQ doesn't implement them yet. However the usual way to prioritize work in celery, is to route high priority tasks to different servers. In the real world this may actually work better than per. message priorities. You can use this in combination with rate limting to achieve a highly performant system. Can I schedule tasks to execute at a specific time? --------------------------------------------------- .. module:: celery.task.base **Answer**: Yes. You can use the ``eta`` argument of :meth:`Task.apply_async`. However, you can't schedule a periodic task at a specific time yet. The good news is, if anyone is willing to implement it, it shouldn't be that hard, some pointers to achieve this has been written here: http://bit.ly/99UQNO How do I shut down ``celeryd`` safely? -------------------------------------- **Answer**: Use the ``TERM`` signal, and celery will finish all currently executing jobs and shut down as soon as possible. No tasks should be lost. You should never stop ``celeryd`` with the ``KILL`` signal (``-9``), unless you've tried ``TERM`` a few times and waited a few minutes to let it get a chance to shut down.