.. _guide-routing: =============== Routing Tasks =============== .. warning:: This document refers to functionality only available in brokers using AMQP. Other brokers may implement some functionality, see their respective documentation for more information, or contact the :ref:`mailing-list`. .. contents:: :local: .. _routing-basics: Basics ====== .. _routing-automatic: Automatic routing ----------------- The simplest way to do routing is to use the :setting:`CELERY_CREATE_MISSING_QUEUES` setting (on by default). With this setting on, a named queue that is not already defined in :setting:`CELERY_QUEUES` will be created automatically. This makes it easy to perform simple routing tasks. 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:: CELERY_ROUTES = {"feed.tasks.import_feed": {"queue": "feeds"}} With this route enabled import feed tasks will be routed to the ``"feeds"`` queue, while all other tasks will be routed to the default queue (named ``"celery"`` for historic reasons). Now you can start server ``z`` to only process the feeds queue like this:: (z)$ celeryd -Q feeds You can specify as many queues as you want, so you can make this server process the default queue as well:: (z)$ celeryd -Q feeds,celery .. _routing-changing-default-queue: Changing the name of the default queue ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can change the name of the default queue by using the following configuration: .. code-block:: python CELERY_QUEUES = {"default": {"exchange": "default", "binding_key": "default"}} CELERY_DEFAULT_QUEUE = "default" .. _routing-autoqueue-details: How the queues are defined ~~~~~~~~~~~~~~~~~~~~~~~~~~ The point with this feature is to hide the complex AMQP protocol for users with only basic needs. However -- you may still be interested in how these queues are declared. A queue named ``"video"`` will be created with the following settings: .. code-block:: python {"exchange": "video", "exchange_type": "direct", "routing_key": "video"} The non-AMQP backends like ``ghettoq`` does not support exchanges, so they require the exchange to have the same name as the queue. Using this design ensures it will work for them as well. .. _routing-manual: Manual routing -------------- 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: .. code-block:: python CELERY_DEFAULT_QUEUE = "default" CELERY_QUEUES = { "default": { "binding_key": "task.#", }, "feed_tasks": { "binding_key": "feed.#", }, } CELERY_DEFAULT_EXCHANGE = "tasks" CELERY_DEFAULT_EXCHANGE_TYPE = "topic" CELERY_DEFAULT_ROUTING_KEY = "task.default" :setting:`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 :setting:`CELERY_DEFAULT_EXCHANGE` and :setting:`CELERY_DEFAULT_EXCHANGE_TYPE` settings. To route a task to the ``feed_tasks`` queue, you can add an entry in the :setting:`CELERY_ROUTES` setting: .. code-block:: python CELERY_ROUTES = { "feeds.tasks.import_feed": { "queue": "feed_tasks", "routing_key": "feed.import", }, } You can also override this using the ``routing_key`` argument to :func:`~celery.execute.apply_async`, or :func:`~celery.execute.send_task`: >>> from feeds.tasks import import_feed >>> import_feed.apply_async(args=["http://cnn.com/rss"], ... queue="feed_tasks", ... routing_key="feed.import") To make server ``z`` consume from the feed queue exclusively you can start it with the ``-Q`` option:: (z)$ celeryd -Q feed_tasks --hostname=z.example.com Servers ``x`` and ``y`` must be configured to consume from the default queue:: (x)$ celeryd -Q default --hostname=x.example.com (y)$ celeryd -Q default --hostname=y.example.com 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:: (z)$ celeryd -Q feed_tasks,default --hostname=z.example.com 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", }, } If you're confused about these terms, you should read up on AMQP. .. seealso:: In addition to the :ref:`amqp-primer` below, there's `Rabbits and Warrens`_, an excellent blog post describing queues and exchanges. There's also AMQP in 10 minutes*: `Flexible Routing Model`_, and `Standard Exchange Types`_. For users of RabbitMQ the `RabbitMQ FAQ`_ could be useful as a source of information. .. _`Rabbits and Warrens`: http://blogs.digitar.com/jjww/2009/01/rabbits-and-warrens/ .. _`Flexible Routing Model`: http://bit.ly/95XFO1 .. _`Standard Exchange Types`: http://bit.ly/EEWca .. _`RabbitMQ FAQ`: http://www.rabbitmq.com/faq.html .. _amqp-primer: AMQP Primer =========== Messages -------- A message consists of headers and a body. Celery uses headers to store the content type of the message and its content encoding. The content type is usually the serialization format used to serialize the message. The body contains the name of the task to execute, the task id (UUID), the arguments to execute it with and some additional metadata -- like the number of retries or an ETA. This is an example task message represented as a Python dictionary: .. code-block:: python {"task": "myapp.tasks.add", "id": "54086c5e-6193-4575-8308-dbab76798756", "args": [4, 4], "kwargs": {}} .. _amqp-producers-consumers-brokers: Producers, consumers and brokers -------------------------------- The client sending messages is typically called a *publisher*, or a *producer*, while the entity receiving messages is called a *consumer*. The *broker* is the message server, routing messages from producers to consumers. You are likely to see these terms used a lot in AMQP related material. .. _amqp-exchanges-queues-keys: Exchanges, queues and routing keys. ----------------------------------- 1. Messages are sent to exchanges. 2. An exchange routes messages to one or more queues. Several exchange types exists, providing different ways to do routing, or implementing different messaging scenarios. 3. The message waits in the queue until someone consumes it. 4. The message is deleted from the queue when it has been acknowledged. The steps required to send and receive messages are: 1. Create an exchange 2. Create a queue 3. Bind the queue to the exchange. Celery automatically creates the entities necessary for the queues in :setting:`CELERY_QUEUES` to work (except if the queue's ``auto_declare`` setting is set to :const:`False`). Here's an example queue configuration with three queues; One for video, one for images and one default queue for everything else: .. code-block:: python CELERY_QUEUES = { "default": { "exchange": "default", "binding_key": "default"}, "videos": { "exchange": "media", "binding_key": "media.video", }, "images": { "exchange": "media", "binding_key": "media.image", } } CELERY_DEFAULT_QUEUE = "default" CELERY_DEFAULT_EXCHANGE_TYPE = "direct" CELERY_DEFAULT_ROUTING_KEY = "default" .. note:: In Celery the ``routing_key`` is the key used to send the message, while ``binding_key`` is the key the queue is bound with. In the AMQP API they are both referred to as the routing key. .. _amqp-exchange-types: Exchange types -------------- The exchange type defines how the messages are routed through the exchange. The exchange types defined in the standard are `direct`, `topic`, `fanout` and `headers`. Also non-standard exchange types are available as plug-ins to RabbitMQ, like the `last-value-cache plug-in`_ by Michael Bridgen. .. _`last-value-cache plug-in`: http://github.com/squaremo/rabbitmq-lvc-plugin .. _amqp-exchange-type-direct: Direct exchanges ~~~~~~~~~~~~~~~~ Direct exchanges match by exact routing keys, so a queue bound by the routing key ``video`` only receives messages with that routing key. .. _amqp-exchange-type-topic: Topic exchanges ~~~~~~~~~~~~~~~ Topic exchanges matches routing keys using dot-separated words, and the wildcard characters: ``*`` (matches a single word), and ``#`` (matches zero or more words). With routing keys like ``usa.news``, ``usa.weather``, ``norway.news`` and ``norway.weather``, bindings could be ``*.news`` (all news), ``usa.#`` (all items in the USA) or ``usa.weather`` (all USA weather items). .. _amqp-api: Related API commands -------------------- .. method:: exchange.declare(exchange_name, type, passive, durable, auto_delete, internal) Declares an exchange by name. :keyword passive: Passive means the exchange won't be created, but you can use this to check if the exchange already exists. :keyword durable: Durable exchanges are persistent. That is - they survive a broker restart. :keyword auto_delete: This means the queue will be deleted by the broker when there are no more queues using it. .. method:: queue.declare(queue_name, passive, durable, exclusive, auto_delete) Declares a queue by name. Exclusive queues can only be consumed from by the current connection. Exclusive also implies ``auto_delete``. .. method:: queue.bind(queue_name, exchange_name, routing_key) Binds a queue to an exchange with a routing key. Unbound queues will not receive messages, so this is necessary. .. method:: queue.delete(name, if_unused=False, if_empty=False) Deletes a queue and its binding. .. method:: exchange.delete(name, if_unused=False) Deletes an exchange. .. note:: Declaring does not necessarily mean "create". When you declare you *assert* that the entity exists and that it's operable. There is no rule as to whom should initially create the exchange/queue/binding, whether consumer or producer. Usually the first one to need it will be the one to create it. .. _amqp-api-hands-on: Hands-on with the API --------------------- Celery comes with a tool called :program:`camqadm` (short for Celery AMQ Admin). It's used for command-line access to the AMQP API, enabling access to administration tasks like creating/deleting queues and exchanges, purging queues or sending messages. You can write commands directly in the arguments to ``camqadm``, or just start with no arguments to start it in shell-mode:: $ camqadm -> connecting to amqp://guest@localhost:5672/. -> connected. 1> Here `1>` is the prompt. The number 1, is the number of commands you have executed so far. Type `help` for a list of commands available. It also supports auto-completion, so you can start typing a command and then hit the `tab` key to show a list of possible matches. Let's create a queue we can send messages to:: 1> exchange.declare testexchange direct ok. 2> queue.declare testqueue ok. queue:testqueue messages:0 consumers:0. 3> queue.bind testqueue testexchange testkey ok. This created the direct exchange ``testexchange``, and a queue named ``testqueue``. The queue is bound to the exchange using the routing key ``testkey``. From now on all messages sent to the exchange ``testexchange`` with routing key ``testkey`` will be moved to this queue. We can send a message by using the ``basic.publish`` command:: 4> basic.publish "This is a message!" testexchange testkey ok. Now that the message is sent we can retrieve it again. We use the ``basic.get`` command here, which pops a single message off the queue, this command is not recommended for production as it implies polling, any real application would declare consumers instead. Pop a message off the queue:: 5> basic.get testqueue {'body': 'This is a message!', 'delivery_info': {'delivery_tag': 1, 'exchange': u'testexchange', 'message_count': 0, 'redelivered': False, 'routing_key': u'testkey'}, 'properties': {}} AMQP uses acknowledgment to signify that a message has been received and processed successfully. If the message has not been acknowledged and consumer channel is closed, the message will be delivered to another consumer. Note the delivery tag listed in the structure above; Within a connection channel, every received message has a unique delivery tag, This tag is used to acknowledge the message. Also note that delivery tags are not unique across connections, so in another client the delivery tag ``1`` might point to a different message than in this channel. You can acknowledge the message we received using ``basic.ack``:: 6> basic.ack 1 ok. To clean up after our test session we should delete the entities we created:: 7> queue.delete testqueue ok. 0 messages deleted. 8> exchange.delete testexchange ok. .. _routing-tasks: Routing Tasks ============= .. _routing-defining-queues: Defining queues --------------- In Celery available queues are defined by the :setting:`CELERY_QUEUES` setting. Here's an example queue configuration with three queues; One for video, one for images and one default queue for everything else: .. code-block:: python CELERY_QUEUES = { "default": { "exchange": "default", "binding_key": "default"}, "videos": { "exchange": "media", "exchange_type": "topic", "binding_key": "media.video", }, "images": { "exchange": "media", "exchange_type": "topic", "binding_key": "media.image", } } CELERY_DEFAULT_QUEUE = "default" CELERY_DEFAULT_EXCHANGE = "default" CELERY_DEFAULT_EXCHANGE_TYPE = "direct" CELERY_DEFAULT_ROUTING_KEY = "default" Here, the :setting:`CELERY_DEFAULT_QUEUE` will be used to route tasks that doesn't have an explicit route. The default exchange, exchange type and routing key will be used as the default routing values for tasks, and as the default values for entries in :setting:`CELERY_QUEUES`. .. _routing-task-destination: Specifying task destination --------------------------- The destination for a task is decided by the following (in order): 1. The :ref:`routers` defined in :setting:`CELERY_ROUTES`. 2. The routing arguments to :func:`~celery.execute.apply_async`. 3. Routing related attributes defined on the :class:`~celery.task.base.Task` itself. It is considered best practice to not hard-code these settings, but rather leave that as configuration options by using :ref:`routers`; This is the most flexible approach, but sensible defaults can still be set as task attributes. .. _routers: Routers ------- A router is a class that decides the routing options for a task. All you need to define a new router is to create a class with a ``route_for_task`` method: .. code-block:: python class MyRouter(object): def route_for_task(self, task, args=None, kwargs=None): if task == "myapp.tasks.compress_video": return {"exchange": "video", "exchange_type": "topic", "routing_key": "video.compress"} return None If you return the ``queue`` key, it will expand with the defined settings of that queue in :setting:`CELERY_QUEUES`:: {"queue": "video", "routing_key": "video.compress"} becomes --> {"queue": "video", "exchange": "video", "exchange_type": "topic", "routing_key": "video.compress"} You install router classes by adding them to the :setting:`CELERY_ROUTES` setting:: CELERY_ROUTES = (MyRouter, ) Router classes can also be added by name:: CELERY_ROUTES = ("myapp.routers.MyRouter", ) For simple task name -> route mappings like the router example above, you can simply drop a dict into :setting:`CELERY_ROUTES` to get the same behavior: .. code-block:: python CELERY_ROUTES = ({"myapp.tasks.compress_video": { "queue": "video", "routing_key": "video.compress" }}, ) The routers will then be traversed in order, it will stop at the first router returning a true value, and use that as the final route for the task.