This document describes Celery 2.4. For development docs, go here.
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 Mailing list.
Basics¶
Automatic routing¶
The simplest way to do routing is to use the
CELERY_CREATE_MISSING_QUEUES
setting (on by default).
With this setting on, a named queue that is not already defined in
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 historical 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
Changing the name of the default queue¶
You can change the name of the default queue by using the following configuration:
CELERY_QUEUES = {"default": {"exchange": "default",
"binding_key": "default"}}
CELERY_DEFAULT_QUEUE = "default"
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:
{"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.
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:
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"
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
and
CELERY_DEFAULT_EXCHANGE_TYPE
settings.
To route a task to the feed_tasks queue, you can add an entry in the
CELERY_ROUTES
setting:
CELERY_ROUTES = {
"feeds.tasks.import_feed": {
"queue": "feed_tasks",
"routing_key": "feed.import",
},
}
You can also override this using the routing_key argument to
apply_async()
, or 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:
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.
See also
In addition to the 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.
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:
{"task": "myapp.tasks.add",
"id": "54086c5e-6193-4575-8308-dbab76798756",
"args": [4, 4],
"kwargs": {}}
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.
Exchanges, queues and routing keys.¶
- Messages are sent to exchanges.
- An exchange routes messages to one or more queues. Several exchange types exists, providing different ways to do routing, or implementing different messaging scenarios.
- The message waits in the queue until someone consumes it.
- The message is deleted from the queue when it has been acknowledged.
The steps required to send and receive messages are:
- Create an exchange
- Create a queue
- Bind the queue to the exchange.
Celery automatically creates the entities necessary for the queues in
CELERY_QUEUES
to work (except if the queue’s auto_declare
setting is set to False
).
Here’s an example queue configuration with three queues; One for video, one for images and one default queue for everything else:
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.
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.
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.
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).
Hands-on with the API¶
Celery comes with a tool called 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 polls for new messages on the queue.
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¶
Defining queues¶
In Celery available queues are defined by the 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:
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 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 CELERY_QUEUES
.
Specifying task destination¶
The destination for a task is decided by the following (in order):
- The Routers defined in
CELERY_ROUTES
. - The routing arguments to
apply_async()
. - Routing related attributes defined on the
Task
itself.
It is considered best practice to not hard-code these settings, but rather leave that as configuration options by using Routers; This is the most flexible approach, but sensible defaults can still be set as task attributes.
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:
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 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 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 CELERY_ROUTES
to get the
same behavior:
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.