This document discusses using Celery and RabbitMQ to handle asynchronous and distributed tasks in Python. It provides an overview of Celery and RabbitMQ and how they can be used together. Examples are given of defining and running tasks, including periodic tasks, task sets, routing, retries, and interacting with results. The benefits of using Celery for slow, memory-intensive, or externally-dependent tasks are highlighted.

2. Celery
Òscar Vilaplana
February 28 2012

3. Outline
self.__dict__
Use task queues
Celery and RabbitMQ
Getting started with RabbitMQ
Getting started with Celery
Periodic tasks
Examples

4. self.__dict__
{'name': 'Òscar Vilaplana',
'origin': 'Catalonia',
'company': 'Paylogic',
'tags': ['developer', 'architect', 'geek'],
'email': 'dev@oscarvilaplana.cat',
}

5. Proposal
Take a slow task.
Decouple it from your system
Call it asynchronously

6. Separate projects
Separate projects allow us to:
Divide your system in sections
e.g. frontend, backend, mailing, reportgenerator. . .
Tackle them individually
Conquer themdeclare them Done:
Clean code
Clean interface
Unit tested
Maintainable
(but this is not only for Celery tasks)

7. Coupled Tasks
In some cases, it may not be possible to decouple some tasks.
Then, we either:
Have some workers in your system's network
with access to the code of your system
with access to the system's database
They handle messages from certain queues, e.g. internal.#

8. Candidates
Processes that:
Need a lot of memory.
Are slow.
Depend on external systems.
Need a limited amount of data to work (easy to decouple).
Need to be scalable.
Examples:
Render complex reports.
Import big les
Send e-mails

9. Example: sending complex emails
Create a in independent project: yourappmail
Generator of complex e-mails.
It needs the templates, images. . .
It doesn't need access to your system's database.
Deploy it in servers of our own, or in Amazon servers
We can add/remove as we need them
On startup:
Join the RabbitMQ cluster
Start celeryd
Normal operation: 1 server is enough
tps peak
On high load: start as many servers as needed (
tps server )

10. yourappmail
A decoupled email generator:
Has a clean API
Decoupled from your system's db: It needs to receive all
information
Customer information
Custom data
Contents of the email
Can be deployed to as many servers as we need
Scalable

11. Not for everything
Task queues are not a magic wand to make things faster
They can be used as such (like cache).
It hides the real problem.

12. Celery
Asynchronous distributed task queue
Based on distributed message passing.
Mostly for real-time queuing
Can do scheduling too.
REST: you can query status and results via URLs.
Written in Python
Celery: Message Brokers and Result Storage

13. Celery's tasks
Tasks can be async or sync
Low latency
Rate limiting
Retries
Each task has an UUID: you can ask for the result back if you
know the task UUID.
RabbitMQ
Messaging system
Protocol: AMQP
Open standard for messaging middleware
Written in Erlang
Easy to cluster!

14. Install the packages from the RabbitMQ website
RabbitMQ Server
Management Plugin (nice HTML interface)
rabbitmq-plugins enable rabbitmq_management
Go to http://localhost:55672/cli/ and download the cli.
HTML interface at http://localhost:55672/

15. Set up a cluster
rabbit1$ rabbitmqctl cluster_status
Cluster status of node rabbit@rabbit1 ...
[{nodes,[{disc,[rabbit@rabbit1]}]},{running_nodes,[rabbit@ra
...done.
rabbit2$ rabbitmqctl stop_app
Stopping node rabbit@rabbit2 ...done.
rabbit2$ rabbitmqctl reset
Resetting node rabbit@rabbit2 ...done.
rabbit2$ rabbitmqctl cluster rabbit@rabbit1
Clustering node rabbit@rabbit2 with [rabbit@rabbit1] ...done
rabbit2$ rabbitmqctl start_app
Starting node rabbit@rabbit2 ...done.

16. Notes
Automatic conguration
Use .config le to describe the cluster.
Change the type of the node
RAM node
Disk node

17. Install Celery
Just pip install

18. Dene a task
Example tasks.py
from celery.task import task
@task
def add(x, y):
print I received the task to add {} and {}.format(x, y
return x + y

19. Congure username, vhost, permissions
$ rabbitmqctl add_user myuser mypassword
$ rabbitmqctl add_vhost myvhost
$ rabbitmqctl set_permissions -p myvhost myuser .* .* .

20. Conguration le
Write celeryconfig.py
BROKER_HOST = localhost
BROKER_PORT = 5672
BROKER_USER = myusername
BROKER_PASSWORD = mypassword
BROKER_VHOST = myvhost
CELERY_RESULT_BACKEND = amqp
CELERY_IMPORTS = (tasks, )

21. Launch daemon
celeryd -I tasks # import the tasks module

22. Schedule tasks
from tasks import add
# Schedule the task
result = add.delay(1, 2)
value = result.get() # value == 3

23. Schedule tasks by name
Sometimes the tasks module is not available on the clients
from tasks import add
# Schedule the task
result = add.delay(1, 2)
value = result.get() # value == 3
print value

24. Schedule the tasks better: apply_async
task.apply_async has more options:
countdown=n: the task will run at least n seconds in the
future.
eta=datetime: the task will run not earlier than than
datetime.
expires=n or expires=datetime the task will be revoked in
n seconds or at datetime
It will be marked as REVOKED
result.get will raise a TaskRevokedError
serializer
pickle: default, unless CELERY_TASK_SERIALIZER says
otherwise.
alternative: json, yaml, msgpack

25. Result
A result has some useful operations:
successful: True if task succeeded
ready: True if the result is ready
revoke: cancel the task.
result: if task has been executed, this contains the result if it
raised an exception, it contains the exception instance
state:
PENDING
STARTED
RETRY
FAILURE
SUCCESS

26. TaskSet
Run several tasks at once. The result keeps the order.
from celery.task.sets import TaskSet
from tasks import add
job = TaskSet(tasks=[
add.subtask((4, 4)),
add.subtask((8, 8)),
add.subtask((16, 16)),
add.subtask((32, 32)),
])
result = job.apply_async()
result.ready() # True -- all subtasks completed
result.successful() # True -- all subtasks successful
values = result.join() # [4, 8, 16, 32, 64]
print values

27. TaskSetResult
The TaskSetResult has some interesting properties:
successful: if all of the subtasks nished successfully (no
Exception)
failed: if any of the subtasks failed.
waiting: if any of the subtasks is not ready yet.
ready: if all of the subtasks are ready.
completed_count: number of completed subtasks.
revoke: revoke all subtasks.
iterate: iterate oer the return values of the subtasks once
they nish (sorted by nish order).
join: gather the results of the subtasks and return them in a
list (sorted by the order on which they were called).

28. Retrying tasks
If the task fails, you can retry it by calling retry()
@task
def send_twitter_status(oauth, tweet):
try:
twitter = Twitter(oauth)
twitter.update_status(tweet)
except (Twitter.FailWhaleError, Twitter.LoginError), exc
send_twitter_status.retry(exc=exc)
To limit the number of retries set task.max_retries.

29. Routing
apply_async accepts the parameter routing to create some
RabbitMQ queues
pdf: ticket.#
import_files: import.#
Schedule the task to the appropriate queue
import_vouchers.apply_async(args=[filename],
routing_key=import.vouchers)
generate_ticket.apply_async(args=barcodes,
routing_key=ticket.generate)

30. celerybeat
from celery.schedules import crontab
CELERYBEAT_SCHEDULE = {
# Executes every Monday morning at 7:30 A.M
every-monday-morning: {
task: tasks.add,
schedule: crontab(hour=7, minute=30,day_of_week=1),
args: (16, 16),
},
}

31. There can be only one celerybeat running
But we can have two machines that check on each other.

32. Import a big le:
tasks.py
def import_bigfile(server, filename):
with create_temp_file() as tmp:
fetch_bigfile(tmp, server, filename)
import_bigfile(tmp)
report_result(...) # e.g. send confirmation e-mail

33. Import big le: Admin interface, server-Side
import tasks
def import_bigfile(filename):
result = tasks.imporg_bigfile.delay(filename)
return result.task_id
class ImportBigfile(View):
def post_ajax(request):
filename = request.get('big_file')
task_id = import_bigfile(filename)
return task_id

34. Import big le: Admin interface, client-side
Post the le asynchronously
Get the task_id back
Put some working. . . message.
Periodically ask Celery if the task is ready and change
working. . . into done!
No need to call Paylogic code: just ask Celery directly
Improvements:
Send the username to the task.
Have the task call back the Admin interface when it's done.
The Backoce can send an e-mail to the user when the task is
done.

35. Do a time-consuming task.
from tasks import do_difficult_thing
...stuff...
# I have all data necessary to do the difficult thing
difficult_result = do_difficult_thing.delay(some, values)
# I don't need the result just yet, I can keep myself busy
... stuff ...
# Now I really need the result
difficult_value = difficult_result.get()