Celery is a task queue powered by RabbitMQ. You can use it for anything that doesn’t need to complete in the current request-response cycle. Or use it wherever Les tells you to use it.

For example, each addon has a current_version cached property. This query on initial run causes strain on our database. We can create a denormalized database field called current_version on the addons table.

We’ll need to populate regularly so it has fairly up-to-date data. We can do this in a process outside the request-response cycle. This is where Celery comes in.



Celery depends on RabbitMQ. If you use homebrew you can install this:

brew install rabbitmq

Setting up rabbitmq involves some configuration. You may want to define the following

# On a Mac, you can find this in System Preferences > Sharing
export HOSTNAME='<laptop name>.local'

Then run the following commands:

# Set your host up so it's semi-permanent
sudo scutil --set HostName $HOSTNAME

# Update your hosts by either:
# 1) Manually editing /etc/hosts
# 2) `echo $HOSTNAME >> /etc/hosts`

# RabbitMQ insists on writing to /var
sudo rabbitmq-server -detached

# Setup rabitty things (sudo is required to read the cookie file)
sudo rabbitmqctl add_user zamboni zamboni
sudo rabbitmqctl add_vhost zamboni
sudo rabbitmqctl set_permissions -p zamboni zamboni ".*" ".*" ".*"

Back in safe and happy django-land you should be able to run:

./manage.py celery worker -Q priority,devhub,images,limited  $OPTIONS

Celery understands python and any tasks that you have defined in your app are now runnable asynchronously.

Celery Tasks

Any python function can be set as a celery task. For example, let’s say we want to update our current_version but we don’t care how quickly it happens, just that it happens. We can define it like so:

def _update_addons_current_version(data, **kw):
    task_log.debug("[%s@%s] Updating addons current_versions." %
                   (len(data), _update_addons_current_version.rate_limit))
    for pk in data:
            addon = Webapp.objects.get(pk=pk)
        except Webapp.DoesNotExist:
            task_log.debug("Missing addon: %d" % pk)

@task is a decorator for Celery to find our tasks. We can specify a rate_limit like 2/m which means celery worker will only run this command 2 times a minute at most. This keeps write-heavy tasks from killing your database.

If we run this command like so:

from celery.task.sets import TaskSet

all_pks = Webapp.objects.all().values_list('pk', flat=True)
ts = [_update_addons_current_version.subtask(args=[pks])
      for pks in mkt.site.utils.chunked(all_pks, 300)]

All the Webapps with ids in pks will (eventually) have their current_versions updated.

Cron Jobs

This is all good, but let’s automate this. In Zamboni we can create cron jobs like so:

def update_addons_current_version():
    """Update the current_version field of the addons."""
    d = Webapp.objects.valid().values_list('id', flat=True)

    with establish_connection() as conn:
        for chunk in chunked(d, 1000):
            print chunk

This job will hit all the addons and run the task we defined in small batches of 1000.

We’ll need to add this to both the prod and preview crontabs so that they can be run in production.

Better than Cron

Of course, cron is old school. We want to do better than cron, or at least not rely on brute force tactics.

For a surgical strike, we can call _update_addons_current_version any time we add a new version to that addon. Celery will execute it at the prescribed rate, and your data will be updated ... eventually.

During Development

celery worker only knows about code as it was defined at instantiation time. If you change your @task function, you’ll need to HUP the process.

However, if you’ve got the @task running perfectly you can tweak all the code, including cron jobs that call it without need of restart.