How to configure Celery

This article assumes that you are already familiar with the basics of using Celery with Django and that you have Celery installed in your application.

If not, please see Celery’s documentation.

Add a Celery service to your application

In the application’s subscription, add the number of Celery workers you require. You can start with just one and add more later if required.

Important

If your environments have not yet been deployed, please deploy each of them. This is required before Celery can be provisioned on the application.

Celery will then be provisioned for your application’s environments by our infrastructure team. This includes configuration of new environment variables it will need.

Once provisioned and deployed, your cloud application will run with new Docker instances for the Celery workers. The containers running Celery components use the same image as the web container, but are started up with a different command.

We provide various cloud containers for Celery:

  • Celery worker containers (multiple containers, according to the subscription)

  • a Celery beat container, to handle scheduling

  • a Celery camera, to provide snapshots for monitoring

Note that if your Divio application is on a plan that pauses due to inactivity, this will also pause the Celery containers.

Application configuration

Your application needs configuration to:

  • read the environment variables we supply and use them as values for configuring Celery

  • start up each Celery container correctly

  • for local development, run Celery’s workers in their own containers to replicate the cloud configuration

These tasks are covered in order below.

Using the broker environment variable

For Celery, we provide a DEFAULT_AMQP_BROKER_URL (the same value is also available as BROKER_URL, provided for legacy Aldryn Celery applications). This provides configuration details for the AMQP message queue that handles Celery tasks. It’s in the form:

transport://userid:password@hostname:port/virtual_host

This configuration will need to be passed to Celery for its broker settings (CELERY_BROKER_URL, for Django).

For applications using Aldryn Celery

Aldryn Celery will take care of configuration. See Aldryn Celery (legacy) below.

Starting the cloud containers

As noted above, these containers are all instances of the same application image, but are started up by different commands.

For the worker and scheduling containers, your application needs an executable at /usr/local/bin/aldryn-celery, containing:

#!/bin/sh
if [ $1 = "beat" ] ; then
    celery -A path.to.celery.app beat --loglevel=INFO
else
    celery -A path.to.celery.app worker --concurrency=4 --loglevel=INFO --without-gossip --without-mingle --without-heartbeat -Ofair
fi

Note the paths that you will need to specify yourself.

Similarly, on deployment the infrastructure invokes (by default) a Django management command python manage.py celerycam to start up the monitoring container.

  • If you don’t want to use a monitoring container, please inform us, so that we can configure your application to start up without issuing the command (deployments will fail if the command fails).

  • If you do want to use a monitoring container, you will need to add a celerycam management command to your application. The command needs to respond to the invocation: python manage.py celerycam --frequency=10 --pidfile=.

For an example of a celerycam management command implementation, see how Aldryn Celery does this via the djcelery.snapshot.Camera class from the Django Celery library.

These entrypoints will be improved in future for developer convenience.

For applications using Aldryn Celery

If using Aldryn Celery, an executable /usr/local/bin/aldryn-celery is provided.

Similarly, a celerycam management command is implemented.

No further action is required on your part.

See Aldryn Celery (legacy) below.

Configure Celery for the local environment

For development purposes you will need to set up Celery in your local environment too, in such a way that it reflects the provision made on our cloud. A complete set-up would include:

function

handled by

on the cloud

local container name

AMPQ message broker service responsible for the creation of task queues

RabbitMQ

CloudAMPQ

rabbitmq

task execution

Celery workers

Celery containers

celeryworker

scheduling

Celery beat

Celery beat container

celerybeat

monitoring

Celery snapshots

Celery camera container

celerycam

Locally, the four new containers will be set up as new services using the docker-compose.yml file.

Note that in the cloud environment, the Celery-related containers are launched automatically. They, and the AMPQ message queue, are not directly accessible. All monitoring and interaction must be handled via the main application running in the web container(s). The docker-compose file is not used on the cloud.

Your application will already have other services listed in its docker-compose.yml. Each of the new services will need to be added in a similar way.

RabbitMQ

Set up the RabbitMQ messaging service, by adding the following lines:

services:

  web:
    [...]

  database_default:
    [...]

  rabbitmq:
    image: rabbitmq:3.5-management
    hostname: rabbitmq
    ports:
      - "15672:15672"
    expose:
      - "15672"

This uses the official Docker RabbitMQ image (the rabbitmq:3.5-management image in turn installs rabbitmq:3.5). It also gives the container a hostname (rabbitmq), maps and exposes the management interface port (15672).

Celery worker

Next add a Celery worker service in the same way. This service needs to run a Django environment almost identical to that used by the web service, as it will use the same codebase, need access to the same database and so on. Its definition will therefore be very similar, with key changes noted here:

celeryworker:
  build: "."
  links:
    - "database_default"
    - "rabbitmq:rabbitmq"
  volumes:
    - ".:/app:rw"
    - "./data:/data:rw"
  command: <startup command>
  env_file: .env-local

Rather than copying the example above, use the actual web service in your docker-compose file as its basis, in case it contains other values that need to be present. There’s no need for the ports option.

You will need to provide a <startup command> based on the one used to start up the cloud workers.

For applications using Aldryn Celery, use command: aldryn-celery worker.

Celery beat

Celery beat needs to be set up in much the same way:

celerybeat:
  build: "."
  links:
    - "database_default"
    - "rabbitmq:rabbitmq"
  volumes:
    - ".:/app:rw"
    - "./data:/data:rw"
  command: <startup command>
  env_file: .env-local

You will need to provide a <startup command> based on the one used to start up the cloud scheduler.

For applications using Aldryn Celery, use command: aldryn-celery beat.

Celery cam

And Celery cam:

celerycam:
  build: "."
  links:
    - "database_default"
    - "rabbitmq:rabbitmq"
  volumes:
    - ".:/app:rw"
    - "./data:/data:rw"
  command: aldryn-celery cam
  env_file: .env-local

You will need to provide a <startup command> based on based on the one used to start up the cloud monitoring container., e.g. python manage.py celerycam --frequency=10 --pidfile=.

For applications using Aldryn Celery, use command: aldryn-celery cam.

The web service

Finally, to the links option in web, you also need to add the link to rabbitmq:

web:
  [...]
  links:
    [...]
    - "rabbitmq:rabbitmq"

Set up local environment variables

In .env-local add:

DEFAULT_AMQP_BROKER_URL="amqp://guest:guest@rabbitmq:5672/"

Note

For legacy Aldryn Celery applications, name the environment variable BROKER_URL instead of DEFAULT_AMQP_BROKER_URL.

Port 5672 of the RabbitMQ server should not be confused with port 15672 of its management interface.

Run the local application

Build the newly-configured application:

docker-compose build

Now docker-compose up will start the services that Celery requires.

Note that although the Django runserver in your web container will restart automatically to load new code whenever you make changes, that will not apply to the other services.

These will need to be restarted manually, for example by stopping and restarting the local application or by running docker-compose restart. (Usually, only the celeryworker container needs to be restarted, so you can do docker-compose restart celeryworker.)

If you make any local changes to a application’s configuration that need to be accessible to the Celery workers, run docker-compose build to rebuild them.

Environment variables

When Celery is enabled for your application, a new environment variable DEFAULT_AMQP_BROKER_URL will be configured. (It’s also provided as BROKER_URL for legacy Aldryn Celery applications.)

The environment variable will have different values in different cloud environments.

The number of Celery workers per Docker instance can be configured with the CELERYD_CONCURRENCY environment variable. The default is 2. This can be increased, but in that case, you will need to monitor your own RAM consumption via the Control Panel.

For applications using Aldryn Celery

Other environment variables used by Aldryn Celery can be found in its aldryn_config.py.

Aldryn Celery (legacy)

Aldryn Celery is an Aldryn Addon wrapper application that installs and configures Celery in your application, exposing multiple Celery settings as environment variables for fine-tuning its configuration.

Aldryn Celery installs components including Celery itself and Django Celery. The addon is no longer updated, and installs an older version of Celery. Applications currently using Aldryn Celery will eventually need to be updated to maintain compatibility with other dependencies of the application.