upstream-zulip/docs/subsystems/queuing.md

Queue processors

Zulip uses RabbitMQ to manage a system of internal queues. These are used for a variety of purposes:

• Asynchronously doing expensive operations like sending email notifications which can take seconds per email and thus would otherwise time out when 100s are triggered at once (e.g., inviting a lot of new users to a realm).

• Asynchronously doing non-time-critical somewhat expensive operations like updating analytics tables (e.g., UserActivityInternal) which don't have any immediate runtime effect.

• Communicating events to push to clients (browsers, etc.) from the main Zulip Django application process to the Tornado-based events system. Example events might be that a new message was sent, a user has changed their subscriptions, etc.

• Processing mobile push notifications and email mirroring system messages.

• Processing various errors, frontend tracebacks, and slow database queries in a batched fashion.

Needless to say, the RabbitMQ-based queuing system is an important part of the overall Zulip architecture, since it's in critical code paths for everything from signing up for account, to rendering messages, to delivering updates to clients.

We use the pika library to interface with RabbitMQ, using a simple custom integration defined in zerver/lib/queue.py.

Adding a new queue processor

To add a new queue processor:

• Define the processor in zerver/worker/ using the @assign_queue decorator; it's pretty easy to get the template for an existing similar queue processor. This suffices to test your queue worker in the Zulip development environment (tools/run-dev will automatically restart the queue processors and start running your new queue processor code). You can also run a single queue processor manually using, for example, ./manage.py process_queue --queue=user_activity.

• So that supervisord will know to run the queue processor in production, you will need to add to the base_queues variable in puppet/zulip/manifests/app_frontend_base.pp; the list there is used to generate /etc/supervisor/conf.d/zulip.conf.

The queue will automatically be added to the list of queues tracked by scripts/nagios/check-rabbitmq-consumers, so Nagios can properly check whether a queue processor is running for your queue. You still need to update the sample Nagios configuration in puppet/kandra manually.

Publishing events into a queue

You can publish events to a RabbitMQ queue using the queue_event_on_commit function defined in zerver/lib/queue.py.

An interesting challenge with queue processors is what should happen when queued events in Zulip's backend tests. Our current solution is that in the tests, queue_event_on_commit will (by default) simple call the consume method for the relevant queue processor. However, queue_event_on_commit also supports being passed a function that should be called in the tests instead of the queue processor's consume method. Where possible, we prefer the model of calling consume in tests since that's more predictable and automatically covers the queue processor's code path, but it isn't always possible.

Clearing a RabbitMQ queue

If you need to clear a queue (delete all the events in it), run ./manage.py purge_queue <queue_name>, for example:

./manage.py purge_queue user_activity

You can also use the amqp tools directly. Install amqp-tools from apt and then run:

amqp-delete-queue --username=zulip --password='...' --server=localhost \
   --queue=user_activity

with the RabbitMQ password from /etc/zulip/zulip-secrets.conf.