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 timeout 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.

• Doing markdown rendering for messages delivered to the Tornado via websockets.

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/queue_processors.py 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.py will automatically restart the queue processors and start running your new queue processor code). You can also run a single queue processor manually using e.g. ./manage.py process_queue --queue=user_activity.

• So that supervisord will known to run the queue processor in production, you will need to define a program entry for it in servers/puppet/modules/zulip/files/supervisor/conf.d/zulip.conf and add it to the zulip-workers group further down in the file.

• For monitoring, you need to add a check that your worker is running to puppet/zulip/files/cron.d/rabbitmq-numconsumers if it's a one-at-a-time consumer like user_activity_internal or a custom nagios check if it is a bulk processor like slow_queries.

Publishing events into a queue

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

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_presence

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