Zuul v3


Zuul v3 is the upcoming release of Zuul. While it is not in production yet, we are running it. Once it goes live, this document should be renamed to Zuul and the old document should go away - so don’t develop any emotional attachments to permalinks to this document.

Zuul is a pipeline-oriented project gating system. It facilitates running tests and automated tasks in response to Code Review events.


The OpenStack project uses a number of pipelines in Zuul:

Newly uploaded patchsets enter this pipeline to receive an initial +/-1 Verified vote.
Changes that have been approved by core developers are enqueued in order in this pipeline, and if they pass tests, will be merged.
This pipeline runs jobs that operate after each change is merged.
This pipeline runs jobs on projects in response to pre-release tags.
When a commit is tagged as a release, this pipeline runs jobs that publish archives and documentation.
This pipeline is used for silently testing new jobs.
This pipeline is used for on-demand testing of new jobs.
This pipeline has jobs triggered on a timer for e.g. testing for environmental changes daily.

Zuul watches events in Gerrit (using the Gerrit “stream-events” command) and matches those events to the pipelines above. If a match is found, it adds the change to the pipeline and starts running related jobs.

The gate pipeline uses speculative execution to improve throughput. Changes are tested in parallel under the assumption that changes ahead in the queue will merge. If they do not, Zuul will abort and restart tests without the affected changes. This means that many changes may be tested in parallel while continuing to assure that each commit is correctly tested.

Zuul’s current status may be viewed at http://zuulv3.openstack.org/.

Zuul’s configuration is stored in project-config: zuul/main.yaml. Anyone may propose a change to the configuration by editing that file and submitting the change to Gerrit for review.

For the full syntax of Zuul’s configuration file format, see the Zuul reference manual.


Zuul has three main subsystems:

  • Zuul Scheduler
  • Zuul Executors
  • Zuul Web

that in OpenStack’s deployment depend on four ‘external’ systems:

  • Nodepool
  • Zookeeper
  • gear
  • MySQL


The Zuul Scheduler and gear are all co-located on a single host, zuulv3.openstack.org.

Zuul is stateless, so the server does not need backing up. However zuul talks through git and ssh so you will need to manually check ssh host keys as the zuul user.


Is this still true or are we managing host keys in puppet now?


sudo su - zuul
ssh -p 29418 review.openstack.org

The Zuul Scheduler talks to Nodepool using Zookeeper and distributes work to the executors using gear.

OpenStack’s Zuul installation is also configured to write job results into a MySQL database via the SQL Reporter plugin. The database for that is a Rackspace Cloud DB and is configured in the mysql entry of the zuul_connection_secrets entry for the zuulv3.openstack.org FQDN.

Restarting the Scheduler

Zuul Scheduler restarts are disruptive, so non-emergency restarts should always be scheduled for quieter times of the day, week and cycle. To be as courteous to developers as possible, just prior to a restart the Zuul Status Page should be checked to see the status of the gate. If there is a series of changes nearly merged, wait until that has been completed.

Since Zuul is stateless, some work needs to be done to save and then re-enqueue patches when restarts are done. To accomplish this, start by running zuul-changes.py to save the check and gate queues:

python /opt/zuul/tools/zuul-changes.py http://zuulv3.openstack.org \
  check >check.sh
python /opt/zuul/tools/zuul-changes.py http://zuulv3.openstack.org \
  gate >gate.sh

These check.sh and gate.sh scripts will be used after the restart to re-enqueue the changes.

Now use service zuul stop to stop zuul and then run ps to make sure the process has actually stopped, it may take several seconds for it to finally go away.

Once you’re ready, use service zuul start to start zuul again.

To re-enqueue saved jobs, first run the gate.sh script and then check.sh to re-enqueue the changes from before the restart:


You may watch the Zuul Status Page to confirm that changes are returning to the queues.


The Zuul Executors are a horizontally scalable set of servers named ze*.openstack.org. They perform git merging operations for the scheduler and execute Ansible playboks to actually run jobs.

Our jobs are configured to upload as much information as possible along with their logs, but if there is an error which can not be diagnosed in that manner, logs are available in the executor-debug log file on the executor host. You may use the Zuul build UUID to track assignment of a given job from the Zuul scheduler to the Zuul executor used by that job.

It is safe, although not free, to restart executors. If an executor goes away the scheduler will reschedule the jobs it was originally running.


Zuul Web is a horizontally scalable service. It is currently running colocated with the scheduler on zuulv3.openstack.org. Zuul Web provides live console streaming and will be the home of various web dashboards such as the status page.

Zuul Web is stateless so is safe to restart, however restarting it will result in a loss of connection for anyone watching a live-stream of a console log when the restart happens.

GitHub Projects

OpenStack does not use GitHub for development purposes, but there are some non-OpenStack projects in the broader ecosystem that we care about who do. When we are interested in setting up jobs in Zuul to test the interaction between OpenStack projects and those ecosystem projects, we can add the OpenStack Zuul GitHub app to those projects, then configure them in Zuul.

In order to add the GitHub app to a project, an admin on that project should nagivate to the OpenStack Zuul app in the GitHub UI. From there they can click “Install”, then choose the project or organization they want to install the App on.

The repository then needs to be added to the zuul/main.yaml file before Zuul can be configured to actually run jobs on it.

Information about the configuration of the OpenStack Zuul App itself can be found on the GitHub page at OpenStack Zuul App.