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


The OpenStack project uses a number of pipelines in Zuul, as defined in project-config: zuul.d/pipelines.yaml.

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 https://zuul.opendev.org/.

Zuul’s configuration is distributed across projects listed in project-config: zuul/main.yaml. Anyone may propose a change to the configuration by editing configuration in those projects and submitting the change to Gerrit for review.

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


Zuul and gear are lightweight - it should be possible to run both on a 1G instance for small deployments. OpenStack’s deployment requires at least a 8G instance at the time of writing, though additional cache memory helps performance.

Zuul is mostly stateless, so the server does not need backing up (though it does rely on a Trove instance for its build history). However zuul talks through git and ssh so you will need to manually check ssh host keys as the zuul user. e.g.:

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

To debug Zuul’s gearman server, SSL is required. Use the following command:

openssl s_client -connect localhost:4730 -cert /etc/zuul/ssl/client.pem  -key /etc/zuul/ssl/client.key


Zuul 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 OpenStack Status Page should be checked to see the status of the OpenStack 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://zuul.openstack.org \
  check >check.sh
python /opt/zuul/tools/zuul-changes.py http://zuul.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-scheduler 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.

When you are satisfied that zuul is up, 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. This frontend is provided by the zuul-web service on the same server, which may also need to be restarted.


Servers with names matching the pattern ze*.openstack.org are Zuul Executors. These are horizontally scalable components of Zuul which run Ansible within a Bubblewrap context and connect to job nodes. They can be started and stopped at will, and new ones added as necessary to accommodate load.


Servers with names matching the pattern zm*.openstack.org are Zuul Mergers. These are horizontally scalable components of Zuul which perform git operations for the benefit of jobs. They can be started and stopped at will, and new ones added as necessary to accommodate load.


In some cases it may be warranted to compare the decrypted plaintext of a secret from job configuration against a reference value while troubleshooting, since random padding means encrypting the same plaintext a second time will result in wholly different ciphertext. In order to avoid unintentional disclosure this should only be done when absolutely necessary, but it’s possible to decrypt a secret locally on the scheduler server with a command like the following (just extract the secret ciphertext from the job configuration first to remove surrounding YAML, there is no need to dedent nor recombine split lines):

cat ciphertext.txt | base64 -d | sudo openssl rsautl -decrypt -oaep -inkey \
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