system-config/doc/source/zuul.rst

:title: Zuul

.. _zuul:

Zuul
####

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

At a Glance
===========

:Hosts:
  * https://zuul.opendev.org
  * zuul*.opendev.org
  * ze*.opendev.org
  * zm*.opendev.org
  * zl*.opendev.org
:Configuration:
  * :config:`zuul/main.yaml`
  * :config:`zuul.d`
:Projects:
  * https://opendev.org/zuul/zuul
:Bugs:
  * https://storyboard.openstack.org/#!/project/zuul/zuul
:Resources:
  * `Zuul Reference Manual`_
:Chat:
  * ``#zuul:opendev.org`` on Matrix

Overview
========

The OpenDev project uses a number of pipelines in Zuul:

**check**
  Newly uploaded patchsets enter this pipeline to receive an initial
  +/-1 Verified vote.

**gate**
  Changes that have been approved by core reviewers are enqueued in
  order in this pipeline, and if they pass tests, will be merged.

**post**
  This pipeline runs jobs that operate after each change is merged.

**pre-release**
  This pipeline runs jobs on projects in response to pre-release tags.

**release**
  When a commit is tagged as a release, this pipeline runs jobs that
  publish archives and documentation.

**silent**
  This pipeline is used for silently testing new jobs.

**experimental**
  This pipeline is used for on-demand testing of new jobs.

**periodic**
  This pipeline has jobs triggered on a timer for e.g. testing for
  environmental changes daily.

**promote**
   This pipeline runs jobs that operate after each change is merged
   in order to promote artifacts generated in the gate
   pipeline.

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 stored in :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`_.

Sysadmin
========

Zuul has three main subsystems:

* Zuul Scheduler
* Zuul Executors
* Zuul Web

that in OpenDev's deployment depend on four 'external' systems:

* Nodepool
* Zookeeper
* gear
* MySQL

Scheduler
---------

The Zuul Scheduler and gear are all co-located on a single host,
referred to by the ``zuul.opendev.org`` CNAME in DNS.

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.

e.g.::

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

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

OpenDev'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 ``zuul-scheduler`` group.

Executors
---------

The Zuul Executors are a horizontally scalable set of servers named
``ze*.opendev.org``. They perform git merging operations for the scheduler
and execute Ansible playbooks 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.

Web
---

Zuul Web is a horizontally scalable service. It is currently running colocated
with the scheduler on ``zuul.opendev.org``. Zuul Web provides live console
streaming and is 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.

Restarting Zuul Services
------------------------

Currently the safest way to restart the Zuul scheduler is to restart all
services at the same time. The reason for this is that if the scheduler is
restarted, but executors are not, then the executors and scheduler can get out
of sync with each other. Note that restarting Zuul Web or a single executor
should continue to be safe as noted above, but this process should generally
be preferred.

Zuul Scheduler restarts are disruptive, so non-emergency restarts should
always be scheduled for quieter times of the day, week and cycle. We should
attempt to be courteous and avoid restarts when project teams are cutting
releases or have other important changes that are about to land.

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 the ``zuul-changes`` script to save the check and gate queues::

  root@zuul02# ~root/zuul-changes.py https://zuul.opendev.org >queues-$(date +%Y%m%d).sh

The resulting script will be executed when Zuul is up and running again to restore
the previous queue contents.

One other thing to consider before restarting all zuul services is you may
want to update all of the zuul docker images. This can be useful if restarting
Zuul to correct a bug that was fixed in the Zuul codebase. To do this run
the ``zuul_pull.yaml`` playbook from bridge::

  root@bridge# ansible-playbook -f20 /home/zuul/src/opendev.org/opendev/system-config/playbooks/zuul_pull.yaml

Once ready to restart all Zuul services you will want to run the
``zuul_restart.yaml`` playbook from bridge to do this::

  root@bridge# ansible-playbook -f20 /home/zuul/src/opendev.org/opendev/system-config/playbooks/zuul_restart.yaml

Once this playbook is done running, the services will have been restarted, but
the Zuul system still needs to load its configs before it is ready to do work.
The `root <https://zuul.opendev.org/>`_ of the Zuul dashboard will show you
loaded tenants. Once all tenants show up on this page, it is safe to proceed
with re-enqueing changes to pipelines with the script we generated earlier.
Note that the OpenStack tenant takes the most time. If you wait for it to
show up in the dashboard you should be ready to go. You can double check
this by loading the OpenStack Zuul `status
<https://zuul.opendev.org/t/openstack/status>`_ and ensuring it doesn't report
an error.

To re-enqueue, execute the previously generated script::

  root@zuul# bash queues-$(date +%Y%m%d).sh

When this has completed you are done with the Zuul restart. Please log
the restart and any Zuul version update with statusbot in IRC.

Secrets
-------

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. The first step is extracting the key data from
our daily key backups::

  root@zuul# jq --raw-output '.keys."/keystorage/gerrit/opendev/opendev%2Fsystem-config".keys[0].private_key' /var/lib/zuul/zuul-keys-backup.json

The name between the double quotes is the path to the project's keys in
ZooKeeper. To construct this you will need to know the Zuul connection name
and full project name. The connection name in the example above is 'gerrit',
replace it with the appropriate connection name for the project you are looking
at. Next is the unique project name. In the example above we start with
``opendev/system-config`` and split it on ``/``. Everything before the first ``/``
is the next component of our name in this case, ``opendev``. Then we take the
entire name ``opendev/system-config`` and URL encode it to get
``opendev%2Fsystem-config`` which becomes our last component.

Save the output of this jq command to a file ``secret.pem``. Then extract the
secret ciphertext from the job configuration to remove surrounding
YAML (there is no need to recombine split lines) and run the following
command to decrypt::

  cat ciphertext.txt | sed 's/^ *//' | base64 -d | sudo openssl rsautl -decrypt -oaep -inkey \
  secret.pem

Debugging Problems
------------------

Occasionally you'll have a job enter an error state or an entire change that
appears to be stuck in a Zuul pipeline. Debugging these problems can be a bit
daunting to start as Zuul's logs are quite verbose. The good news is that once
you learn a few tricks those verbose logs become quite the powerful tool.

Often the best place to start is grepping the Zuul scheduler debug log for
the pipeline entry identifier (eg change number, tag, or ref sha1)::

  you@zuul02$ grep 123456 /var/log/zuul/debug.log
  you@zuul02$ grep c6229660cda0af42ecd5afbe7fefdb51136a0436 /var/log/zuul/debug.log

In many of these log lines you'll see Zuul event IDs like
``[e: 1718628fe39643e1bd6a88a9a1477b4f]``. This ID identifies the event that
triggered Zuul to take action for these changes and is logged through all
the Zuul services. It can be very powerful to do a grep on this event ID and
trace through the actions that the scheduler took for this event::

  you@zuul02$ grep 1718628fe39643e1bd6a88a9a1477b4f /var/log/zuul/debug.log

This might lead you to look at executor logs where you can use the same
ID to grep for actions related to this even on the executor::

  you@ze01$ grep 1718628fe39643e1bd6a88a9a1477b4f /var/log/zuul/executor-debug.log

As you trace through the logs related to a change or event ID you can look for
``ERROR`` or ``Traceback`` messages to try and identify the underlying source of
the problem. Note that ``Traceback`` messages are not prefixed with the event
ID which means you'll have to grep with additional context, for example using
``grep -B20 -A20``.

Another useful debugging tool is Zuul's SIGUSR2 handler. This signal handler
produces a thread dump in the debug log and toggles the yappi python profiler.
Each Zuul service supports the signal handler and it can be triggered via::

  you@zuul02$ sudo kill -USR2 $ZUUL_PID

To determine ``$ZUUL_PID`` you can run ``ps`` against the ``zuul-*`` service
that you are interested in getting information from. For example::

  you@zuul02$ ps -ef | grep zuul-scheduler
  zuuld    1893030 1893010  0 08:33 ?        00:00:00 /usr/bin/dumb-init -- /usr/local/bin/zuul-scheduler -f
  zuuld    1893052 1893030 69 08:33 ?        07:57:42 /usr/local/bin/python /usr/local/bin/zuul-scheduler -f
  zuuld    1893198 1893052  0 08:33 ?        00:03:22 /usr/local/bin/python /usr/local/bin/zuul-scheduler -f

All of the zuul services are run under ``dumb-init``. The process to send
SIGUSR2 to is the child of the ``dumb-init`` process. In the example above
``$ZUUL_PID`` would be ``1893052``.

The first time you run it you will turn on the yappi profiler. This profiler
does incur a runtime cost which can significantly slow down Zuul's processing
of pipelines. Be sure to resend the signal once you have let Zuul run long
enough to collect a representative set of profiler data. In most cases a minute
or two should be sufficient. Slow memory leaks may require hours, but running
Zuul under yappi for hours isn't practical.

.. _zuul_github_projects:

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
OpenDev 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
navigate to the `OpenDev 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 OpenDev Zuul App itself can be
found on the :ref:`github` page at :ref:`openstack_zuul_app`.

.. _OpenDev Zuul: https://github.com/apps/opendev-zuul
.. _Zuul Reference Manual: https://zuul-ci.org/docs/zuul
.. _Zuul Status Page: https://zuul.opendev.org