Spare a thought for CentOS users, it's been a bumpy ride for the last few
years. CentOS 7 to 8 migrations, CentOS Linux 8 end-of-life, CentOS Stream. New
RHEL clones such as Rocky Linux and Alma Linux stepping up to fill the void.
How to respond?
In this blog post, we'll look at how these events affected us and our clients,
and the steps we took to mitigate the risks involved in using CentOS Stream.
Here is a brief timeline of the major events in the CentOS saga.
- 24th September 2019
- CentOS Linux 8 general availability. CentOS Stream 8 is introduced at the
same time, described as an alternative, parallel distribution, upstream of
RHEL, with a rolling release model.
- 16th October 2019
- OpenStack Train release general availability.
- 1st January 2020
- The end of life of Python 2 on January 1 2020 adds pressure to reinstall
CentOS 7 systems with the then shiny new CentOS Linux 8.
- 8th December 2020
- CentOS project accelerates CentOS Linux end-of-life, bringing it forward by
8 years to 31st December 2021. CentOS Stream remains.
- 11th December 2020
- CentOS co-founder Gregory Kurtzer announces Rocky Linux, a RHEL clone.
- 31st December 2021
- RIP CentOS Linux :(
OpenStack Train release
The OpenStack Train release (not to be confused with the StackHPC Release
train!) was a big one for the Kolla projects. With CentOS 7 providing
insufficient support for Python 3, we needed to get to CentOS 8 to support
Python 3 before the end-of-life of Python 2. Train therefore needed to support
both CentOS 7 (with Python 2) and CentOS 8 (with Python 3), and to provide a route
for migration. Major OS version upgrades are not supported in CentOS, so a reinstall is
required. The Kolla Ansible spec
provides some of the gory details.
It took significant development effort to make this migration possible.
While the automation provided by Kayobe and Kolla Ansible helps significantly
to perform the migration, it still involves significant operator effort to
reinstall each host and keep the cloud running.
CentOS Linux EOL
Having fairly recently migrated our clients' systems to CentOS 8, the
announcement of the end-of-life of CentOS Linux came as quite a blow.
It also required us to make a decision about how to proceed.
Should we jump into the Stream? CentOS Stream seemed to divide the community,
with some claiming it to be not production-ready, and others having faith in
its CI testing.
Should we try out one of the new RHEL clones? Rocky Linux seemed promising,
but would not be released until June 2021, and would need to build a
sustainable community around it to be viable.
Should we switch to another distribution entirely? We started development of
Ubuntu support for Kayobe around this time. Tempting, but migrating from
CentOS to Ubuntu could be a bigger challenge than migrating from CentOS 7 to 8,
and a hard sell for our existing clients.
Taking the plunge
In the end, with a little encouragement from the direction taken by CERN, we
learned to stop worrying and deploy CentOS Stream. Of course, in order to stop
worrying, we needed to mitigate the risks of using a rolling release
CentOS Linux was a rebuild of RHEL, meaning that it followed the same release
cycle, albeit delayed by a few weeks. The main CentOS mirrors served the latest
minor release (e.g. 8.2), and were updated every few months with a new minor
release. Security patches and critical bug fixes were applied between minor
releases. While the minor releases sometimes introduced issues that needed to
be fixed or worked around, this model provided fairly long periods of
With CentOS Stream, there is a constant trickle of updates, and the state at
any point lies somewhere between one minor release of RHEL and the next. In
fact, it may be worse than this, if packages are upgraded then downgraded again
before the next minor RHEL release. There is some CI testing around changes to
Stream, however clearly it won't be as rigorous as the testing applied to a
RHEL minor release.
Islands in the Stream
How to cope with this instability? For StackHPC, the answer lies in creating
snapshots of the CentOS Stream package repositories. Stable, tested islands in
This approach is one we had been starting to apply to some of our client
deployments, even with CentOS Linux. Having a local mirror with repository
snapshots improves repeatability, as well as reducing dependence on an
external resource (the upstream mirrors), and avoiding a fan-in effect on
package downloads from the Internet.
This approach worked well for a while, but it soon became clear that having
separate package repositories for each deployment was not optimal. Each site
was using a different set of packages, a different set of locally-built
container images, and no common source of configuration beyond that provided by
Kayobe and Kolla Ansible. Without some changes, this would soon become a
scaling bottleneck for the company.
As they say, never waste a good crisis. CentOS Stream gave us an opportunity to take the StackHPC Release Train
project off of our (long) backlog. It's an effort to make our OpenStack
deployments more consistent, reproducible and reliable, by releasing a set of
tested artifacts. This includes:
- Package repositories
- Kolla container images
- Binary artifacts (images, etc.)
- Source code repositories
- Kayobe and Kolla configuration
We are initially targeting CentOS Stream, but we will likely add support for
We use Pulp to manage and host content for the
release train. Pulp is a content server that manages repositories of software
packages and facilitates their distribution to content consumers.
It is implemented in Python using the Django framework, and is composed of a
for the various content types (e.g. RPMs, containers). A key feature of Pulp
is fine-grained control over snapshots of repositories.
It's useful to have an awareness of the core concepts in Pulp, since they are
common to the various plugins.
- A pulp repository of artifacts & metadata
- A remote repository
- Repository version
- A snapshot of a repository at a point in time
- Sync a repository with a remote, creating a repository version
- A published repository version, including metadata
- Content made available to users
For example, the RPM plugin might have a centos-baseos repository, which
syncs from the upstream centos-baseos remote at
http://mirror.centos.org/centos/8-stream/BaseOS/, creating a new repository
version when new content is available. In order to make the content available,
we might create a publication of the new repository version, then associate
a centos-baseos-development distribution with the publication.
Another centos-baseos-production might be associated with an older
publication and repository version. Pulp can serve both distributions at
the same time, allowing us to test a new snapshot of the repository without
affecting users of existing snapshots.
Automation & Continuous Integration (CI)
Automation and CI are key aspects of the release train. The additional control
provided by the release train comes at a cost in maintenance and complexity,
which must be offset via automation and CI. In general, Ansible is used for
automation, and GitHub Actions provide CI.
The intention is to have as much as possible of the release train automated and
run via CI. Typically, workflows may go through the following stages as they
- automated via Ansible, manually executed
- executed by GitHub Actions workflows, manually triggered by workflow
- executed by GitHub Actions workflows, automatically triggered by an event
e.g. pull request or another workflow
This sequence discourages putting too much automation into the GitHub Actions
workflows, ensuring it is possible to run them manually.
We typically use Ansible to drive the Pulp API, and have made several
the squeezer collection. We also created the stackhpc.pulp Ansible collection
as a higher level interface to define repositories, remotes, and other Pulp
Ark is our production Pulp server, and is hosted
on Leafcloud. It comprises a single compute
instance, with content stored in Leafcloud's object storage. We use
pulp_installer to deploy Pulp.
It is the master copy of development and released content. Access to the API
and artifacts is controlled via client certificates and passwords.
Clients access Ark via a Pulp service deployed on their local infrastructure.
Content is synced from Ark to the local Pulp service, and control plane hosts
acquire the content from there.
A test Pulp service runs on the SMS lab cloud.
Content is synced from Ark to the test Pulp service, where it is used to build
container images and run tests. In some respects, the test Pulp service may be
considered a client.
Various different types of content are hosted by Pulp, including:
We also anticipate supporting the following content:
Some of this content may be mirrored from upstream sources, while others are
the result of release train build processes.
Access to released Pulp content is restricted to clients with a support
agreement. Build and test processes also need access to unreleased content.
Access to package repositories is controlled via Pulp x509 cert guards. A HashiCorp Vault service acts as a Certificate Authority (CA)
for the cert guards. Two cert guards are in use - development and
release. The development cert guard is assigned to unreleased content,
while the release cert guard is assigned to released content. Clients are
provided with a client certificate which they use when syncing package
repositories in their local Pulp service with Ark. Clients' client certificates
are authorised to access content protected by the release cert guard. Build
and test processes are provided with a client certificate that is authorised to
access both the development and release cert guards. The latter is made
possible via the CA chain.
Access to container images is controlled by token authentication, which uses
Django users in the backend. Two container namespaces are in use -
stackhpc-dev and stackhpc. The stackhpc-dev namespace is used for
unreleased content, while the stackhpc namespace is used for released
content. Clients are provided with a set of credentials, which they use when
syncing container image repositories in their local Pulp service with Ark.
Clients' credentials are authorised to pull from the stackhpc namespace.
Build and test processes are provided with credentials that are authorised to
push to the stackhpc-dev namespace.
Syncing package repositories
The stackhpc-release-train repository provides
Ansible-based automation and Github Actions workflows for the release train.
A Github Actions workflow syncs CentOS Stream and other upstream package
repositories nightly into Ark, creating new snapshots when there are changes.
The workflow may also be run on demand. Publications and distributions are
created using the development content guard, to ensure that untested
content is not accessible to clients. We sync using the immediate
policy, to ensure content remains available if it is removed from upstream
mirrors. This workflow also syncs the content to the test Pulp service.
Package repository distributions are versioned based on the date/time stamp at
the beginning of the sync workflow, e.g. 20211122T102435. This version
string is used as the final component of the path at which the corresponding
distribution is hosted. For example, a CentOS Stream 8 BaseOS snapshot may be
The rationale behind using a date/time stamp is that there is no sane way to
version a large collection of content, such as a repository, in a way in which
the version reflects changes in the content (e.g. SemVer). While the timestamp
used is fairly arbitrary, it does at least provide a reasonable guarantee of
ordering, and is easily automated.
Building container images
Kolla container images are built from the test Pulp service package
repositories and pushed to the Ark container registry.
Build and test processes run on SMS cloud, to avoid excessive running costs.
All content in Ark that is required by the build and test processes is synced
to the test Pulp service running in SMS cloud, minimising data egress from Ark.
Kolla container images are built via Kayobe, using a builder environment in
StackHPC Kayobe config.
The configuration uses the package repositories in Ark when building
containers. Currently this is run manually, but will eventually run as a CI
job. The stackhpc-dev namespace in Ark contains container push
repositories, which are
pushed to using Kayobe. (Currently this is rather slow due to a Pulp bug.)
A Github Actions workflow runs on demand, syncing container repositories in
test Pulp service with those in Ark. It also configures container image
distributions to be private, since they are public by default.
Kolla container images are versioned based on the OpenStack release name and
the date/time stamp at the beginning of the build workflow, e.g.
wallaby-20211122T102435. This version string is used as the image tag.
Unlike package repositories, container image tags allow multiple versions to be
present in a distribution of a container repository simultaneously. We
therefore use separate namespaces for development (stackhpc-dev) and
Release Train content is tested via a Kayobe deployment of OpenStack. An
aio environment in StackHPC Kayobe config provides a converged
control/compute host for testing. Currently this is run manually, but will
eventually run as a CI job.
StackHPC maintains a base Kayobe configuration which includes settings
required to consume the release train, as well as various generally applicable
configuration changes. Release Train consumers merge this configuration into
their own, and apply site and environment-specific changes. This repository
provides configuration and playbooks to:
- deploy a local Pulp service as a container on the seed
- define which package repository versions to use
- define which container image tags to use
- sync all necessary content from Ark into the local Pulp service
- use the local Pulp repository mirrors on control plane hosts
- use the local Pulp container registry on control plane hosts
This configuration is in active development and is expected to evolve over the
coming releases. It currently supports the OpenStack Victoria and Wallaby
Further documentation of this configuration is available in the readme.
In the second half of 2021, with the end-of-life of CentOS Linux looming large,
the pressure to switch our clients to CentOS Stream was increasing. We had to
make some tough calls in order to implement the necessary parts of the release
train and put them into production. The StackHPC team then had to work hard
migrating our clients' systems to our new release train and CentOS Stream.
Of course, there were some teething problems, but overall, the adoption of the
release train went fairly smoothly.
Release Train day 2
Clouds don't stand still, and at some point something will require a patch. How
well does the release train cope with changes? A test soon came in the form of
a zero-day Grafana exploit, CVE-2021-43798.
Grafana applied fixes and cut some releases. We needed to get the updated
packages into our Grafana container image, and roll it out to affected
deployments. This turned out to be a little more clunky than I'd hoped, with
repository snapshot versions tracked in multiple places, and various pull
requests. It looked a bit like this:
- Manual package repository sync (the previous night's sync had failed)
- Update test repo versions, commit only grafana version bump in
- Sync and publish repos in test pulp service
- Bump grafana repository version in stackhpc-kayobe-config repository
- Build & push grafana image
- Promote container images with the new tag
- Sync container images to test Pulp service
- Bump grafana container image tag in stackhpc-kayobe-config repository
- Test the new container
Needless to say, this could be simpler. However, it was a good catalyst for
some head scratching and a whiteboard session with Matt Anson, resulting in
a design for release train mk2.
Release Train mk2
The changes in mk2 are mostly quite subtle, but the key shift was to using
stackhpc-kayobe-config as the single source of truth for all repository
versions and container image tags. The stackhpc-release-train repository would
not include any 'live' state, just a set of Ansible playbooks and Github
Actions workflows to drive the various workflows.
Another change in mk2 is around the branching model for stackhpc-kayobe-config,
adding the ability to stage changes for an upcoming release, without making
them available to clients. There are also some usability improvements to the
automation, making it easier to perform actions changes on specific package
repositories or container images.
Where next for the release train? Within the current scope of package
repositories and container images, there is plenty of room for improvement.
Usability enhancements, more automation and CI, and better automated building
security scanning and testing of changes before they are released. We'll also
likely expand to support other distributions than CentOS Stream.
Release Train also has ambitions around our source code, improving our
development processes with better CI, more automation for our (few)
repositories with downstream changes, and increased reproducibility of builds
Another avenue for growth of the release train is moving up the stack to our
platforms, such as Slurm and Kubernetes.
CentOS Stream in retrospect
Overall, our experience with CentOS Stream has been acceptable, given the steps
we have taken to mitigate risks of unexpected changes. Indeed, there have been
a few occasions where changes in Stream have broken the Kolla upstream CI jobs,
that we have been insulated from.
There are some other downsides to Stream. Mellanox does not publish OFED
packages built against Stream, only RHEL minor releases. While it is generally
possible to force the use of a compatible kernel in Stream, this kernel will
not receive the security updates that a RHEL minor release kernel would.
In many cases it may be possible to use the in-box Mellanox drivers, however
these may lack support for some of the latest hardware or features. This is
a consideration for the points where we make our snapshots.
Kayobe and Kolla Ansible have recently added support for Rocky Linux as a host
OS, as well as the ability to run libvirt as a host daemon rather than in a
container. This reduces the coupling between the host and containers, making it
safer to mix their OS distributions, e.g. running Rocky hosts with CentOS Stream
Get in touch
If you would like to get in touch we would love to hear
from you. Reach out to us via Twitter
or directly via our contact page.