Moving TripleO repos to independent release model¶
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The TripleO repos 3 mostly follow the cycle-with-intermediary release model, for example tripleo-heat-templates at 4. Mostly because some of tripleo repos use the independent release model, for example tripleo-upgrade at 5. A description of the different release models can be found at 6.
By following the cycle-with-intermediary release model, TripleO is bound to produce a release for each OpenStack development cycle and a corresponding stable/branch in the tripleo repos. However as we have seen this causes an ongoing maintenance burden; consider that currently TripleO supports 5 active branches - Train, Ussuri, Victoria, Wallaby and Xena (current master). In fact until very recently that list contained 7 branches, including Stein and Queens (currently moving to End Of Life 7).
This creates an ongoing maintenance and resource burden where for each branch we are backporting changes, implementing, running and maintaining upstream CI and ensuring compatibility with the rest of OpenStack with 3rd party CI and the component and integration promotion pipelines 8, on an ongoing bases.
Finally, changes in the underlying OS between branches means that for some branches we maintain two “types” of CI job; for stable/train we have to support both Centos 7 and Centos 8. With the coming stable/xena, we would likely have to support Centos-Stream-8 as well as Centos-Stream-9 in the event that Stream-9 is not fully available by the xena release, which further compounds the resource burden. By adopting the proposal laid out here we can choose to skip the Xena branch thus avoiding this increased CI and maintenance cost.
The proposal is for all TripleO repos that are currently using the cycle-with-intermediary release model to switch to independent. This will allow us to choose to skip a particular release and more importantly skip the creation of the given stable/branch on those repos.
This would allow the TripleO community to focus our resources on those branches that are most ‘important’ to us, namely the ‘FFU branches’. That is, the branches that are part of the TripleO Fast Forward Upgrade chain (currently these are Train -> Wallaby -> Z?). For example it is highly likely that we would not create a Xena branch.
Developers will be freed from having to backport changes across stable/branches and this will have a dramatic effect on our upstream CI resource consumption and maintenance burden.
We can continue to create all the stable/branches and use the same release model we currently have. This would mean we would continue to have an increased maintenance burden and would have to address that with increased resources.
For upgrades it would mean that TripleO would no longer directly support all OpenStack stable branches. So if we decide not to create stable/xena for example then you cannot upgrade from wallaby to xena using TripleO. In some respects this would more closely match reality since the focus of the active tripleo developer community has typically been on ensuring the Fast Forward Upgrade (e.g. train to wallaby) and less so on ensuring the point to point upgrade between 2 branches.
Other End User Impact¶
TripleO would no longer be able to deploy all versions of OpenStack. One idea that was brough forth in the discussions around this topic thus far, is that we can attempt to address this by designating a range of git tags as compatible with a particular OpenStack stable branch.
For example if TripleO doesn’t create a stable/xena, but during the xena cycle makes releases for the various Tripleo repos then those releases will be compatible for deploying OpenStack stable/xena. We can maintain and publicise a set of compatible tags for each of the affected repos (e.g., tripleo-heat-templates versions 15.0.0 to 15.999.999 are compatible with OpenStack stable/xena).
Some rules around tagging will help us. Generally we can keep doing what we currently do with respect to tagging; For major.minor.patch (e.g. 15.1.1) in the release tag, we will always bump major to signal a new stable branch.
One problem with this solution is that there is no place to backport fixes to. For example if you are using tripleo-heat-templates 15.99.99 to deploy OpenStack Xena (and there is no stable/xena for tht) then you’d have to apply any fixes to the top of the 15.99.99 tag and use it. There would be no way to commit these fixes into the code repo.
Other Deployer Impact¶
There were concerns raised in the openstack-discuss thread  about RDO packaging and how it would be affected by this proposal. As was discussed there are no plans for RDO to stop building packages for any branch. For the building of tripleo repos we would have to rely on the latest compatible git tag, as outlined above in Other End User Impact.
Will have less stable/branches to backport fixes to. It is important to note however that by skipping some branches, resulting backports across multiple branches will result in a larger code diff and so be harder for developers to implement. That is, there will be increased complexity in resulting backports if we skip intermediate branches.
As noted in the Other End User Impact section above, for those branches that tripleo decides not to create, there will be no place for developers to commit any branch specific fixes to. They can consume particular tagged releases of TripleO repos that are compatible with the given branch, but will not be able to commit those changes to the upstream repo since the branch will not exist.
Besides posting the review against the releases repo 9 we will need to update documentation to reflect and inform about this change.
Yes we will at least need to add some section to the docs to explain this. We may also add some landing page to show the currently ‘active’ or supported TripleO branches.