Move Ironic to a feature-based release model

https://blueprints.launchpad.net/ironic/+spec/feature-based-releases

At the Liberty design summit in Vancouver, the Ironic team generally agreed to move the Ironic project to a feature-based release model.[0] This is an informational spec to lay out why we are doing this and how it will work.

Problem description

Ironic currently uses the traditional OpenStack release model.[1] This involves:

  • One release every six months.
  • A feature freeze prior to cutting this release, typically for a duration of six weeks. Only bug fix work and docs work should be done in this period.
  • An optional spec freeze a few weeks prior to the feature freeze.
  • After the feature freeze period, a stable branch is forked from master and a release candidate period begins. Critical bugs fixed on master during this period may be backported to the stable branch for the final release.
  • After about four weeks of iterating on release candidates, the final version is released. This stable release may receive critical bug fixes for a longer period of 9-15 months.

This model creates a few problems. The primary problem created is that development happens in peaks and valleys. Developers and vendors realize that the feature freeze is coming up, and attempt to merge features quickly before the freeze. This creates a flurry of both patches and reviews, which has the tendency to burn out core reviewers. Then feature freeze begins. Features cannot be landed and development slows, almost to a halt. This typically takes six weeks. Once the next cycle opens for feature development, code starts to trickle through but most folks are working on summit planning and taking a break from the stress of the last release. This essentially causes 10 weeks of developer downtime per cycle, or 20/52 weeks per year.

Proposed change

We should release Ironic (roughly) following the independent release model defined by the OpenStack governance.[2]

Note

As of July 15, 2015 [10], ironic follows the cycle-with-intermediary release model [11]. This model, which better matches this specification, was created by the OpenStack governance after this specification had been approved.

There are a few things of note here.

  • We should release Ironic when major features or bug fixes are complete on the master branch. This will be at the discretion of Ironic’s PTL or a designated release manager.

  • We should continue to release a “final” release every six months, to continue to be part of the coordinated release.[3]

  • In lieu of a feature freeze, Ironic’s “stable” release should come from a “normal” Ironic release that happens around the same time that other integrated projects begin the release candidate period. For example, for Liberty[4], Ironic should aim to make a release in the second half of September. This will become equivalent to the stable/liberty branch. Release candidates should be built from this branch and may receive bug fixes. Stable releases should be eligible for receiving backports following the current process.

  • Ironic releases should roughly follow SemVer[5] – the difference being that the major version should be bumped for significant changes in Ironic’s functionality or code. Ironic should never do a release without an upgrade path from a prior release, so in traditional SemVer the major version would never change. We should bump minor and patch versions as needed for minor feature and bug fix releases.

  • Ironic releases should be published to PyPI.

  • Specs should be no longer be targeted to a particular release – folks should just work on specs and code continuously. Features will be released as they land. The ironic-specs repo should change to:

    • One “approved” directory where all specs live initially.
    • Separate $version-implemented directories that house the specs that were implemented in $version of Ironic. Specs are moved to this directory when the work is completed.
    • Create a placeholder in the old location which indicates which release of Ironic the work was completed in, with a link to the new location. This will keep older links from breaking.
  • Leading up to all releases, reviewers should honor a “soft freeze” period of a few days to a week. Code that is risky in terms of breakage should probably not land at this time; quick bug fixes, driver changes, and other less risky changes should be okay to land in most cases. The PTL or designated release manager must be sure to communicate upcoming releases and these freezes well.

  • Ironic may need to decouple from global requirements during Dep Freeze[7]. During OpenStack’s feature freeze, the master branch of global-requirements is locked. It should be okay to accept changes to Ironic’s requirements.txt on the master branch that are blocked in global-requirements by the Dep Freeze, as the stable branch has already been cut by this point. This should only happen on an as-needed basis as the problem arises; not by default. We also may need to temporarily drop the “gate-ironic-requirements” job during this time. Finally, any patches to Ironic changing requirements.txt should also have a patch to global-requirements with a general “looks good” from a global-requirements core reviewer.

  • Folks have discussed using feature branches for larger chunks of work. While these can be useful, we must be sure to only use them when absolutely necessary as feature branches can carry a large amount of pain. We should prefer feature flags[9] over feature branches where possible.

Alternatives

Continue on the status quo. :(

Data model impact

None.

State Machine Impact

None.

REST API impact

None.

Client (CLI) impact

None. The client will continue to release independently, and likely more often than the server.

RPC API impact

None.

Driver API impact

None.

Nova driver impact

As the Nova driver is released with Nova, it will not change in terms of the way it is released.

Security impact

As only stable releases will receive backports, security bugs in other releases should be fixed and released ASAP. An advisory should be published that encourages users to upgrade to the new release.

Stable branches should continue to receive backports for security bug fixes.

Intermediate releases will not receive backports for security patches. Any security bug in an intermediate release should be fixed and released with the appropriate version bump. Whether the version change is major/minor/patch may depend on what else has landed on master and will be released with the patch.

Other end user impact

End users will get features shipped to them more quickly.

Scalability impact

None.

Performance Impact

None.

Other deployer impact

Deployers will receive changes and features more quickly. Those that do not wish to do so may continue to consume the six-month integrated release.

Developer impact

All the productivity.

Implementation

Assignee(s)

PTL, designated release manager (if one exists), and core reviewers.

Work Items

  • Switch to semver.
  • Start releasing independently.
  • Document the process in the developer docs.[8]

Dependencies

None.

Testing

None.

Upgrades and Backwards Compatibility

This should cause upgrades to be smaller and thus less impactful.

Documentation Impact

We should write a page in our developer docs about the process, including the changes to the specs process.