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stable: initial stable-branching documentation
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Branch and release maintenance for the Kata Containers project.
# Overview
As detailed in the [release documentation](https://github.com/kata-containers/documentation/blob/master/Releases.md),
the Kata Containers project makes use of semantic versioning. A release version is described
by MAJOR.MINOR.PATCH. Early in the project, we will introduce many new features, which require
minor version changes as well as bug fixes. To facilitate a stable user environment, Kata will
begin to provide stable branch-based releases and a master branch release.
## Stable branch patch criteria
No new features should be introduced to stable branches. This is intended to limit risk to users,
providing only bug and security fixes.
## Branch Management
Kata Containers will maintain two stable release branches in addition to the master branch.
Once a new MAJOR or MINOR release is created from master, a new stable branch is created for
the prior MAJOR or MINOR release and the older stable branch is no longer maintained. End of
maintainence for a branch is announced on the Kata Containers mailing list. Users can determine
the version currently installed by running `kata-runtime kata-env`. It is recommended to use the
latest stable branch available.
A couple of examples follow to help clarify this process.
### New bug fix introduced
A bug fix is submitted against the runtime which does not introduce new inter-component dependencies.
This fix is applied to both the master and stable branches, and there is no need to create a new
stable branch.
| Branch | Original version | New version |
|--|--|--|
| `master` | `1.3.0-rc0` | `1.3.0-rc1` |
| `stable-1.2` | `1.2.0` | `1.2.1` |
| `stable-1.1` | `1.1.2` | `1.1.3` |
### New release made feature or change adding new inter-component dependency
A new feature is introduced, which adds a new inter-component dependency. In this case a new stable
branch is created (stable-1.3) starting from master and the older stable branch (stable-1.1)
is dropped from maintenance.
| Branch | Original version | New version |
|--|--|--|
| `master` | `1.3.0-rc1` | `1.3.0` |
| `stable-1.3` | N/A| `1.3.0` |
| `stable-1.2` | `1.2.1` | `1.2.2` |
| `stable-1.1` | `1.1.3` | (unmaintained) |
Note, the stable-1.1 branch will still exist with tag 1.1.3, but under current plans it is
not maintained further. The next tag applied to master will be 1.4.0-rc0.
## Backporting Process
Development that occurs against the master branch and applicable code commits should also be submitted
against the stable branches. Some guidelines for this process follow::
1. Only bug and security fixes which do not introduce inter-component dependencies are
candidates for stable branches. These PRs should be marked with "bug" in GitHub.
2. Once a PR is created against master which meets requirement of (1), a comparable one
should also be submitted against the stable branches. It is the responsibility of the submitter
to apply their pull request against stable, and it is the responsibility of the
reviewers to help identify stable-candidate pull requests.
## Continuous Integration Testing
The test repository is forked to create stable branches from master. Full CI
runs on each stable and master PR using its respective tests repository branch.
### An alternative method for CI testing:
Ideally, the continuous integration infrastructure will run the same test suite on both master
and the stable branches. When tests are modified or new feature tests are introduced, explicit
logic should exist within the testing CI to make sure only applicable tests are executed against
stable and master. While this is not in place currently, it should be considered in the long term.
## Release Management
### Patch releases
Releases are made on a weekly cadence for patch releases, which include a GitHub release as
well as binary packages. These patch releases are made for both stable branches, and a 'release candidate'
for the next `MAJOR` or `MINOR` is created from master. If there are no changes across all the repositories, no
release is created and an announcement is made on the developer mailing list to highlight this.
If a release is being made, each repository is tagged for this release, regardless
of whether changes are introduced.
## Minor releases
### Frequency
Minor releases are less frequent in order to provide a more stable baseline for users. They are currently
running on a six week cadence. As the Kata Containers code base matures it is expected this will become
a much longer duration.
### Compatibility
Kata guarantees compatibility between components that are within one minor release of each other.
This is critical for dependencies which cross between host (runtime, shim, proxy) and
the guest (hypervisor, rootfs and agent). For example, consider a cluster with a long-running
deployment, workload-never-dies, all on kata version 1.1.3 components. If the operator updates
the Kata components to the next new minor release (i.e. 1.2.0), we need to guarantee that the 1.2.0
runtime still communicates with 1.1.3 agent within workload-never-dies.
Handling live-update is out of the scope of this document. See this [kata-runtime issue](https://github.com/kata-containers/runtime/issues/492) for details.