# Versioning and Branching in controller-runtime

*NB*: this also applies to controller-tools.

## TL;DR:

### Users

- We follow [Semantic Versioning (semver)](https://semver.org)
- Use releases with your dependency management to ensure that you get
  compatible code
- The master branch contains all the latest code, some of which may break
  compatibility (so "normal" `go get` is not recommended)

### Contributors

- All code PR must be labeled with :bug: (patch fixes), :sparkles:
  (backwards-compatible features), or :warning: (breaking changes)

- Breaking changes will find their way into the next major release, other
  changes will go into an semi-immediate patch or minor release

- Please *try* to avoid breaking changes when you can.  They make users
  face difficult decisions ("when do I go through the pain of
  upgrading?"), and make life hard for maintainers and contributors
  (dealing with differences on stable branches).

### Mantainers

Don't be lazy, read the rest of this doc :-)

## Overview

controller-runtime (and friends) follow [Semantic
Versioning](https://semver.org).  I'd recommend reading the aforementioned
link if you're not familiar, but essentially, for any given release X.Y.Z:

- an X (*major*) release indicates a set of backwards-compatible code.
  Changing X means there's a breaking change.

- a Y (*minor*) release indicates a minimum feature set.  Changing Y means
  the addition of a backwards-compatible feature.

- a Z (*patch*) release indicates minimum set of bugfixes.  Changing
  Z means a backwards-compatible change that doesn't add functionality.

*NB*: If the major release is `0`, any minor release may contain breaking
changes.

These guarantees extend to all code exposed in public APIs of
controller-runtime. This includes code both in controller-runtime itself,
*plus types from dependencies in public APIs*.  Types and functions not in
public APIs are not considered part of the guarantee.

In order to easily maintain the guarantees, we have a couple of processes
that we follow.

## Branches

controller-runtime contains two types of branches: the *master* branch and
*release-X* branches.

The *master* branch is where development happens.  All the latest and
greatest code, including breaking changes, happens on master.

The *release-X* branches contain stable, backwards compatible code.  Every
major (X) release, a new such branch is created.  It is from these
branches that minor and patch releases are tagged.  If some cases, it may
be neccessary open PRs for bugfixes directly against stable branches, but
this should generally not be the case.

The maintainers are responsible for updating the contents of this branch;
generally, this is done just before a release using release tooling that
filters and checks for changes tagged as breaking (see below).

### Tooling

* [release-notes.sh](hack/release-notes.sh): generate release notes
  for a range of commits, and check for next version type (***TODO***)

* [verify-commit-messages.sh](hack/verify-commit-messages.sh): check that
  your PR and/or commit messages have the right versioning icon
  (***TODO***).

## PR Process

Every PR should be annotated with an icon indicating whether it's
a:

- Breaking change: :warning: (`:warning:`)
- Non-breaking feature: :sparkles: (`:sparkles:`)
- Patch fix: :bug: (`:bug:`)
- Docs: :book: (`:book:`)
- Infra/Tests/Other: :running: (`:running:`)

You can also use the equivalent emoji directly, since GitHub doesn't
render the `:xyz:` aliases in PR titles.

Individual commits should not be tagged separately, but will generally be
assumed to match the PR. For instance, if you have a bugfix in with
a breaking change, it's generally encouraged to submit the bugfix
separately, but if you must put them in one PR, mark the commit
separately.

### Commands and Workflow

controller-runtime follows the standard Kubernetes workflow: any PR needs
`lgtm` and `approved` labels, PRs authors must have signed the CNCF CLA,
and PRs must pass the tests before being merged.  See [the contributor
docs](https://github.com/kubernetes/community/blob/master/contributors/guide/pull-requests.md#the-testing-and-merge-workflow)
for more info.

We use the same priority and kind labels as Kubernetes.  See the labels
tab in GitHub for the full list.

The standard Kubernetes comment commands should work in
controller-runtime.  See [Prow](https://prow.k8s.io/command-help) for
a command reference.

## Release Process

Minor and patch releases are generally done immediately after a feature or
bugfix is landed, or sometimes a series of features tied together.

Minor releases will only be tagged on the *most recent* major release
branch, except in exceptional circumstances.  Patches will be backported
to maintained stable versions, as needed.

Major releases are done shortly after a breaking change is merged -- once
a breaking change is merged, the next release *must* be a major revison.
We don't intend to have a lot of these, so we may put off merging breaking
PRs until a later date.

### Exact Steps

Follow the release-specific steps below, then follow the general steps
after that.

#### Minor and patch releases

1. Update the release-X branch with the latest set of changes by calling
   `git rebase master` from the release branch.

#### Major releases

1. Create a new release branch named `release-X` (where `X` is the new
   version) off of master.

#### General

2. Generate release notes using the release note tooling.

3. Add a release for controller-runtime on GitHub, using those release
   notes, with a title of `vX.Y.Z`.
 
4. Do a similar process for
   [controller-tools](https://github.com/kubernetes-sigs/controller-tools)

5. Announce the release in `#kubebuilder` on Slack with a pinned message.

6. Potentially update
   [kubebuilder](https://github.com/kubernetes-sigs/kubebuilder) as well.

### Breaking Changes

Try to avoid breaking changes.  They make life difficult for users, who
have to rewrite their code when they eventually upgrade, and for
maintainers/contributors, who have to deal with differences between master
and stable branches.

That being said, we'll occaisonally want to make breaking changes. They'll
be merged onto master, and will then trigger a major release (see [Release
Proccess](#release-process)).  Because breaking changes induce a major
revision, the maintainers may delay a particular breaking change until
a later date when they are ready to make a major revision with a few
breaking changes.

If you're going to make a breaking change, please make sure to explain in
detail why it's helpful.  Is it necessary to cleanly resolve an issue?
Does it improve API ergonomics?

Maintainers should treat breaking changes with caution, and evaluate
potential non-breaking solutions (see below).

Note that API breakage in public APIs due to dependencies will trigger
a major revision, so you may occaisonally need to have a major release
anyway, due to changes in libraries like `k8s.io/client-go` or
`k8s.io/apimachinery`.

*NB*: Pre-1.0 releases treat breaking changes a bit more lightly.  We'll
still consider carefully, but the pre-1.0 timeframe is useful for
converging on a ergonomic API.

#### Avoiding breaking changes

##### Solutions to avoid

- **Confusingly duplicate methods, functions, or variables.**

  For instance, suppose we have an interface method `List(ctx
  context.Context, options *ListOptions, obj runtime.Object) error`, and
  we decide to switch it so that options come at the end, parametrically.
  Adding a new interface method `ListParametric(ctx context.Context, obj
  runtime.Object, options... ListOption)` is probably not the right
  solution:

   - Users will intuitively see `List`, and use that in new projects, even
     if it's marked as deprecated.
   
   - Users who don't notice the deprecation may be confused as to the
     difference between `List` and `ListParametric`.
   
   - It's not immediately obvious in isolation (e.g. in surrounding code)
     why the method is called `ListParametric`, and may cause confusion
     when reading code that makes use of that method.

  In this case, it may be better to make the breaking change, and then
  eventually do a major release.

## Why don't we...

### Use "next"-style branches

Development branches:

- don't win us much in terms of maintenance in the case of breaking
  changes (we still have to merge/manage multiple branches for development
  and stable) 
  
- can be confusing to contributors, who often expect master to have the
  latest changes.

### Never break compatibility

Never doing a new major release could be an admirable goal, but gradually
leads to API cruft.

Since one of the goals of controller-runtime is to be a friendly and
intuitive API, we want to avoid too much API cruft over time, and
occaisonal breaking changes in major releases help accomplish that goal.

Furthermore, our dependency on Kubernetes libraries makes this difficult
(see below)

### Always assume we've broken compatibility

*a.k.a. k8s.io/client-go style*

While this makes life easier (a bit) for maintainers, it's problematic for
users.  While breaking changes arrive sooner, upgrading becomes very
painful.

Furthermore, we still have to maintain stable branches for bugfixes, so
the maintenance burden isn't lessened by a ton.

### Extend compatibility guarantees to all dependencies

This is very difficult with the number of Kubernetes dependencies we have.
Kubernetes dependencies tend to either break compatibility every major
release (e.g. k8s.io/client-go, which loosely follows semver), or at
a whim (many other Kubernetes libraries).

If we limit to the few objects we expose, we can better inform users about
how *controller-runtime itself* has changed in a given release.  Then,
users can make informed decisions about how to proceed with any direct
uses of Kubernetes dependencies their controller-runtime-based application
may have.
