This document details the versioning and release plan for containerd. Stability is a top goal for this project and we hope that this document and the processes it entails will help to achieve that. It covers the release process, versioning numbering, backporting, API stability and support horizons.

If you rely on containerd, it would be good to spend time understanding the areas of the API that are and are not supported and how they impact your project in the future.

This document will be considered a living document. Supported timelines, backport targets and API stability guarantees will be updated here as they change.

If there is something that you require or this document leaves out, please reach out by filing an issue.

Releases of containerd will be versioned using dotted triples, similar to Semantic Version. For the purposes of this document, we will refer to the respective components of this triple as <major>.<minor>.<patch>. The version number may have additional information, such as alpha, beta and release candidate qualifications. Such releases will be considered "pre-releases".

Major and minor releases of containerd will be made from master. Releases of containerd will be marked with GPG signed tags and announced at https://github.com/containerd/containerd/releases. The tag will be of the format v<major>.<minor>.<patch> and should be made with the command git tag -s v<major>.<minor>.<patch>.

After a minor release, a branch will be created, with the format release/<major>.<minor> from the minor tag. All further patch releases will be done from that branch. For example, once we release v1.0.0, a branch release/1.0 will be created from that tag. All future patch releases will be done against that branch.

Pre-releases, such as alphas, betas and release candidates will be conducted from their source branch. For major and minor releases, these releases will be done from master. For patch releases, these pre-releases should be done within the corresponding release branch.

While pre-releases are done to assist in the stabilization process, no guarantees are provided.

The upgrade path for containerd is such that the 0.0.x patch releases are always backward compatible with its major and minor version. Minor (0.x.0) version will always be compatible with the previous minor release. i.e. 1.2.0 is backwards compatible with 1.1.0 and 1.1.0 is compatible with 1.0.0. There is no compatibility guarantees for upgrades that span multiple, minor releases. For example, 1.0.0 to 1.2.0 is not supported. One should first upgrade to 1.1, then 1.2.

There are no compatibility guarantees with upgrades to major versions. For example, upgrading from 1.0.0 to 2.0.0 may require resources to migrated or integrations to change. Each major version will be supported for at least 1 year with bug fixes and security patches.

The activity for the next release will be tracked in the milestones. If your issue or PR is not present in a milestone, please reach out to the maintainers to create the milestone or add an issue or PR to an existing milestone.

Support horizons will be defined corresponding to a release branch, identified by <major>.<minor>. Releases branches will be in one of several states:

• Next: The next planned release branch.
• Active: The release is currently supported and accepting patches.
• End of Life: The release branch is no longer supported and no new patches will be accepted.

Releases will be supported up to one year after a minor release. This means that we will accept bug reports and backports to release branches until the end of life date. If no new minor release has been made, that release will be considered supported until the next minor is released or one year, whichever is longer.

The current state is available in the following table:

Note that branches and release from before 1.0 may not follow these rules.

This table should be updated as part of the release preparation process.

Backports in containerd are community driven. As maintainers, we'll try to ensure that sensible bugfixes make it into active release, but our main focus will be features for the next minor or major release. For the most part, this process is straightforward and we are here to help make it as smooth as possible.

If there are important fixes that need to be backported, please let use know in one of three ways:

1. Open an issue.
2. Open a PR with cherry-picked change from master.
3. Open a PR with a ported fix.

If you are reporting a security issue, please reach out discreetly at security@containerd.io. Remember that backported PRs must follow the versioning guidelines from this document.

Any release that is "active" can accept backports. Opening a backport PR is fairly straightforward. The steps differ depending on whether you are pulling a fix from master or need to draft a new commit specific to a particular branch.

To cherry pick a straightforward commit from master, simply use the cherry pick process:

1. Pick the branch to which you want backported, usually in the format release/<minor>.<major>. The following will create a branch you can use to open a PR:

   $ git checkout -b my-backport-branch release/<major>.<minor>.

2. Find the commit you want backported.

3. Apply it to the release branch:

   $ git cherry-pick -xsS <commit>

4. Push the branch and open up a PR against the release branch:

   $ git push -u stevvooe my-backport-branch
   

   Make sure to replace stevvooe with whatever fork you are using to open the PR. When you open the PR, make sure to switch master with whatever release branch you are targeting with the fix.

If there is no existing fix in master, you should first fix the bug in master, or ask us a maintainer or contributor to do it via an issue. Once that PR is completed, open a PR using the process above.

Only when the bug is not seen in master and must be made for the specific release branch should you open a PR with new code.

The following table provides an overview of the components covered by containerd versions:

From the version stated in the above table, that component must adhere to the stability constraints expected in release versions.

Unless explicitly stated here, components that are called out as unstable or not covered may change in a future minor version. Breaking changes to "unstable" components will be avoided in patch versions.

The primary product of containerd is the GRPC API. As of the 1.0.0 release, the GRPC API will not have any backwards incompatible changes without a major version jump.

To ensure compatibility, we have collected the entire GRPC API symbol set into a single file. At each minor release of containerd, we will move the current next.pb.txt file to a file named for the minor version, such as 1.0.pb.txt, enumerating the support services and messages. See api/ for details.

Note that new services may be added in minor releases. New service methods and new fields on messages may be added if they are optional.

The metrics API that outputs prometheus style metrics will be versioned independently, prefixed with the API version. i.e. /v1/metrics, /v2/metrics.

The metrics API version will be incremented when breaking changes are made to the prometheus output. New metrics can be added to the output in a backwards compatible manner without bumping the API version.

containerd is based on a modular design where plugins are implemented to provide the core functionality. Plugins implemented in tree are supported by the containerd community unless explicitly specified as non-stable. Out of tree plugins are not supported by the containerd maintainers.

Currently, the Windows runtime and snapshot plugins are not stable and not supported. Please refer to the github milestones for Windows support in a future release.

Error codes will not change in a patch release, unless a missing error code causes a blocking bug. Error codes of type "unknown" may change to more specific types in the future. Any error code that is not "unknown" that is currently returned by a service will not change without a major release or a new version of the service.

If you find that an error code that is required by your application is not well-documented in the protobuf service description or tested explicitly, please file and issue and we will clarify.

Unless explicitly stated, the formats of certain fields may not be covered by this guarantee and should be treated opaquely. For example, don't rely on the format details of a URL field unless we explicitly say that the field will follow that format.

The Go client API, documented in godoc, is currently considered unstable. It is recommended to vendor the necessary components to stabilize your project build. Note that because the Go API interfaces with the GRPC API, clients written against a 1.0 Go API should remain compatible with future 1.x series releases.

We intend to stabilize the API in a future release when more integrations have been carried out.

Any changes to the API should be detectable at compile time, so upgrading will be a matter of fixing compilation errors and moving from there.

The CRI (Container Runtime Interface) GRPC API is used by a Kubernetes kubelet to communicate with a container runtime. This interface is used to manage container lifecycles and container images. Currently this API is under development and unstable across Kubernetes releases. Each Kubernetes release only supports a single version of CRI and the CRI plugin only implements a single version of CRI.

Each minor release will support one version of CRI and at least one version of Kubernetes. Once this API is stable, a minor will be compatible with any version of Kubernetes which supports that version of CRI.

The ctr tool provides the ability to introspect and understand the containerd API. At this time, it is not considered a primary offering of the project. It may be completely refactored or have breaking changes in minor releases.

We will try not break the tool in patch releases.

As a general rule, anything not mentioned in this document is not covered by the stability guidelines and may change in any release. Explicitly, this pertains to this non-exhaustive list of components:

• File System layout
• Storage formats
• Snapshot formats

Between upgrades of subsequent, minor versions, we may migrate these formats. Any outside processes relying on details of these file system layouts may break in that process. Container root file systems will be maintained on upgrade.

We may make exceptions in the interest of security patches. If a break is required, it will be communicated clearly and the solution will be considered against total impact.