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

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

Major and minor releases of containerd will be made from main. 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

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 main. 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.

Upgrade Path

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.

Next Release

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 Horizon

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

• Next: The next planned release branch.
• Active: The release is a stable branch which is currently supported and accepting patches.
• Extended: The release branch is only accepting security patches.
• LTS: The release is a long term stable branch which 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 at least 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 6 months after the next minor is released or one year, whichever is longer. Additionally, releases may have an extended security support period after the end of the active period to accept security backports. This timeframe will be decided by maintainers before the end of the active status.

Long term stable (LTS) releases will be supported for at least three years after their initial minor release. These branches will accept bug reports and backports until the end of life date. They may also accept a wider range of patches than non-LTS releases to support the longer term maintainability of the branch, including library dependency, toolchain (including Go) and other version updates which are needed to ensure each release is built with fully supported dependencies and remains usable by containerd clients. LTS releases can also accept feature backports to support new Kubernetes releases. The default action has to be reject it though, for long-term stability. This is still negotiable when the feature is a hard dependency for a new release of Kubernetes. There should be at least a 6-month overlap between the end of life of an LTS release and the initial release of a new LTS release. Up to 6 months before the announced end of life of an LTS branch, the branch may convert to a regular Active release with stricter backport criteria.

The current state is available in the following tables:

NOTE containerd v1.7 will end of life at the same time as v1.6 LTS. Due to Minimal Version Selection used by Go modules, 1.7 must be supported until EOL of all 1.x releases. Once 1.7 is in extended support, it will continue to accept security patches in addition to client changes relevant for package importers using the 1.6 LTS daemon.

Kubernetes Support

The Kubernetes version matrix represents the versions of containerd which are recommended for a Kubernetes release. Any actively supported version of containerd may receive patches to fix bugs encountered in any version of Kubernetes, however, our recommendation is based on which versions have been the most thoroughly tested. See the Kubernetes test grid for the list of actively tested versions. Kubernetes only supports n-3 minor release versions and containerd will ensure there is always a supported version of containerd for every supported version of Kubernetes.

Deprecated containerd and kubernetes versions

** Note: containerd v1.6., and v1.7. support CRI v1 and v1alpha2 through EOL as those releases continue to support older versions of k8s, cloud providers, and other clients using CRI v1alpha2. CRI v1alpha2 is deprecated in v1.7 and will be removed in containerd v2.0.

Backporting

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 us know in one of three ways:

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

If you are reporting a security issue:

Please follow the instructions at containerd/project

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 main or need to draft a new commit specific to a particular branch.

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

1. Pick the branch to which you want backported, usually in the format release/<major>.<minor>. 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>
   

   If all of the work from a particular PR/set of PRs is wanted, cherry-pick the individual commits instead of the merge commit. Take #8624 for example, 82ec62b is favored over 9e834e7.

   (Optional) If other commits exist in the main branch which are related to the cherry-picked commit; eg: fixes to the main PR. It is recommended to cherry-pick those commits also into this same my-backport-branch.

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 main with whatever release branch you are targeting with the fix. Make sure the PR title has [<release branch>] prefixed. e.g.:

   [release/1.4] Fix foo in bar
   

If there is no existing fix in main, you should first fix the bug in main, 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 main and must be made for the specific release branch should you open a PR with new code.

Public API Stability

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.

Go client API stability includes the client, defaults and version package as well as all packages under pkg, core, api and protobuf. All packages under cmd, contrib, integration, and internal are not considered part of the stable client API.

GRPC API

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.

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.

*.pb.txt files are generated at each API release. They prevent unintentional changes to the API by having a diff that the CI can run. These files are not intended to be consumed or used by clients.

As of containerd 2.0, the API version diverges from the main containerd version. While containerd 2.0 is a major version jump for containerd, the API will remain on 1.x to remain backwards compatible with prior releases and existing clients. The 2.0 release adds the API to a separate Go module which can remain as the github.com/containerd/containerd/api Go package and imported separately from the rest of containerd.

The API minor version will continue to be incremented for each major and minor version release of containerd. However, the API is tagged directly out of the main branch with the minor version incrementing earlier in the next release cycle rather than at the end. This means that after the containerd 2.0 release, the next API change is tagged as api/v1.9.0 prior to any containerd 2.1 release. The latest API version should be backported to all supported versions and patch releases for prior API versions should be avoided if possible.

Metrics API

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.

Plugins API

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

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 an issue and we will clarify.

Opaque Fields

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.

Go client API

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.

CRI GRPC API

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.

ctr tool

The ctr tool provides the ability to introspect and understand the containerd API. It is not considered a primary offering of the project and is unsupported in that sense. While we understand its value as a debug tool, it may be completely refactored or have breaking changes in minor releases.

Targeting ctr for feature additions reflects a misunderstanding of the containerd architecture. Feature addition should focus on the client Go API and additions to ctr may or may not be accepted at the discretion of the maintainers.

We will do our best to not break compatibility in the tool in patch releases.

Daemon Configuration

The daemon’s configuration file, commonly located in /etc/containerd/config.toml is versioned and backwards compatible. The version field in the config file specifies the config’s version. If no version number is specified inside the config file then it is assumed to be a version 1 config and parsed as such. The latest version is version = 2. The main branch is being prepared to support the next config version 3. The configuration is automatically migrated to the latest version on each startup, leaving the configuration file unchanged. To avoid the migration and optimize the daemon startup time, use containerd config migrate to output the configuration as the latest version. Version 1 is no longer deprecated and is supported by migration, however, it is recommended to use at least version 2.

Migrating a configuration to the latest version will limit the prior versions of containerd in which the configuration can be used. It is suggested not to migrate your configuration file until you are confident you do not need to quickly rollback your containerd version. Use the table of configuration versions to containerd releases to know the minimum version of containerd for each configuration version.

Not Covered

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.

Exceptions

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.

Deprecated features

The deprecated features are shown in the following table:

• Pulling Schema 1 images has been disabled in containerd v2.0, but it still can be enabled by setting an environment variable CONTAINERD_ENABLE_DEPRECATED_PULL_SCHEMA_1_IMAGE=1 until containerd v2.1. ctr users have to specify --local too (e.g., ctr images pull --local). Users of CRI clients (such as Kubernetes and crictl) have to specify this environment variable on the containerd daemon (usually in the systemd unit).

Deprecated config properties

The deprecated properties in config.toml are shown in the following table:

Note

CNI Config Template (plugins."io.containerd.grpc.v1.cri".cni.conf_template) was once deprecated in v1.7.0, but its deprecation was cancelled in v1.7.3.

version = 2

# OLD
# [plugins."io.containerd.grpc.v1.cri"]
#   systemd_cgroup = true

# NEW
[plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc.options]
  SystemdCgroup = true

version = 2

# OLD
# [plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc]
#   runtime_root = "/path/to/runc/root"

# NEW
[plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc.options]
  Root = "/path/to/runc/root"

Experimental features

Experimental features are new features added to containerd which do not have the same stability guarantees as the rest of containerd. An effort is made to avoid breaking interfaces between versions, but changes to experimental features before being fully supported is possible. Users can still expect experimental features to be high quality and are encouraged to use new features to help them stabilize more quickly.