Kubernetes Deprecation Policy

This document details the deprecation policy for various facets of the system.

Kubernetes is a large system with many components and many contributors. As with any such software, the feature set naturally evolves over time, and sometimes a feature may need to be removed. This could include an API, a flag, or even an entire feature. To avoid breaking existing users, Kubernetes follows a deprecation policy for aspects of the system that are slated to be removed.

Deprecating parts of the API

Since Kubernetes is an API-driven system, the API has evolved over time to reflect the evolving understanding of the problem space. The Kubernetes API is actually a set of APIs, called "API groups", and each API group is independently versioned. API versions fall into 3 main tracks, each of which has different policies for deprecation:

A given release of Kubernetes can support any number of API groups and any number of versions of each.

The following rules govern the deprecation of elements of the API. This includes:

• REST resources (aka API objects)
• Fields of REST resources
• Annotations on REST resources, including "beta" annotations but not including "alpha" annotations.
• Enumerated or constant values
• Component config structures

These rules are enforced between official releases, not between arbitrary commits to master or release branches.

Rule #1: API elements may only be removed by incrementing the version of the API group.

Once an API element has been added to an API group at a particular version, it can not be removed from that version or have its behavior significantly changed, regardless of track.

Rule #2: API objects must be able to round-trip between API versions in a given release without information loss, with the exception of whole REST resources that do not exist in some versions.

For example, an object can be written as v1 and then read back as v2 and converted to v1, and the resulting v1 resource will be identical to the original. The representation in v2 might be different from v1, but the system knows how to convert between them in both directions. Additionally, any new field added in v2 must be able to round-trip to v1 and back, which means v1 might have to add an equivalent field or represent it as an annotation.

Rule #3: An API version in a given track may not be deprecated in favor of a less stable API version.

• GA API versions can replace beta and alpha API versions.
• Beta API versions can replace earlier beta and alpha API versions, but may not replace GA API versions.
• Alpha API versions can replace earlier alpha API versions, but may not replace GA or beta API versions.

Rule #4a: API lifetime is determined by the API stability level

• GA API versions may be marked as deprecated, but must not be removed within a major version of Kubernetes
• Beta API versions are deprecated no more than 9 months or 3 minor releases after introduction (whichever is longer), and are no longer served 9 months or 3 minor releases after deprecation (whichever is longer)
• Alpha API versions may be removed in any release without prior deprecation notice

This ensures beta API support covers the maximum supported version skew of 2 releases, and that APIs don't stagnate on unstable beta versions, accumulating production usage that will be disrupted when support for the beta API ends.

Rule #4b: The "preferred" API version and the "storage version" for a given group may not advance until after a release has been made that supports both the new version and the previous version

Users must be able to upgrade to a new release of Kubernetes and then roll back to a previous release, without converting anything to the new API version or suffering breakages (unless they explicitly used features only available in the newer version). This is particularly evident in the stored representation of objects.

All of this is best illustrated by examples. Imagine a Kubernetes release, version X, which introduces a new API group. A new Kubernetes release is made every approximately 4 months (3 per year). The following table describes which API versions are supported in a series of subsequent releases.

REST resources (aka API objects)

Consider a hypothetical REST resource named Widget, which was present in API v1 in the above timeline, and which needs to be deprecated. We document and announce the deprecation in sync with release X+1. The Widget resource still exists in API version v1 (deprecated) but not in v2alpha1. The Widget resource continues to exist and function in releases up to and including X+8. Only in release X+9, when API v1 has aged out, does the Widget resource cease to exist, and the behavior get removed.

Starting in Kubernetes v1.19, making an API request to a deprecated REST API endpoint:

1. Returns a Warning header (as defined in RFC7234, Section 5.5) in the API response.

2. Adds a "k8s.io/deprecated":"true" annotation to the audit event recorded for the request.

3. Sets an apiserver_requested_deprecated_apis gauge metric to 1 in the kube-apiserver process. The metric has labels for group, version, resource, subresource that can be joined to the apiserver_request_total metric, and a removed_release label that indicates the Kubernetes release in which the API will no longer be served. The following Prometheus query returns information about requests made to deprecated APIs which will be removed in v1.22:

   apiserver_requested_deprecated_apis{removed_release="1.22"} * on(group,version,resource,subresource) group_right() apiserver_request_total
   

Fields of REST resources

As with whole REST resources, an individual field which was present in API v1 must exist and function until API v1 is removed. Unlike whole resources, the v2 APIs may choose a different representation for the field, as long as it can be round-tripped. For example a v1 field named "magnitude" which was deprecated might be named "deprecatedMagnitude" in API v2. When v1 is eventually removed, the deprecated field can be removed from v2.

Enumerated or constant values

As with whole REST resources and fields thereof, a constant value which was supported in API v1 must exist and function until API v1 is removed.

Component config structures

Component configs are versioned and managed similar to REST resources.

Future work

Over time, Kubernetes will introduce more fine-grained API versions, at which point these rules will be adjusted as needed.

Deprecating a flag or CLI

The Kubernetes system is comprised of several different programs cooperating. Sometimes, a Kubernetes release might remove flags or CLI commands (collectively "CLI elements") in these programs. The individual programs naturally sort into two main groups - user-facing and admin-facing programs, which vary slightly in their deprecation policies. Unless a flag is explicitly prefixed or documented as "alpha" or "beta", it is considered GA.

CLI elements are effectively part of the API to the system, but since they are not versioned in the same way as the REST API, the rules for deprecation are as follows:

Rule #5a: CLI elements of user-facing components (e.g. kubectl) must function after their announced deprecation for no less than:

• GA: 12 months or 2 releases (whichever is longer)
• Beta: 3 months or 1 release (whichever is longer)
• Alpha: 0 releases

Rule #5b: CLI elements of admin-facing components (e.g. kubelet) must function after their announced deprecation for no less than:

• GA: 6 months or 1 release (whichever is longer)
• Beta: 3 months or 1 release (whichever is longer)
• Alpha: 0 releases

Rule #5c: Command line interface (CLI) elements cannot be deprecated in favor of less stable CLI elements

Similar to the Rule #3 for APIs, if an element of a command line interface is being replaced with an alternative implementation, such as by renaming an existing element, or by switching to use configuration sourced from a file instead of a command line argument, that recommended alternative must be of the same or higher stability level.

Rule #6: Deprecated CLI elements must emit warnings (optionally disable) when used.

Deprecating a feature or behavior

Occasionally a Kubernetes release needs to deprecate some feature or behavior of the system that is not controlled by the API or CLI. In this case, the rules for deprecation are as follows:

Rule #7: Deprecated behaviors must function for no less than 1 year after their announced deprecation.

If the feature or behavior is being replaced with an alternative implementation that requires work to adopt the change, there should be an effort to simplify the transition whenever possible. If an alternative implementation is under Kubernetes organization control, the following rules apply:

Rule #8: The feature of behavior must not be deprecated in favor of an alternative implementation that is less stable

For example, a generally available feature cannot be deprecated in favor of a Beta replacement. The Kubernetes project does, however, encourage users to adopt and transitions to alternative implementations even before they reach the same maturity level. This is particularly important for exploring new use cases of a feature or getting an early feedback on the replacement.

Alternative implementations may sometimes be external tools or products, for example a feature may move from the kubelet to container runtime that is not under Kubernetes project control. In such cases, the rule cannot be applied, but there must be an effort to ensure that there is a transition path that does not compromise on components' maturity levels. In the example with container runtimes, the effort may involve trying to ensure that popular container runtimes have versions that offer the same level of stability while implementing that replacement behavior.

Deprecation rules for features and behaviors do not imply that all changes to the system are governed by this policy. These rules applies only to significant, user-visible behaviors which impact the correctness of applications running on Kubernetes or that impact the administration of Kubernetes clusters, and which are being removed entirely.

An exception to the above rule is feature gates. Feature gates are key=value pairs that allow for users to enable/disable experimental features.

Feature gates are intended to cover the development life cycle of a feature - they are not intended to be long-term APIs. As such, they are expected to be deprecated and removed after a feature becomes GA or is dropped.

As a feature moves through the stages, the associated feature gate evolves. The feature life cycle matched to its corresponding feature gate is:

• Alpha: the feature gate is disabled by default and can be enabled by the user.
• Beta: the feature gate is enabled by default and can be disabled by the user.
• GA: the feature gate is deprecated (see "Deprecation") and becomes non-operational.
• GA, deprecation window complete: the feature gate is removed and calls to it are no longer accepted.

Deprecation

Features can be removed at any point in the life cycle prior to GA. When features are removed prior to GA, their associated feature gates are also deprecated.

When an invocation tries to disable a non-operational feature gate, the call fails in order to avoid unsupported scenarios that might otherwise run silently.

In some cases, removing pre-GA features requires considerable time. Feature gates can remain operational until their associated feature is fully removed, at which point the feature gate itself can be deprecated.

When removing a feature gate for a GA feature also requires considerable time, calls to feature gates may remain operational if the feature gate has no effect on the feature, and if the feature gate causes no errors.

Features intended to be disabled by users should include a mechanism for disabling the feature in the associated feature gate.

Versioning for feature gates is different from the previously discussed components, therefore the rules for deprecation are as follows:

Rule #9: Feature gates must be deprecated when the corresponding feature they control transitions a lifecycle stage as follows. Feature gates must function for no less than:

• Beta feature to GA: 6 months or 2 releases (whichever is longer)
• Beta feature to EOL: 3 months or 1 release (whichever is longer)
• Alpha feature to EOL: 0 releases

Rule #10: Deprecated feature gates must respond with a warning when used. When a feature gate is deprecated it must be documented in both in the release notes and the corresponding CLI help. Both warnings and documentation must indicate whether a feature gate is non-operational.

Deprecating a metric

Each component of the Kubernetes control-plane exposes metrics (usually the /metrics endpoint), which are typically ingested by cluster administrators. Not all metrics are the same: some metrics are commonly used as SLIs or used to determine SLOs, these tend to have greater import. Other metrics are more experimental in nature or are used primarily in the Kubernetes development process.

Accordingly, metrics fall under three stability classes (ALPHA, BETA STABLE); this impacts removal of a metric during a Kubernetes release. These classes are determined by the perceived importance of the metric. The rules for deprecating and removing a metric are as follows:

Rule #11a: Metrics, for the corresponding stability class, must function for no less than:

• STABLE: 4 releases or 12 months (whichever is longer)
• BETA: 2 releases or 8 months (whichever is longer)
• ALPHA: 0 releases

Rule #11b: Metrics, after their announced deprecation, must function for no less than:

• STABLE: 3 releases or 9 months (whichever is longer)
• BETA: 1 releases or 4 months (whichever is longer)
• ALPHA: 0 releases

Deprecated metrics will have their description text prefixed with a deprecation notice string '(Deprecated from x.y)' and a warning log will be emitted during metric registration. Like their stable undeprecated counterparts, deprecated metrics will be automatically registered to the metrics endpoint and therefore visible.

On a subsequent release (when the metric's deprecatedVersion is equal to current_kubernetes_version - 3)), a deprecated metric will become a hidden metric. Unlike their deprecated counterparts, hidden metrics will no longer be automatically registered to the metrics endpoint (hence hidden). However, they can be explicitly enabled through a command line flag on the binary (--show-hidden-metrics-for-version=). This provides cluster admins an escape hatch to properly migrate off of a deprecated metric, if they were not able to react to the earlier deprecation warnings. Hidden metrics should be deleted after one release.

Exceptions

No policy can cover every possible situation. This policy is a living document, and will evolve over time. In practice, there will be situations that do not fit neatly into this policy, or for which this policy becomes a serious impediment. Such situations should be discussed with SIGs and project leaders to find the best solutions for those specific cases, always bearing in mind that Kubernetes is committed to being a stable system that, as much as possible, never breaks users. Exceptions will always be announced in all relevant release notes.