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api_compatibility_policy.md

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API compatibility policy

This document proposes a policy regarding making updates to the Tekton API surface in this repo. Users should be able to build on the APIs in these projects with a clear idea of what they can rely on and what should be considered in progress and therefore likely to change.

What is the API?

The API is considered to consist of:

  • The structure of the CRDs including the spec and status sections, as well as:
    • The ordering of the steps within the status
    • The naming of the step containers within the status
    • The labels propagated from PipelineRuns to TaskRuns and TaskRuns to Pods.
  • The structure of the directories created in executing containers by Tekton
  • The order that PipelineResources declared within a Task are applied in
  • The interfaces of the images that are built as part of Tekton Pipelines, i.e. images in cmd which are used as part of PipelineResources

This policy is about changes to any of the above facets of the API.

A backwards incompatible change would be a change that requires a user to update existing instances of these CRDs in clusters where they are deployed and/or cause them to change their CRD definitions to account for changes in the above.

CRD API Versions

The apiVersion field in a Tekton CRD determines whether the overall API (and its default behaviors) is considered to be in alpha, beta, or GA. The use of alpha, beta and GA (aka stable) follows the corresponding Kubernetes API stages definition.

Within a stable CRD, certain opt-in features or API fields gated may be considered alpha or beta. Similarly, within a beta CRD, certain opt-in features may be considered alpha. See the section on Feature Gates for details.

The following CRDs are considered beta, though features may be introduced that are alpha:

  • Task
  • TaskRun
  • ClusterTask
  • Pipeline
  • PipelineRun

Alpha CRDs

  • For Alpha CRDs, the apiVersion contains the alpha (e.g. v1alpha1)

  • Features may be dropped at any time, though you will be given at least one release worth of warning.

Beta CRDs

  • The apiVersion field of the CRD contains contain beta (for example, v1beta1).

  • These features will not be dropped, though the details may change.

  • Any backwards incompatible changes must be introduced in a backwards compatible manner first, with a deprecation warning in the release notes and migration instructions.

  • Users will be given at least 9 months to migrate before a backward incompatible change is made. This means an older beta API version will continue to be supported in new releases for a period of at least 9 months from the time a newer version is made available.

GA CRDs

  • The apiVersion field of the CRD is vX where X is an integer.

  • Stable API versions remain available for all future releases within a semver major version.

  • Stable features may be marked as deprecated but may only be removed by incrementing the api version (i.e v1 to v2).

  • We will not make any backwards incompatible changes to fields that are stable without incrementing the api version.

  • Alpha and Beta features may be present within a stable API version. However, they will not be enabled by default and must be enabled by setting enable-api-fields to alpha or beta.

Feature Gates

CRD API versions gate the overall stability of the CRD and its default behaviors. Within a particular CRD version, certain opt-in features may be at a lower stability level as described in TEP-33. These fields may be disabled by default and can be enabled by setting the right enable-api-fields feature-flag as described in TEP-33:

  • stable (default) - This value indicates that only fields of the highest stability level are enabled; For beta CRDs, this means only beta stability fields are enabled, i.e. alpha fields are not enabled. For GA CRDs, this means only GA fields are enabled by defaultd, i.e. beta and alpha fields would not be enabled.
  • beta - This value indicates that only fields which are of beta (or greater) stability are enabled, i.e. alpha fields are not enabled.
  • alpha - This value indicates that fields of all stability levels are enabled, specifically alpha, beta and GA.
Feature Versions -> v1 beta alpha
stable x
beta x x
alpha x x x

See the current list of alpha features and beta features.

Alpha features

  • Alpha feature in beta or GA CRDs are disabled by default and must be enabled by setting enable-api-fields to alpha

  • These features may be dropped or backwards incompatible changes made at any time though will be given at least one release worth of warning

  • Alpha features are reviewed for promotion to beta at a regular basis. However, there is no guarantee that they will be promoted to beta.

Beta features

  • Beta features in GA CRDs are disabled by default and must be enabled by setting enable-api-fields to beta. In beta API versions, beta features are enabled by default.

  • Beta features may be deprecated or changed in a backwards incompatible way by following the same process as Beta CRDs i.e. by providing a 9 month support period.

  • Beta features are reviewed for promotion to GA/Stable on a regular basis. However, there is no guarantee that they will be promoted to GA/stable.

  • For beta API versions, beta is the highest level of stability possible for any feature.

GA/Stable features

  • GA/Stable features are present in a GA CRD only.

  • GA/Stable features are enabled by default

  • GA/Stable features will not be removed or changed in a backwards incompatible manner without incrementing the API Version.

Approving API changes

API changes must be approved by OWNERS. The policy is slightly different for additive changes vs. backwards incompatible changes.

Additive changes

Additive changes are changes that add to the API and do not cause problems for users of previous versions of the API.

These changes must be approved by at least 2 OWNERS.

Backwards incompatible changes

Backwards incompatible changes change the API, e.g. by removing fields from a CRD spec. These changes will mean that folks using a previous version of the API will need to adjust their usage in order to use the new version. Adding a new field to the CustomRun API that all CustomRun controllers are expected to support is also a backwards incompatible change, as CustomRun controllers that were valid before the change would be invalid after the change.

These changes must be approved by more than half of the project OWNERS (i.e. 50% + 1).

Deprecated Features

Tekton Pipelines maintains a list of features that have been deprecated which includes the earliest date each feature will be removed.

Go libraries compatibility policy

Tekton Pipelines may introduce breaking changes to its Go client libraries, as long as these changes do not impact users' yaml/json CRD definitions. For example, a change that renames a field of a CRD would need to follow the policy on backwards incompatible changes, but a change that renames the Go struct type for that field is allowed.