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title authors reviewers creation-date last-updated status see-also replaces superseded-by
Insulate users from kubeadm API version changes
@fabriziopandini
@vincepri
2021-02-10
2021-02-10
implementable
/docs/proposals/20190610-machine-states-preboot-bootstrapping.md

Insulate users from kubeadm API version changes

Table of Contents

Glossary

Refer to the Cluster API Book Glossary.

Summary

Make CABPK and KCP to use more recent versions of the kubeadm API and insulate users from kubeadm API version changes.

Motivation

Cluster bootstrap provider for kubeadm (CABPK) and the control plane provider for kubeadm (KCP) API still relying on kubeadm v1beta1 API, which has been deprecated, and it is going to be removed ASAP.

While moving to a more recent version of the kubeadm API is a required move, Cluster API should take this opportunity to stop relying on the assumption that the kubeadm API types in the KubeadmConfig/KubeadmControlPlane specs are supported(1) by all the Kubernetes/kubeadm version in the support range.

This would allow to separate what users fill in the KubeadmConfig/KubeadmControlPlane from which kubeadm API version Cluster API end up using in the bootstrap data.

(1) Supported in this context means that the serialization format of the types is the same, because types are already different (see background info in the implementation details paragraph).

Goals

  • Define a stop-gap for using the most recent version of the kubeadm API in Cluster API v1alpha3 - introducing any breaking changes.
  • Define how to stop to exposing the kubeadm v1betax types in the KubeadmConfig/KubeadmControlPlane specs for v1alpha4.
  • Define how to use the right version of the kubeadm types generating our kubeadm yaml file and when interacting with the kubeadm-config ConfigMap.
  • Ensure a clean and smooth v1alpha3 to v1aplha4 upgrade experience.

Non-Goals

  • Adding or removing fields in KubeadmConfig/KubeadmControlPlane spec.
  • Introduce behavioral changes in CABPK or KCP.

Future work

  • Evaluate improvements for the Cluster API owned version of KubeadmConfig/KubeadmControlPlane specs types.
  • Make it possible for the Cluster API users to take benefit of changes introduced in recent versions of the Kubeadm API.

Proposal

User Stories

Story 1

As a user, I want to user kubeadm providers for Cluster API (CABPK & KPC) in the same way across the entire spectrum of supported kubernetes versions.

Story 2

As the kubeadm tool, I want the kubeadm providers for Cluster API (CABPK & KPC) to use the latest kubeadm API supported by the target kubeadm/kubernetes version.

Requirements

R1 - avoid breaking changes in v1alpha3 R2 - ensure a clean v1alpha3 to v1alpha4 upgrade

Implementation Details/Notes/Constraints

Background info about kubeadm API version

kubeadm v1beta1 types:

  • Introduced in v1.13.
  • Deprecation cycle started with v1.17.
  • Removal scheduled for v1.20 but then postponed to v1.21.

kubeadm v1beta2 types:

  • Introduced in v1.15.
  • Changes from the previous version are minimal:
    • Support for IgnorePreflightErrors into the NodeRegistrationOptions.
    • Support for CertificateKey into InitConfiguration and JoinConfiguration configuration (not relevant for Cluster API because it is not using the automatic certificate copy feature).
    • improved serialization support (in practice a set of omitempty fixes).

Background info about kubeadm types into the KubeadmConfig/KubeadmControlPlane specs

Given the fact that importing kubeadm (which is part of the Kubernetes codebase) in Cluster API is impractical, Cluster API hosts a mirror of kubeadm API types.

kubeadm v1beta1 mirror-types:

  • Hosted in bootstrap/kubeadm/types/upstreamv1beta1.
  • Diverged from the original v1beta1 types for better CRD support (in practice a set of +optional fixes, few omitempty differences).

kubeadm v1beta2 mirror-types:

  • Hosted in bootstrap/kubeadm/types/upstreamv1beta2.
  • Currently, not used in the Cluster API codebase.
  • Does not include changes for better CRD support introduced in kubeadm v1beta1 mirror-types.

Cluster API v1alpha3 changes

Changes to cluster API v1alpha3 release should be minimal and no breaking change should be introduced while implementing this proposal.

According to this principle and to the feedback to this proposal, we are going to implement alternative 2 described below.

Alternative 1:

Keep kubeadm v1beta1 types as a Hub type (1); implement conversion to kubeadm API version f(Kubernetes Version) when generating the kubeadm config for init/join ( e.g convert to kubeadm API v1beta2 for Kubernetes version >= v1.15, convert to kubeadm API v1beta1 for Kubernetes version < v1.15).

This alternative is the more clean, robust and forward looking, but it requires much more work than alternative 2.

Alternative 2:

Keep kubeadm v1beta1 types as a Hub type (1); only change the apiVersion to kubeadm.k8s.io/v1beta2 in the generated kubeadm config for init/join.

This alternative is based on following considerations:

  • kubeadm v1beta1 mirror types are "compatible" with kubeadm v1beta2 types
    • Having support for IgnorePreflightErrors into the NodeRegistrationOptions is not required for Cluster API v1alpha3
    • The automatic certificate copy feature is not used by Cluster API
    • Improved serialization support has been already applied to the kubeadm v1beta1 mirror-types (and further extended).
  • the minimal Kubernetes version supported by Cluster API is v1.16, and kubeadm v1.16 could work with v1beta2 types. Same for all the other versions up to latest(1.20) and next (1.21).
  • limitations: this approach is not future proof, and it should be reconsidered whenever a new version of kubeadm types is created while v1alpha3 is still supported.

Common considerations for both alternatives

KCP is modifying the kubeadm-config Config Map generated by kubeadm, and ideally also this bit of code should be made kubeadm version aware.

However, given that the current implementation currently uses unstructured, and a change for using the kubeadm types, requires a big refactor, the proposed approach for v1alpha3 is to limit the changes to only upgrade the apiVersion when required.

  • limitations: this approach is not future proof, and it should be reconsidered whenever a new version of kubeadm types is changing one of the fields edited during upgrades.

(1) See https://book.kubebuilder.io/multiversion-tutorial/conversion-concepts.html for a definition of Hub or spoke types/version.

Cluster API v1alpha4 changes

Changes to cluster API v1alpha4 could be more invasive and seek for a forward-looking solution.

Planned actions are:

  • introduce a Cluster API owned version of the kubeadm config types (starting from kubeadm v1beta1) to be used by KubeadmConfig/KubeadmControlPlane specs; this should also act as a serialization/deserialization hub (1). Please note that those types will be part of Cluster API types, and thus initially versioned as v1alpha4; once conversion will be in place, those types are not required anymore to have the same serialization format of the real kubeadm types.
  • preserve bootstrap/kubeadm/types/upstreamv1beta1 as a serialization/deserialization spoke (1)(2) for v1alpha4 (also, this will be used by v1alpha3 API types until removal)
  • preserve bootstrap/kubeadm/types/upstreamv1beta2 as serialization/deserialization spoke (1)(2) for v1alpha4
  • implement hub/spoke conversions (1)
  • make CABPK to use conversion while generating the kubeadm config file for init/join
  • make KCP to use conversion while serializing/deserializing the kubeadm-config Config Map
  • make KCP to use the Cluster API owned version of the kubeadm config types instead of using Unstructured for the kubeadm-config Config Map handling
  • add the IgnorePreflightError field to the Cluster API owned types; this field will be silently ignored when converting to v1beta1 (because this version does not support this field). Note: we are not planning to add CertificateKey to the Cluster API owned types because this field is not relevant for Cluster API.

(1) See https://book.kubebuilder.io/multiversion-tutorial/conversion-concepts.html for a definition of Hub or spoke types/version. (2) As soon as it will be possible to vendor kubeadm types, we should drop this copy and use kubeadm library as a source of truth. Hover there is no concrete plan for this yet.

Security Model

This proposal does not introduce changes the existing security model.

Risks and Mitigations

  • Time crunch

This change has been postponed several times for different reasons, and now it is being worked with a strict deadline before kubeadm type removal. Mitigation: Changes to the KubeadmConfig/KubeadmControlPlane specs types considered as a future work.

Alternatives

The Alternatives section is used to highlight and record other possible approaches to delivering the value proposed by a proposal.

Upgrade Strategy

Given the requirement to provide a clean upgrade path from v1alpha3 to v1alpha4, upgrades should be handles using conversion web-hooks only. No external upgrade tool/additional manual steps should be required for upgrade.

Additional Details

Test Plan

For v1alpha4 this will be tested by the periodic jobs testing creating cluster with different Kubernetes releases, doing upgrades to the next version and running Kubernetes conformance.

For v1alpha3 there are no such jobs. We should explore if to backport all the required changes to the test framework (complex) or if to create ad-hoc test for this using what is supported by the v1alpha3 version of the test framework (possible limitations).

Implementation History

  • 2021-02-10: First draft and round of feedback from community