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kubeadm phases to beta

Table of Contents

Summary

We are defining the road map for graduating kubeadm alpha phase commands to beta, addressing concerns/lessons learned so far about the additional effort for maintenance of this feature.

Motivation

The kubeadm phase command was introduced in v1.8 cycle under the kubeadm alpha phase command with the goal of providing users with an interface for invoking individually any task/phase of the kubeadm init workflow.

During this period, kubeadm phase proved to be a valuable and composable API/toolbox that can be used by any IT automation tool or by an advanced user for creating custom clusters.

However, the existing separation of kubeadm init and kubeadm phase in the code base required a continuous effort to keep the two things in sync, with a proliferation of flags, duplication of code or in some case inconsistencies between the init and phase implementation.

Goals

  • To define the approach for graduating to beta the kubeadm phase user interface.
  • To significantly reduces the effort for maintaining phases up to date with kubeadm init.
  • To support extension of the "phases" concept to other kubeadm workflows.
  • To enable re-use of phases across different workflows e.g. the cert phase used by the kubeadm init and by the kubeadm join workflows.

Non-goals

  • This proposal doesn't include any changes of improvements to the actual kubeadm init workflow.
  • This proposal doesn't include a plan for implementation of phases in workflows different than the kubeadm init workflow; such plans will be defined by the sig during release planning for each cycle, and then documented in this KEP.
    • v1.14 implementation of phases in the kubeadm join workflow
    • v1.15 implementation of phases in the kubeadm upgrade node workflow
    • v1.17 implementation of phases in the kubeadm upgrade apply workflow

Proposal

User Stories

  • As a kubernetes administrator/IT automation tool, I want to run all the phases of the kubeadm init workflow.
  • As a kubernetes administrator/IT automation tool, I want to run only one or some phases of the kubeadm init workflow.
  • As a kubernetes administrator/IT automation tool, I want to run all the phases of the kubeadm init workflow except some phases.
  • Same as above, but for the kubeadm join workflow
  • As a kubernetes administrator/IT automation tool, I used kubeadm init/join and excluded some phases, I will want to run all the phases of kubeadm upgrade except for the equivalent phases.

Implementation Details

The core of the new phase design consist into a simple, lightweight workflow manager to be used for implementing composable kubeadm workflows.

Composable kubeadm workflows are build by an ordered sequence of phases; each phase can have it's own, nested, ordered sequence of phases. For instance:

  preflight       Run master pre-flight checks
  certs           Generates all PKI assets necessary to establish the control plane
    /ca             Generates a self-signed kubernetes CA
    /apiserver      Generates an API server serving certificate and key
    ...
  kubeconfig      Generates all kubeconfig files necessary to establish the control plane
    /admin          Generates a kubeconfig file for the admin to use and for kubeadm itself
    /kubelet        Generates a kubeconfig file for the kubelet to use.
    ...
  ...

The above list of ordered phases should be made accessible from all the command supporting phases via the command help, e.g. kubeadm init --help (and eventually in the future kubeadm join --help and kubeadm upgrade apply --help etc.)

Additionally we are going to improve consistency between the command outputs/logs with the name of phases defined in the above list. This will be achieved by enforcing that the prefix of each output/log should match the name of the corresponding phase, e.g. [certs/ca] Generated ca certificate and key. instead of the current [certificates] Generated ca certificate and key..

Single phases will be made accessible to the users via a new phase sub command that will be nested in the command supporting phases, e.g. kubeadm init phase and kubeadm join phase (and eventually in the future kubeadm upgrade apply phase etc.). e.g.

kubeadm init phases certs [flags]

kubeadm init phases certs ca [flags]

Additionally we are going also to allow users to skip phases from the main workflow, via the --skip-phases flag. e.g.

kubeadm init --skip-phases addons/proxy

The above UX will be supported by a new components, the PhaseRunner that will be responsible of running phases according to the given order; nested phases will be executed immediately after their parent phase.

The PhaseRunner will be instantiated by kubeadm commands with the configuration of the specific list of ordered phases; the PhaseRunner in turn will dynamically generate all the phase sub commands for the phases.

Phases invoked by the PhaseRunner should be designed in order to ensure reuse across different workflows e.g. reuse of phase certs in both kubeadm init and kubeadm join workflows.

Graduation Criteria

  • To create a periodic E2E test that bootstraps a cluster using phases
  • To document the new user interface for phases in kubernetes.io

Implementation History

  • #61631 First prototype implementation (now outdated)
  • v1.13 implementation of phases in the kubeadm init workflow
  • v1.14 implementation of phases in the kubeadm join workflow
  • v1.15 implementation of phases in the kubeadm upgrade node workflow
  • v1.17 implementation of phases in the kubeadm upgrade apply workflow (planned)
  • v1.24 retroactively promoted to Beta. upgrade apply might need a separate KEP.

Drawbacks

It would not be possible to expose to the users phases which are not part of kubeadm workflows ("extra" phases should be hosted on dedicated kubeadm alpha command and follow the normal graduation process).

This is considered an acceptable trade-off in light of the benefits of the suggested approach.

Alternatives

It is possible to graduate phases by simply moving corresponding command to first level, but this approach provide less opportunities for reducing the effort for maintaining phases up to date with the changes of kubeadm.