Contributing Guidelines

Read the following guide if you’re interested in contributing to cluster-api.

Contributor License Agreements

We’d love to accept your patches! Before we can take them, we have to jump a couple of legal hurdles.

Please fill out either the individual or corporate Contributor License Agreement (CLA). More information about the CLA and instructions for signing it can be found here.

NOTE: Only original source code from you and other people that have signed the CLA can be accepted into the *repository.

Finding Things That Need Help

If you’re new to the project and want to help, but don’t know where to start, we have a semi-curated list of issues that should not need deep knowledge of the system. Have a look and see if anything sounds interesting. Before starting to work on the issue, make sure that it doesn’t have a lifecycle/active label. If the issue has been assigned, reach out to the assignee. Alternatively, read some of the docs on other controllers and try to write your own, file and fix any/all issues that come up, including gaps in documentation!

Contributing a Patch

  1. If you haven’t already done so, sign a Contributor License Agreement (see details above).
  2. If working on an issue, signal other contributors that you are actively working on it using /lifecycle active.
  3. Fork the desired repo, develop and test your code changes.
  4. Submit a pull request.
    1. All code PR must be labeled with one of
      • ⚠️ (:warning:, major or breaking changes)
      • ✨ (:sparkles:, feature additions)
      • 🐛 (:bug:, patch and bugfixes)
      • 📖 (:book:, documentation or proposals)
      • 🌱 (:seedling:, minor or other)

All changes must be code reviewed. Coding conventions and standards are explained in the official developer docs. Expect reviewers to request that you avoid common go style mistakes in your PRs.

Documentation changes

The documentation is published in form of a book at:

The source for the book is this folder containing markdown files and we use mdBook to build it into a static website.

After making changes locally you can run make serve-book which will build the HTML version and start a web server so you can preview if the changes render correctly at http://localhost:3000; the preview auto-updates when changes are detected.

Note: you don’t need to have mdBook installed, make serve-book will ensure appropriate binaries for mdBook and any used plugins are downloaded into hack/tools/bin/ directory.

When submitting the PR remember to label it with the 📖 (:book:) icon.


Cluster API uses GitHub milestones to track releases.

  • Minor versions CAN be planned and scheduled twice in a calendar year.
    • Each minor version is preceded with one or more planning session.
    • Planning consists of one or more backlog grooming meetings, roadmap amendments, and CAEP proposal reviews.
  • Patch versions CAN be planned and scheduled each month for each of the currently supported series (usually N and N-1).
  • Code freeze is in effect 72 hours (3 days) before a release.
    • Maintainers should communicate the code freeze date at a community meeting preceding the code freeze date.
    • Only critical bug fixes may be merged in between freeze & release.
      • Each bug MUST be associated with an open issue and properly triaged.
      • PRs MUST be approved by at least 2 project maintainers.
        • First approver should /approve and /hold.
        • Second approver should /approve and /hold cancel.
    • E2E Test grid SHOULD be green before cutting a release.
  • Dates in a release are approximations and always subject to change.
  • Next milestone is for work that has been triaged, but not prioritized/accepted for any release.

Proposal process (CAEP)

The Cluster API Enhacement Proposal is the process this project uses to adopt new features, changes to the APIs, changes to contracts between components, or changes to CLI interfaces.

The template, and accepted proposals live under docs/proposals.

  • Proposals or requests for enhacements (RFEs) MUST be associated with an issue.
    • Issues can be placed on the roadmap during planning if there is one or more folks that can dedicate time to writing a CAEP and/or implementating it after approval.
  • A proposal SHOULD be introduced and discussed during the weekly community meetings, Kubernetes SIG Cluster Lifecycle mailing list, or discuss forum.
  • A proposal in a Google Doc MUST turn into a Pull Request.
  • Proposals MUST be merged and in implementable state to be considered part of a major or minor release.

Triaging E2E test failures

When you submit a change to the Cluster API repository as set of validation jobs is automatically executed by prow and the results report is added to a comment at the end of your PR.

Some jobs run linters or unit test, and in case of failures, you can repeat the same operation locally using make test lint [etc..] in order to investigate and potential issues. Prow logs usually provide hints about the make target you should use (there might be more than one command that needs to be run).

End-to-end (E2E) jobs create real Kubernetes clusters by building Cluster API artifacts with the latest changes. In case of E2E test failures, usually it’s required to access the “Artifacts” link on the top of the prow logs page to triage the problem.

The artifact folder contains:

  • A folder with the clusterctl local repository used for the test, where you can find components yaml and cluster templates.
  • A folder with logs for all the clusters created during the test. Following logs/info are available:
    • Controller logs (only if the cluster is a management cluster).
    • Dump of the Cluster API resources (only if the cluster is a management cluster).
    • Machine logs (only if the cluster is a workload cluster)

In case you want to run E2E test locally, please refer to the Testing guide. An overview over our e2e test jobs (and also all our other jobs) can be found in Jobs.

Reviewing a Patch


Parts of the following content have been adapted from

Any Kubernetes organization member can leave reviews and /lgtm a pull request.

Code reviews should generally look at:

  • Design: Is the code well-designed and consistent with the rest of the system?
  • Functionality: Does the code behave as the author (or linked issue) intended? Is the way the code behaves good for its users?
  • Complexity: Could the code be made simpler? Would another developer be able to easily understand and use this code when they come across it in the future?
  • Tests: Does the code have correct and well-designed tests?
  • Naming: Did the developer choose clear names for variable, types, methods, functions, etc.?
  • Comments: Are the comments clear and useful? Do they explain the why rather than what?
  • Documentation: Did the developer also update relevant documentation?

See Code Review in Cluster API for a more focused list of review items.


Please see the Kubernetes community document on pull requests for more information about the merge process.

  • A PR is approved by one of the project maintainers and owners after reviews.
  • Approvals should be the very last action a maintainer takes on a pull request.

Backporting a Patch

Cluster API maintains older versions through release-X.Y branches. We accept backports of bug fixes and non breaking features to the most recent release branch. Backports MUST not be breaking for both API and behavioral changes. We generally do not accept PRs against older release branches.

As an example:

Let’s assume that the most recent release branch is release-0.3 and the main branch is under active development for the next release. A pull request that has been merged in the main branch can be backported to the release-0.3 if at least one maintainer approves the cherry pick, or asks the PR’s author to backport.

Features and bugs

Open issues to report bugs, or minor features.

For big feature, API and contract amendments, we follow the CAEP process as outlined below.


Proof of concepts, code experiments, or other initiatives can live under the exp folder and behind a feature gate.

  • Experiments SHOULD not modify any of the publicly exposed APIs (e.g. CRDs).
  • Experiments SHOULD not modify any existing CRD types outside of the experimental API group(s).
  • Experiments SHOULD not modify any existing command line contracts.
  • Experiments MUST not cause any breaking changes to existing (non-experimental) Go APIs.
  • Experiments SHOULD introduce utility helpers in the go APIs for experiments that cross multiple components and require support from bootstrap, control plane, or infrastructure providers.
  • Experiments follow a strict lifecycle: Alpha -> Beta prior to Graduation.
    • Alpha-stage experiments:
      • SHOULD not be enabled by default and any feature gates MUST be marked as ‘Alpha’
      • MUST be associated with a CAEP that is merged and in at least a provisional state
      • MAY be considered inactive and marked as deprecated if the following does not happen within the course of 1 minor release cycle:
        • Transition to Beta-stage
        • Active development towards progressing to Beta-stage
        • Either direct or downstream user evaluation
      • Any deprecated Alpha-stage experiment MAY be removed in the next minor release.
    • Beta-stage experiments:
      • SHOULD be enabled by default, and any feature gates MUST be marked as ‘Beta’
      • MUST be associated with a CAEP that is at least in the experimental state
      • MUST support conversions for any type changes
      • MUST remain backwards compatible unless updates are coinciding with a breaking Cluster API release
      • MAY be considered inactive and marked as deprecated if the following does not happen within the course of 1 minor release cycle:
        • Graduate
        • Active development towards Graduation
        • Either direct or downstream user consumption
      • Any deprecated Beta-stage experiment MAY be removed after being deprecated for an entire minor release.
  • Experiment Graduation MUST coincide with a breaking Cluster API release
  • Experiment Graduation checklist:
    • MAY provide a way to be disabled, any feature gates MUST be marked as ‘GA’
    • MUST undergo a full Kubernetes-style API review and update the CAEP with the plan to address any issues raised
    • CAEP MUST be in an implementable state and is fully up to date with the current implementation
    • CAEP MUST define transition plan for moving out of the experimental api group and code directories
    • CAEP MUST define any upgrade steps required for Existing Management and Workload Clusters
    • CAEP MUST define any upgrade steps required to be implemented by out-of-tree bootstrap, control plane, and infrastructure providers.

Breaking Changes

Breaking changes are generally allowed in the main branch, as this is the branch used to develop the next minor release of Cluster API.

There may be times, however, when main is closed for breaking changes. This is likely to happen as we near the release of a new minor version.

Breaking changes are not allowed in release branches, as these represent minor versions that have already been released. These versions have consumers who expect the APIs, behaviors, etc. to remain stable during the life time of the patch stream for the minor release.

Examples of breaking changes include:

  • Removing or renaming a field in a CRD
  • Removing or renaming a CRD
  • Removing or renaming an exported constant, variable, type, or function
  • Updating the version of critical libraries such as controller-runtime, client-go, apimachinery, etc.
    • Some version updates may be acceptable, for picking up bug fixes, but maintainers must exercise caution when reviewing.

There may, at times, need to be exceptions where breaking changes are allowed in release branches. These are at the discretion of the project’s maintainers, and must be carefully considered before merging. An example of an allowed breaking change might be a fix for a behavioral bug that was released in an initial minor version (such as v0.3.0).

API conventions

This project follows the Kubernetes API conventions. Minor modifications or additions to the conventions are listed below.

Optional vs. Required

  • Status fields MUST be optional. Our controllers are patching selected fields instead of updating the entire status in every reconciliation.

  • If a field is required (for our controllers to work) and has a default value specified via OpenAPI schema, but we don’t want to force users to set the field, we have to mark the field as optional. Otherwise, the client-side kubectl OpenAPI schema validation will force the user to set it even though it would be defaulted on the server-side.

Optional fields have the following properties:

  • An optional field MUST be marked with +optional and include an omitempty JSON tag.
  • Fields SHOULD be pointers if the nil and the zero values (by Go standards) have semantic differences.
    • Note: This doesn’t apply to map or slice types as they are assignable to nil.


When using ClusterClass, the semantic difference is important when you have a field in a template which will have instance-specific different values in derived objects. Because in this case it’s possible to set the field to nil in the template and then the value can be set in derived objects without being overwritten by the cluster topology controller.


  • Fields in root objects should be kept as scaffolded by kubebuilder, e.g.:

    type Machine struct {
      metav1.TypeMeta   `json:",inline"`
      metav1.ObjectMeta `json:"metadata,omitempty"`
      Spec   MachineSpec   `json:"spec,omitempty"`
      Status MachineStatus `json:"status,omitempty"`
    type MachineList struct {
      metav1.TypeMeta `json:",inline"`
      metav1.ListMeta `json:"metadata,omitempty"`
      Items           []Machine `json:"items"`
  • Top-level fields in status must always have the +optional annotation. If we want the field to be always visible even if it has the zero value, it must not have the omitempty JSON tag, e.g.:

    • Replica counters like availableReplicas in the MachineDeployment
    • Flags expressing progress in the object lifecycle like infrastructureReady in Machine

CRD additionalPrinterColumns

All our CRD objects should have the following additionalPrinterColumns order (if the respective field exists in the CRD):

  • Namespace (added automatically)
  • Name (added automatically)
  • Cluster
  • Other fields
  • Replica-related fields
  • Phase
  • Age (mandatory field for all CRDs)
  • Version
  • Other fields for -o wide (fields with priority 1 are only shown with -o wide and not per default)

NOTE: The columns can be configured via the kubebuilder:printcolumn annotation on root objects. For examples, please see the ./api package.


$ kubectl get kubeadmcontrolplane
quick-start-d5ufye   quick-start-ntysk0-control-plane   true          true                   1          1       1                       2m44s   v1.22.0
$ kubectl get machinedeployment
NAMESPACE            NAME                      CLUSTER              REPLICAS   READY   UPDATED   UNAVAILABLE   PHASE       AGE     VERSION
quick-start-d5ufye   quick-start-ntysk0-md-0   quick-start-ntysk0   1                  1         1             ScalingUp   3m28s   v1.22.0

Google Doc Viewing Permissions

To gain viewing permissions to google docs in this project, please join either the kubernetes-dev or kubernetes-sig-cluster-lifecycle google group.

Issue and Pull Request Management

Anyone may comment on issues and submit reviews for pull requests. However, in order to be assigned an issue or pull request, you must be a member of the Kubernetes SIGs GitHub organization.

If you are a Kubernetes GitHub organization member, you are eligible for membership in the Kubernetes SIGs GitHub organization and can request membership by opening an issue against the kubernetes/org repo.

However, if you are a member of any of the related Kubernetes GitHub organizations but not of the Kubernetes org, you will need explicit sponsorship for your membership request. You can read more about Kubernetes membership and sponsorship here.

Cluster API maintainers can assign you an issue or pull request by leaving a /assign <your Github ID> comment on the issue or pull request.