`+kubebuilder:conversion:hub` undocumented / unimplemented

[pmalek]

Problem statement

Reading through the documentation about the conversion webhooks, I stumbled across the // +kubebuilder:conversion:hub which I couldn't get much more information on.

It seems that this is used in couple of places in tests in kubebuilder: https://github.com/search?q=repo%3Akubernetes-sigs%2Fkubebuilder%20%2Bkubebuilder%3Aconversion%3Ahub&type=code but not much more than that.

What is the meaning of this code marker? Should it produce any changes in the generated code when applied?

[camilamacedo86]

It is a good catcher.
We need to verify whether this one serves a purpose or not, and if it doesn't, it should be removed.

[camilamacedo86]

Hi @pmalek,

Thank you so much for bringing this up — I really appreciate it!

I checked, and you're right — the marker is not actually used.
It looks like the feature was only partially implemented a while ago (before I joined the project), so I don’t have full context. My guess is that someone added the marker intending to use it for scaffolding, but it turns out we don’t really need it for that.

As part of a bigger effort to implement the webhook conversion properly, we delivered its automation for 4.x. It passed, but I missed this detail — so thanks again for catching it!

We’ll go ahead and remove the marker in the next release.

Best,

[pmalek]

Thanks for getting back to me on this.

As part of a bigger effort to implement the webhook conversion properly, we delivered its automation for 4.x.

Can you shed some more light on this? What automation do you have in mind?

[camilamacedo86]

Sure, users can scaffold a conversion webhook by

    # Create API to check webhook --conversion from v1 to v2
    $ kubebuilder create api --group example.com --version v1 --kind Wordpress --controller=true --resource=true
    $ kubebuilder create api --group example.com --version v2 --kind Wordpress --controller=false --resource=true
    $ kubebuilder create webhook --group example.com --version v1 --kind Wordpress --conversion --spoke v2

Since the release: https://github.com/kubernetes-sigs/kubebuilder/releases/tag/v4.4.0

[camilamacedo86]

Out of Scope

Additionally, I strongly recommend the following:

• Stick to the default scaffold provided by the tool: https://book.kubebuilder.io/versions_compatibility_supportability#project-customizations
• Always use the tool (if possible) to generate your APIs, controllers, and webhooks, so that all automations around them will work properly and make your life easier.
  One of our goals is to reduce the burden on developers — at least for the most common use cases. If you have a case that lacks sufficient support but appears to be very common, please feel free to raise an issue so that we can analyse whether it could fit.

Following these practices will make it much easier to upgrade your project in the future using the new kubebuilder alpha generate helper:
https://book.kubebuilder.io/reference/rescaffold

Soon, we also plan to introduce automation that will allow your project to receive pull requests automatically, helping keep your solution aligned with standard practices and ecosystem updates.
More details here: #4302

Moreover, we would greatly appreciate your contribution and help. 🚀
Feel free to raise issues and/or contribute pull requests if you see that they fit and identify any opportunities for us to improve our documentation or tooling.