kubeadm: add KEP for customization with patches

[neolit123]

this KEP comes with a proposal that replaces the existing Alpha feature for patching using Kustomize.
note that the new KEP template format is used.

xref:
kubernetes/kubeadm#2046
#1739

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[neolit123]

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[fabriziopandini]

@neolit123 I'm wondering if/how we can get feedback from users around this change, because this introduce a discuntinuity hard to manage for users and higher level tools relying on this feature

[neolit123]

I'm wondering if/how we can get feedback from users around this change, because this introduce a discuntinuity hard to manage for users and higher level tools relying on this feature

for reference, core k8s removes Alpha features without a deprecation period. in kubeadm we are nicer and usually allow users to transition for at least one release (as in this KEP).

i disagreed that there will be discontinuity given there will be a replacement, all that users need to do is:

• rename the patch files to match the new patch name format.
• pass the new flag.

not editing the patch files and leaving kustomization.yaml is not really breaking.

i do agree however that there will be discontinuity if we remove something and don't provide an alternative or an easy workaround.

[rosti]

Thanks @neolit123 !
I like the idea, but the UX is a bit strange to me in the following aspects:

• It's not a good practice to encode machine readable information in file names. Something better should be employed.
• It's not a good idea to dump a directory full of stuff and expect kubeadm to consume everything inside that meets certain patterns.

I would advise on having an --experimental-patch-strategic, --experimental-patch-json, and --experimental-patch-merge options. Each can take one file and multiple options can be used in the command line. The patches would be applied in the order of appearance.

That said, patches are local node customizations. As such, they are a big candidate for being included in the NodeRegistrationOptions or as separate kinds in the config. Possibly, coming up with a solution for this should be a requirement to get this feature to beta.

[rosti]

Thanks @neolit123 !
/lgtm
/hold
for @fabriziopandini 's review.

[fabriziopandini]

/lgtm
I agree with @rosti that long term we should move patches from flags to config, but considering we are still alpha and that Kustomize is not doing progress in k/k and kubectl, I'm +1 to explore this alternative and gather user feedback on this solution

[fabriziopandini]

/lgtm

[rosti]

I think that we have reached an agreement here. We can always reiterate if needed.

/hold cancel

[rosti]

/lgtm