Structured-merge-diff support

[erictune]

I want KubeBuilder to support these comment tags:

// +listMapKey=XXX
// +listType=atomic
// +listType=set
// +listType=map

These are supported by https://github.com/kubernetes/kube-openapi (see here ) and I believe they are supported by Server-side apply for CRDs in 1.16.

KubeBuilder should be able to emit these properties when generating CRDs.

Related issues:

• Documentation request for these tags: Within Server-side apply, document map and list merging behaviors. kubernetes/website#16725
• Issue about checking what happens if a CRD is updated to add these tags after the fact: Document how to use list-type and list-keys in a CRD, including in existing clusters. structured-merge-diff#115

Requires 1.16

/kind feature

[erictune]

Note that uses who are currently referring to code which uses these tags already, such as in pod templates within custom controllers, will see a change to their generated CRD when they upgrade their KubeBuilder version.

Need to determine how to explain this to users.

To do that, need to understand what will happen.
kubernetes-sigs/structured-merge-diff#115 is about that.

[DirectXMan12]

Does the actual desierialization of CRD validation schemas support it yet? IIRC, that was the blocker.

[DirectXMan12]

otherwise, yeah, we should open an issue in controller-tools for this

[apelisse]

I think it's been done in 1.16. Let me double-check that.

[apelisse]

kubernetes/kubernetes#77354

[apelisse]

Specifically: https://github.com/kubernetes/kubernetes/pull/77354/files#diff-ca188b89d2fc7104865fe95be2114e81

[apelisse]

cc @jennybuckley

[jennybuckley]

Yes. This is done in 1.16 so this is no longer blocked

[DirectXMan12]

cool, we should cross-file on CT then

[droot]

I am wondering how we can do a better job of educating the users about using these markers when they use a list in their API definition. One idea that comes to mind is some sort of linter command in controller-tools that give tips to users when we encounter a field of list-type that doesn't have these markers specified.

[apelisse]

(btw, I'm working on the implementation right now)
/assign

Yes, I'm trying to look at the documentation holistically right now, there are lots of gaps in that area. Making it obvious to CRD authors that this is a thing would help by calling out this specifically in CRD docs (I'm also working on this).

A fairly obvious way to make a linter is to find specific patterns (@pwittrock already used that principle) like search for lists that have a field name, etc. You could also search for map[string]substructure which is another common pattern. I'm clearly not opposed to the idea.

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[apelisse]

This was absolutely fixed by kubernetes-sigs/controller-tools#347