[Question] Status of $patch (Strategic Merge Patch) support for Custom Resources

[yanniszark]

Question

Do CustomResources support Strategic Merge Patch in kustomize? Specifically for $patch: delete.
Can I rely on this feature? It seems to work on 4.0.4 but not on 3.2.0. I couldn't find an issue / pr solving it (except #2056 which doesn't include a closing PR), so I'm not sure if this was solved by accident or is a supported feature.

Scenario

Given the kustomization:

kustomization.yaml

resources:
- example-cr.yaml

patches:
- patch: |-
    $patch: delete
    apiVersion: "*"
    kind: "*"
    metadata:
      name: "*"
  target:
    kind: ExampleCR

example-cr.yaml

apiVersion: example/v1
kind: ExampleCR
metadata:
  name: example
spec:
  example: example

Outcomes

Version 3.2.0 outputs:

$patch: delete
apiVersion: example/v1
kind: ExampleCR
metadata:
  name: example
spec:
  example: example

Version 4.0.4 outputs nothing (as expected).

[yanniszark]

cc'ing @monopole because of comments in #2056

[Shell32-Natsu]

I am not sure about the custom resources. I am afraid is not a good idea to rely on this behavior.

[yanniszark]

@Shell32-Natsu @monopole shouldn't kustomize support it though, as part of supporting SMP?
It seems the current versions already work. How about establishing the intention to support this feature, by including a unit test?
This is an immensely useful feature that is otherwise missing from kustomize.

[Shell32-Natsu]

Not familiar with this, but IMO a correct strategic merge patch with directive needs schema for the CRD. kustomize hasn't fully supported it yet.

[KnVerey]

I agree with @Shell32-Natsu that the schema is required to correctly implement SMPs in general. I'm guessing that $delete started working in v4 because of the switch to kyaml merge libraries under the hood. There is work in progress to allow users to provide OpenAPI schemas for their resources and get full, correct SMP support. Please see the KEP here: https://github.com/kubernetes/enhancements/tree/master/keps/sig-cli/2206-openapi-features-in-kustomize

[natasha41575]

Closing this issue because using custom OpenAPI schemas is now implemented , see this example here. (More documentation is coming.) SMPs for CRDs will be supported as long as users are able to provide their own schema files.

[yhrn]

@natasha41575 even though Kustomize now supports OpenAPI schemas I'm not sure that fully solves the CRD problem. From what I can see CustomResourceDefinition does not support x-kubernetes-patch-merge-key or x-kubernetes-patch-strategy. I got it to work by using kustomize openapi fetch the get the schemas from the cluster and then patching the schema to add the aforementioned properties but that is not a scalable solution for us.

[KnVerey]

Re: support for those fields in CRD, please follow kubernetes/kubernetes#82942.