[POC] Defaulting CR values without defaulting webhook [TB: 1d]

[k15r]

Prepare a quick demo for defaulting mechanics without a defaulting webhook:

• defaulting of simple types
• defaulting of complex types
• which version of kubernetes is required
• how to implement in a kubebuilder environment

[friedrichwilken]

We can use kubebuilder default annotation to do this.

type MyCRD struct {
// +kubebuilder:default:={myProperty:{myValue:3}}
Spec MySpec `json:"spec"`
}

type MySpec struct {
// +kubebuilder:default:={myValue:3}
MyProperty `json:"myProperty"`
}

type MyProperty struct {
// +kubebuilder:default:=3
MyValue int `json:"myValue"`
}

If one deploys a CR that misses myValue, myPropery.myValue, spec.myProperty.myValue the CR will be automatically have spec.myProperty.myValue=3. The same will happen if one deletes any of the aforementioned fields.

So, this can be used for simple types as ints, complex objects like spec. This works at least on Kubernetes v1.25 and above, which is the version used by SKR and Gardener. Older versions were not tested.

[friedrichwilken]

As a follow up, let's define the required defaults for the NATS-CR and then let's validate, if this approach is sufficient.