1. Define Go types for CRD

Create a directory.
```
mkdir -p pkg/apis/example.com/v1alpha1
```
Create pkg/apis/example.com/v1alpha1/doc.go.
```
package v1alpha1
```

Create pkg/apis/example.com/v1alpha1/types.go.

package v1alpha1

import metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"

// Foo is a specification for a Foo resource
type Foo struct {
    metav1.TypeMeta   `json:",inline"`
    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec   FooSpec   `json:"spec"`
    Status FooStatus `json:"status"`
}

// FooSpec is the spec for a Foo resource
type FooSpec struct {
    DeploymentName string `json:"deploymentName"`
    Replicas       *int32 `json:"replicas"`
}

// FooStatus is the status for a Foo resource
type FooStatus struct {
    AvailableReplicas int32 `json:"availableReplicas"`
}

// FooList is a list of Foo resources
type FooList struct {
    metav1.TypeMeta `json:",inline"`
    metav1.ListMeta `json:"metadata"`

    Items []Foo `json:"items"`
}

Create pkg/apis/example.com/v1alpha1/register.go.

package v1alpha1

import (
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/apimachinery/pkg/runtime"
    "k8s.io/apimachinery/pkg/runtime/schema"
)

var (
    // SchemeBuilder initializes a scheme builder
    SchemeBuilder = runtime.NewSchemeBuilder(addKnownTypes)
    // AddToScheme is a global function that registers this API group & version to a scheme
    AddToScheme = SchemeBuilder.AddToScheme
)

// SchemeGroupVersion is group version used to register these objects.
var SchemeGroupVersion = schema.GroupVersion{
    Group:   "example.com",
    Version: "v1alpha1",
}

func Resource(resource string) schema.GroupResource {
    return SchemeGroupVersion.WithResource(resource).GroupResource()
}

func addKnownTypes(scheme *runtime.Scheme) error {
    scheme.AddKnownTypes(SchemeGroupVersion,
        &Foo{},
        &FooList{},
    )
    metav1.AddToGroupVersion(scheme, SchemeGroupVersion)
    return nil
}

clientset, which we’ll generate in the next section, will uses AddToScheme to register our custom resource Foo.
Scheme defines methods for serializing and deserializing API objects, a type registry for converting group, version, and kind information to and from Go schemas, and mappings between Go schemas of different versions.

Run go mod tidy.
go mod tidy ensures that the go.mod file matches the source code in the module. It adds any missing module requirements necessary to build the current module’s packages and dependencies, and it removes requirements on modules that don’t provide any relevant packages. It also adds any missing entries to go.sum and removes unnecessary entries.
go mod tidy

※ At this point, pkg/apis/example.com/v1alpha1/register.go would have an error cannot use &(FooList literal) (value of type *FooList) as "k8s.io/apimachinery/pkg/runtime".Object value in argument to scheme.AddKnownTypes: missing method DeepCopyObject as DeepCopyObject will be generated in the next step.

Docs

1. Define Go types for CRD

1. Define Go types for CRD