How Does a Deployment Pointer Satisfy the runtime.Object Interface in Kubernetes?

How does &deployment satisfy the runtime.Object type in Kubernetes code?

In Kubernetes' kubectl/run.go code, the Generate function returns a result list that includes two types: runtime.Object and error. However, the last line of the function is return &deployment, nil, which initially raises the question of how &deployment can satisfy the runtime.Object type.

Runtime Object Interface

runtime.Object is an interface defined in the k8s.io/apimachinery/pkg/runtime package. It represents an abstract object in the Kubernetes API. To implement the interface, a type must have the following methods:

• GetObjectKind() schema.ObjectKind: Returns metadata about the object's type.
• DeepCopyObject() Object: Returns a deep copy of the object.

Type Implementation

In this case, deployment is a local variable of type extensionsv1beta1.Deployment, which is a Kubernetes API type. extensionsv1beta1.Deployment extends metav1.TypeMeta, which provides the GetObjectKind() method. Additionally, extensionsv1beta1.Deployment also implements the DeepCopyObject() method directly.

Pointer Satisfies Interface

The & operator in Go creates a pointer to the variable. Pointers have a distinct type from their base type. However, in this case:

• deployment has a method set that includes both GetObjectKind() and DeepCopyObject() with pointer receivers.
• ObjectMeta embeds within Deployment, which means a pointer to Deployment also inherits the GetObjectKind() method from ObjectMeta.

According to the Go specification, an interface type can store a value of any type with a method set that is a superset of the interface. In this case, the method set of *extensionsv1beta1.Deployment is a superset of the method set of runtime.Object. Therefore, &deployment can be assigned to a variable of type runtime.Object, satisfying the return type of the Generate function.

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