Groups and Versions and Kinds, oh my!

Actually, before we get started with our API, we should talk terminology a bit.

When we talk about APIs in Kubernetes, we often use 4 terms: groups, versions, kinds, and resources.

Groups and Versions

An API Group in Kubernetes is simply a collection of related functionality. Each group has one or more versions, which, as the name suggests, allow us to change how an API works over time.

Kinds and Resources

Each API group-version contains one or more API types, which we call Kinds. While a Kind may change forms between versions, each form must be able to store all the data of the other forms, somehow (we can store the data in fields, or in annotations). This means that using an older API version won’t cause newer data to be lost or corrupted. See the Kubernetes API guidelines for more information.

You’ll also hear mention of resources on occasion. A resource is simply a use of a Kind in the API. Often, there’s a one-to-one mapping between Kinds and resources. For instance, the pods resource corresponds to the Pod Kind. However, sometimes, the same Kind may be returned by multiple resources. For instance, the Scale Kind is returned by all scale subresources, like deployments/scale or replicasets/scale. This is what allows the Kubernetes HorizontalPodAutoscaler to interact with different resources. With CRDs, however, each Kind will correspond to a single resource.

Notice that resources are always lowercase, and by convention are the lowercase form of the Kind.

So, how does that correspond to Go?

When we refer to a kind in a particular group-version, we’ll call it a GroupVersionKind, or GVK for short. Same with resources and GVR. As we’ll see shortly, each GVK corresponds to a given root Go type in a package.

Now that we have our terminology straight, we can actually create our API!

Err, but what’s that Scheme thing?

The Scheme we saw before is simply a way to keep track of what Go type corresponds to a given GVK (don’t be overwhelmed by its godocs).

For instance, suppose we mark that the "tutorial.kubebuilder.io/api/v1".CronJob{} type as being in the batch.tutorial.kubebuilder.io/v1 API group (implicitly saying it has the Kind CronJob).

Then, we can later construct a new &CronJob{} given some JSON from the API server that says

{
    "kind": "CronJob",
    "apiVersion": "batch.tutorial.kubebuilder.io/v1",
    ...
}

or properly look up the group-version when we go to submit a &CronJob{} in an update.