Writing a custom resource definition in Kubernetes:

Purpose: Applications hosted on Kubernetes want to introduce their own Kubernetes object so that it can be used like any other and leverage supported features such as usage from command-line, Kubernetes API and secured with role-based access security. It is also stored in the etcd. This article explains the steps to create a custom resource in Kubernetes using the operators written for it.

A stock custom resource definition is a yaml configuration file that has the attributes

Kind: “CustomResourceDefinition”

And specification with group, version and scope fields where group defines the api collection that relates the objects, the version as usually “v1alpha1” or one of the supported strings, and scope as whether the object is available within a namespace or cluster wide. The object is also given names to be called in singular, plural and by type. The metadata of the object is usually constructed from its plural name and group. The definition then includes the properties specific to the resource.


The programmatic way of doing this allows us to do a little bit more than the declarations. It lets us introuduce dynamic behavior. We do this by calling the methods from the client-go package for create, update and delete of the resource to use with Kubernetes API server. These methods are referred to as the ‘clientset’ because they use the corresponding methods from the Kubernetes apiextensions library. We connect to the API server from within the cluster using InClusterConfig method from the package. So far all of these methods are just calls with appropriate parameter and checks of return values.

The code-generator package allows us to create the package:
# vendor/k8s.io/code-generator/generate-groups.sh
Usage: generate-groups.sh <generators> <output-package> <apis-package> <groups-versions> ...

  <generators>        the generators comma separated to run (deepcopy,defaulter,client,lister,informer) or "all".
  <output-package>    the output package name (e.g. github.com/example/project/pkg/generated).
  <apis-package>      the external types dir (e.g. github.com/example/api or github.com/example/project/pkg/apis).
  <groups-versions>   the groups and their versions in the format "groupA:v1,v2 groupB:v1 groupC:v2", relative
                      to <api-package>.
  ...                 arbitrary flags passed to all generator binaries.


Examples:
  generate-groups.sh all             github.com/example/project/pkg/client github.com/example/project/pkg/apis "foo:v1 bar:v1alpha1,v1beta1"
  generate-groups.sh deepcopy,client github.com/example/project/pkg/client github.com/example/project/pkg/apis "foo:v1 bar:v1alpha1,v1beta1"

For example,
vendor/k8s.io/code-generator/generate-groups.sh deepcopy,client path/to/project//pkg/apis project:v1alpha1

The most interesting part of these object creation is the specification of other objects as properties and the chaining of their ‘ownerReference’ which allows us to introduce hierarchy, composition, and scoped actions.