This post from Gianluca Arbezzano contains both theory and code with a complete working application to understand how to build your own shared informer to extend Kubernetes beyond applying YAML via kubectl.

Kubernetes increases in popularity every day but I don’t think we use all its power just applying YAML via kubectl. Kubernetes is a framework and as every framework, it exposes powerful interfaces and API usable to extend its capability with our needs. Shared Informers are what I see as the easy way to enjoy k8s as an extendible tool to programmatically build and ship containers.