featuregates

cases in the field

k8s.io/apiserver
  rapid-reset, defaulted to off.  In repos vendoring, I needed to enable/disable

k8s.io/client-go
  aggregated-discovery - had a bug and needed  way for every vendoring binary we cannot recompile to disable. We added env vars.
  streaming-list - desire to have different default in kube-controller-manager versus kubelet

kube-controller-manager 
  client-go/streaming-list - disabled by default in library, kcm wants it enabled by default and controlled via flag.
  
other controllers and operators
  client-go/streaming-list - disabled by default in library,operator wants it enabled, leave as env var

kubernetes-sigs/structured-merge-diff
  how would we like to put featuregates in it?  registered where and how? perhaps select a different cadence?

intersection with the API compatibility version
  other projects (controllers and apiservers) vendor k8s.io/apiserver and k8s.io/cient-go.
  other projects (especially apiservers) define their own featuregates
  other projects specify their own versions
  the featuregates in k8s.io/apiserver and k8s.io/client-go need to keep their gates separate to understand their compatibility version separately.

openshift binaries have their own featuregates and vendor kube

comparison to flags

registering into a global variable produces problems: glog's usage golang global flags
global flags age less well than cobra flags
as libraries compose with other libraries, referring to local instances allows opt-in versus try to find a way to opt-out

possible different register.go

func init() {
  gates := featuregate.New()
   
  runtime.Must(gates.Add(utilfeature.All))
  runtime.Must(gates.Add(clientfeatures.All))
	
  runtime.Must(utilfeature.SetDefault(gates))
	runtime.Must(clientfeatures.SetDefault(gates))
}