Revert "add NamespaceSelector to select namespaces for Initializers"

This reverts commit ee1291784bf373e22cf972d72c3556835fc033c5.

Author: liggitt

api-machinery/dynamic-admission-control-configuration.md

@@ -24,8 +24,6 @@ default admission controls. This document hashes out the implementation details.
 * Do not block the entire cluster if the initializers/webhooks are not ready
   after registration.
 
-* Admin can enforce initializers to specific namespaces.
-
 ## Specification
 
 We assume initializers could be "fail open". We need to update the extensible
@@ -75,13 +73,6 @@ type Initializer struct {
     // if the timeout is reached. The default timeout for each initializer is
     // 5s.
     FailurePolicy *FailurePolicyType `json:"failurePolicy,omitempty"`
-
-    // Selects Namespaces using cluster scoped-labels. This
-    // matches all pods in all namespaces selected by this label selector.
-    // This field follows standard label selector semantics.
-    // If present but empty, this selector selects all namespaces.
-    // +optional
-    NamespaceSelector *metav1.LabelSelector `json:"namespaceSelector,omitempty"`
 }
 
 // Rule is a tuple of APIGroups, APIVersion, and Resources.It is recommended 
@@ -282,23 +273,6 @@ crashes, so it is rare.
 
 See [Considered but REJECTED alternatives](#considered-but-rejected-alternatives) for considered alternatives.
 
-## Enforce initializers to specific namespaces
-
-Current `InitializerConfiguration` is at the cluster level and all of the to-be-created resources (such as rc and deployments) defined in `Rules`
-will be appended with the pending initializers automatically during creation, regardless of the namespace.
-There is no way to only apply the initializers to specific namespaces.
-
-For example, when running a multi-tenant cluster, it'd be quite useful to only apply the rules in just certain namespaces. Sometimes we
-don't want to enforce in "kube-*" related namespaces as well.
-
-With the help of `NamespaceSelector`, we can
-
-* Apply initializer to ALL namespaces (by default);
-* Apply initializer to limited namespaces using label selector;
-
-Since most users won't add extra labels for namespaces explicitly when creating new resources, the selector matching should only be applied to
-`labels.Set(map[string]string{"namespace": namespace})` instead of widely-used `metadata.Labels`.
-
 ## Future work
 
 1. Figuring out a better schema to represent the order among