([#123001](https://github.com/kubernetes/kubernetes/pull/123001), [@tkashem](https://github.com/tkashem))
- Introduced a feature gate mechanism to `client-go`. Depending on the actual implementation, users can control features via environmental variables or command line options. ([#122555](https://github.com/kubernetes/kubernetes/pull/122555), [@p0lyn0mial](https://github.com/p0lyn0mial))

- Introduces new alpha functionality to the reflector, allowing user to enable API streaming.
  
  To activate this feature, users can set the `ENABLE_CLIENT_GO_WATCH_LIST_ALPHA` environmental variable.
  It is important to note that the server must support streaming for this feature to function properly.
  If streaming is not supported by the server, the reflector will revert to the previous method

	// will be used as the certificates for workloads.
	// The default value is "" and when caName="", the CA will be configured by other
	// mechanisms (e.g., environmental variable CA_PROVIDER).
	CaName string `protobuf:"bytes,65,opt,name=caName,proto3" json:"caName,omitempty"`
	// TODO: remove this?
	// No longer used.

- Fixed a regression in default configurations, which enabled `PodDisruptionConditions`
  by default, that prevented the control plane's pod garbage collector from deleting
  pods that contained duplicated field keys (environmental variables with repeated keys or
  container ports). ([#121103](https://github.com/kubernetes/kubernetes/pull/121103), [@mimowo](https://github.com/mimowo))

}

// EnvVar represents an environment variable present in a Container.
message EnvVar {
  // Name of the environment variable. Must be a C_IDENTIFIER.
  optional string name = 1;

  // Variable references $(VAR_NAME) are expanded
  // using the previously defined environment variables in the container and
  // any service environment variables. If a variable cannot be resolved,

}

var map_EnvVar = map[string]string{
	"":          "EnvVar represents an environment variable present in a Container.",
	"name":      "Name of the environment variable. Must be a C_IDENTIFIER.",

	// +optional
	MountPropagation *MountPropagationMode
	// Expanded path within the volume from which the container's volume should be mounted.
	// Behaves similarly to SubPath but environment variable references $(VAR_NAME) are expanded using the container's environment.
	// Defaults to "" (volume's root).
	// SubPathExpr and SubPath are mutually exclusive.
	// +optional
	SubPathExpr string
}

a consequence, all volume mounts in containers are now `rslave` on Linux by default. To make this default work in all Linux environments the entire mount tree should be marked as shareable, e.g. via `mount --make-rshared /`. All Linux distributions that use systemd already have the root directory mounted as rshared and hence they need not do anything. In Linux environments without systemd we recommend running `mount --make-rshared /` during boot before docker is started, ([@jsafrane](https://github.com/jsafrane))...

}

// EnvVar represents an environment variable present in a Container.
message EnvVar {
  // Name of the environment variable. Must be a C_IDENTIFIER.
  optional string name = 1;

  // Variable references $(VAR_NAME) are expanded
  // using the previously defined environment variables in the container and
  // any service environment variables. If a variable cannot be resolved,

### ConfigMap
* Volumes and environment variables populated from ConfigMap and Secret objects can now tolerate the named source object or specific keys being missing, by adding `optional: true` to the volume or environment variable source specifications. ([#39981](https://github.com/kubernetes/kubernetes/pull/39981), [@fraenkel](https://github.com/fraenkel))