Immutable secrets doc (#19297)

content/en/docs/concepts/configuration/secret.md

@@ -676,6 +676,37 @@ A container using a Secret as a
 Secret updates.
 {{< /note >}}
 
+{{< feature-state for_k8s_version="v1.18" state="alpha" >}}
+
+The Kubernetes alpha feature _Immutable Secrets and ConfigMaps_ provides an option to set
+individual Secrets and ConfigMaps as immutable. For clusters that extensively use Secrets
+(at least tens of thousands of unique Secret to Pod mounts), preventing changes to their
+data has the following advantages:
+
+- protects you from accidental (or unwanted) updates that could cause applications outages
+- improves performance of your cluster by significantly reducing load on kube-apiserver, by
+closing watches for secrets marked as immutable.
+
+To use this feature, enable the `ImmutableEmphemeralVolumes`
+[feature gate](/docs/reference/command-line-tools-reference/feature-gates/) and set
+your Secret or ConfigMap `immutable` field to `true`. For example:
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  ...
+data:
+  ...
+immutable: true
+```
+
+{{< note >}}
+Once a Secret or ConfigMap is marked as immutable, it is _not_ possible to revert this change
+nor to mutate the contents of the `data` field. You can only delete and recreate the Secret.
+Existing Pods maintain a mount point to the deleted Secret - it is recommended to recreate
+these pods.
+{{< /note >}}
+
 ### Using Secrets as environment variables
 
 To use a secret in an {{< glossary_tooltip text="environment variable" term_id="container-env-variables" >}}

content/en/docs/reference/command-line-tools-reference/feature-gates.md

@@ -105,6 +105,7 @@ different Kubernetes components.
 | `HPAScaleToZero` | `false` | Alpha | 1.16 | |
 | `HugePageStorageMediumSize` | `false` | Alpha | 1.18 | |
 | `HyperVContainer` | `false` | Alpha | 1.10 | |
+| `ImmutableEphemeralVolumes` | `false` | Alpha | 1.18 | |
 | `KubeletPodResources` | `false` | Alpha | 1.13 | 1.14 |
 | `KubeletPodResources` | `true` | Beta | 1.15 | |
 | `LegacyNodeRoleBehavior` | `true` | Alpha | 1.16 | |
@@ -403,6 +404,7 @@ Each feature gate is designed for enabling/disabling a specific feature:
 - `HugePageStorageMediumSize`: Enable support for multiple sizes pre-allocated [huge pages](/docs/tasks/manage-hugepages/scheduling-hugepages/).
 - `HyperVContainer`: Enable [Hyper-V isolation](https://docs.microsoft.com/en-us/virtualization/windowscontainers/manage-containers/hyperv-container) for Windows containers.
 - `HPAScaleToZero`: Enables setting `minReplicas` to 0 for `HorizontalPodAutoscaler` resources when using custom or external metrics.
+- `ImmutableEphemeralVolumes`: Allows for marking individual Secrets and ConfigMaps as immutable for better safety and performance.
 - `KubeletConfigFile`: Enable loading kubelet configuration from a file specified using a config file.
   See [setting kubelet parameters via a config file](/docs/tasks/administer-cluster/kubelet-config-file/) for more details.
 - `KubeletPluginsWatcher`: Enable probe-based plugin watcher utility to enable kubelet