Update ServiceAccount tasks in light of TokenRequest

Now that TokenRequest is the default way to get a service account token
for a Pod, update the task pages that relate to this.

content/en/docs/reference/access-authn-authz/service-accounts-admin.md

@@ -40,21 +40,129 @@ kubectl create namespace examplens
 Kubernetes distinguishes between the concept of a user account and a service account
 for a number of reasons:
 
-- User accounts are for humans. Service accounts are for processes, which run
-  in pods.
-- User accounts are intended to be global. Names must be unique across all
-  namespaces of a cluster. In Kubernetes, service accounts are namespaced.
-- Typically, a cluster's user accounts might be synced from a corporate
+- User accounts are for humans. Service accounts are for application processes,
+  which (for Kubernetes) run in containers that are part of pods.
+- User accounts are intended to be global: names must be unique across all
+  namespaces of a cluster. No matter what namespace you look at, a particular
+  username that represents a user represents the same user.
+  In Kubernetes, service accounts are namespaced: two different namespaces can
+  contain ServiceAccounts that have identical names.
+- Typically, a cluster's user accounts might be synchronised from a corporate
   database, where new user account creation requires special privileges and is
-  tied to complex business processes. Service account creation is intended to be
-  more lightweight, allowing cluster users to create service accounts for
-  specific tasks by following the principle of least privilege.
-- Auditing considerations for humans and service accounts may differ.
-- A config bundle for a complex system may include definition of various service
+  tied to complex business processes. By contrast, service account creation is
+  intended to be more lightweight, allowing cluster users to create service accounts
+  for specific tasks on demand. Separating ServiceAccount creation from the steps to
+  onboard human users makes it easier for workloads to following the principle of
+  least privilege.
+- Auditing considerations for humans and service accounts may differ; the separation
+  makes that easier to achieve.
+- A configuration bundle for a complex system may include definition of various service
   accounts for components of that system. Because service accounts can be created
-  without many constraints and have namespaced names, such config is portable.
+  without many constraints and have namespaced names, such configuration is
+  usually portable.
 
-## ServiceAccount admission controller
+## Bound service account token volume mechanism {#bound-service-account-token-volume}
+
+{{< feature-state for_k8s_version="v1.22" state="stable" >}}
+
+By default, the Kubernetes control plane (specifically, the
+[ServiceAccount admission controller](#service-account-admission-controller))
+adds a [projected volume](/docs/concepts/storage/projected-volumes/) to Pods,
+and this volume includes a token for Kubernetes API access.
+
+Here's an example of how that looks for a launched Pod:
+
+```yaml
+...
+  - name: kube-api-access-<random-suffix>
+    projected:
+      defaultMode: 420 # decimal equivalent of octal 0644
+      sources:
+        - serviceAccountToken:
+            expirationSeconds: 7200
+            audience: api
+            path: token
+        - configMap:
+            items:
+              - key: ca.crt
+                path: ca.crt
+            name: kube-root-ca.crt
+        - downwardAPI:
+            items:
+              - fieldRef:
+                  apiVersion: v1
+                  fieldPath: metadata.namespace
+                path: namespace
+```
+
+That manifest snippet defines a projected volume that consists of three sources. In this case,
+each source also represents a single path within that volume. The three sources are:
+
+1. A `serviceAccountToken` source, that contains a token that the kubelet acquires from kube-apiserver
+   The kubelet fetches time-bound tokens using the TokenRequest API. A token served for a TokenRequest expires
+   either when the pod is deleted or after a defined lifespan (by default, that is 1 hour).
+   The token is bound to the specific Pod and has the kube-apiserver as its audience.
+   This mechanism superseded an earlier mechanism that added a volume based on a Secret,
+   where the Secret represented the ServiceAccount for the Pod, but did not expire.
+1. A `configMap` source. The ConfigMap contains a bundle of certificate authority data. Pods can use these
+   certificates to make sure that they are connecting to your cluster's kube-apiserver (and not to middlebox
+   or an accidentally misconfigured peer).
+1. A `downwardAPI` source that looks up the name of thhe namespace containing the Pod, and makes
+   that name information available to application code running inside the Pod.
+
+Any container within the Pod that mounts this particular volume can access the above information.
+
+{{< note >}}
+There is no specific mechanism to invalidate a token issued via TokenRequest. If you no longer
+trust a bound service account token for a Pod, you can delete that Pod. Deleting a Pod expires
+its bound service account tokens.
+{{< /note >}}
+
+## Manual Secret management for ServiceAccounts
+
+Versions of Kubernetes before v1.22 automatically created credentials for accessing
+the Kubernetes API. This older mechanism was based on creating token Secrets that
+could then be mounted into running Pods.
+
+In more recent versions, including Kubernetes v{{< skew currentVersion >}}, API credentials
+are [obtained directly](#bound-service-account-token-volume) using the
+[TokenRequest](/docs/reference/kubernetes-api/authentication-resources/token-request-v1/) API,
+and are mounted into Pods using a projected volume.
+The tokens obtained using this method have bounded lifetimes, and are automatically
+invalidated when the Pod they are mounted into is deleted.
+
+You can still [manually create](/docs/tasks/configure-pod-container/configure-service-account/#manually-create-an-api-token-for-a-serviceaccount) a Secret to hold a service account token; for example, if you need a token that never expires.
+
+Once you manually create a Secret and link it to a ServiceAccount, the Kubernetes control plane automatically populates the token into that Secret.
+
+{{< note >}}
+Although the manual mechanism for creating a long-lived ServiceAccount token exists,
+using [TokenRequest](/docs/reference/kubernetes-api/authentication-resources/token-request-v1/)
+to obtain short-lived API access tokens is recommended instead.
+{{< /note >}}
+
+## Control plane details
+
+### Token controller
+
+The service account token controller runs as part of `kube-controller-manager`.
+This controller acts asynchronously. It:
+
+- watches for ServiceAccount deletion and deletes all corresponding ServiceAccount
+  token Secrets.
+- watches for ServiceAccount token Secret addition, and ensures the referenced
+  ServiceAccount exists, and adds a token to the Secret if needed.
+- watches for Secret deletion and removes a reference from the corresponding
+  ServiceAccount if needed.
+
+You must pass a service account private key file to the token controller in
+the `kube-controller-manager` using the `--service-account-private-key-file`
+flag. The private key is used to sign generated service account tokens.
+Similarly, you must pass the corresponding public key to the `kube-apiserver`
+using the `--service-account-key-file` flag. The public key will be used to
+verify the tokens during authentication.
+
+### ServiceAccount admission controller
 
 The modification of pods is implemented via a plugin
 called an [Admission Controller](/docs/reference/access-authn-authz/admission-controllers/).
@@ -78,10 +186,14 @@ it does the following when a Pod is created or modified:
 1. If the spec of the incoming Pod does already contain any `imagePullSecrets`, then the
    admission controller adds `imagePullSecrets`, copying them from the `ServiceAccount`.
 
-### Bound service account token volume mechanism {#bound-service-account-token-volume}
+### TokenRequest API
 
 {{< feature-state for_k8s_version="v1.22" state="stable" >}}
 
+You use the [TokenRequest](/docs/reference/kubernetes-api/authentication-resources/token-request-v1/)
+subresource of a ServiceAccount to obtain a time-bound token for that ServiceAccount. Usually you don't
+need to call this API: the kubelet sets this up for you using a _projected volume_.
+
 The Kubernetes control plane (specifically, the ServiceAccount admission controller)
 adds a projected volume to Pods, and the kubelet ensures that this volume contains a token
 that lets containers authenticate as the right ServiceAccount.
@@ -131,11 +243,10 @@ See more details about [projected volumes](/docs/tasks/configure-pod-container/c
 
 ## Create additional API tokens {#create-token}
 
-The control plane ensures that a Secret with an API token exists for each
-ServiceAccount. To create additional API tokens for a ServiceAccount, create a
+To create the first, or subsequent, long lived API token for a ServiceAccount, create a
 Secret of type `kubernetes.io/service-account-token` with an annotation
-referencing the ServiceAccount, and the control plane will update that Secret with a
-generated token.
+referencing the ServiceAccount. The control plane then generates a long-lived token and
+updates that Secret with that token data.
 
 Here is a sample manifest for such a Secret:
 
@@ -268,3 +379,7 @@ flag. The private key is used to sign generated service account tokens.
 Similarly, you must pass the corresponding public key to the `kube-apiserver`
 using the `--service-account-key-file` flag. The public key will be used to
 verify the tokens during authentication.
+
+## {{% heading "whatsnext" %}}
+
+- Read more details about [projected volumes](/docs/concepts/storage/projected-volumes/).

content/en/docs/tasks/configure-pod-container/configure-service-account.md

@@ -164,8 +164,38 @@ kubectl delete serviceaccount/build-robot
 
 ## Manually create an API token for a ServiceAccount
 
+Suppose you have an existing service account named "build-robot" as mentioned earlier.
+
+You can get a time-limited API token for that ServiceAccount using `kubectl`:
+
+```shell
+kubectl create token admin-user
+```
+
+The output from that command is a token that you can use to authenticate as that
+ServiceAccount. You can request a specific token duration using the `--duration`
+command line argument to `kubectl create token` (the actual duration of the issued
+token might be shorter, or could even be longer).
+
+{{< note >}}
+Versions of Kubernetes before v1.22 automatically created credentials for accessing
+the Kubernetes API. This older mechanism was based on creating token Secrets that
+could then be mounted into running Pods.
+In more recent versions, including Kubernetes v{{< skew currentVersion >}}, API credentials
+are obtained directly by using the [TokenRequest](/docs/reference/kubernetes-api/authentication-resources/token-request-v1/) API,
+and are mounted into Pods using a [projected volume](/docs/reference/access-authn-authz/service-accounts-admin/#bound-service-account-token-volume).
+The tokens obtained using this method have bounded lifetimes, and are automatically
+invalidated when the Pod they are mounted into is deleted.
+
+You can still manually create a service account token Secret; for example, if you need a token that never expires.
+However, using the [TokenRequest](/docs/reference/kubernetes-api/authentication-resources/token-request-v1/)
+subresource to obtain a token to access the API is recommended instead.
+{{< /note >}}
+
+### Manually create a long-lived API token for a ServiceAccount
+
 If you want to obtain an API token for a ServiceAccount, you create a new Secret
-with a special annotation, `kubernetes.io/service-account.name`
+with a special annotation, `kubernetes.io/service-account.name`.
 
 ```shell
 kubectl apply -f - <<EOF
@@ -213,9 +243,12 @@ token:          ...
 ```
 
 {{< note >}}
-By design, `kubectl describe` hides the value of the token.
+By design, `kubectl describe` hides the confidential data in a Secret.
 {{< /note >}}
 
+When you delete a ServiceAccount that has an associated Secret, the Kubernetes
+control plane automatically cleans up the long-lived token from that Secret.
+
 ## Add ImagePullSecrets to a service account
 
 First, [create an imagePullSecret](/docs/concepts/containers/images/#specifying-imagepullsecrets-on-a-pod).