Document restarts (#478)

* Document service restarts

* Fix xref and some cleanup

* Update modules/concepts/pages/operations/cluster_operations.adoc

Co-authored-by: Natalie <nat@nullable.se>

* Update modules/concepts/pages/operations/cluster_operations.adoc

Co-authored-by: Natalie <nat@nullable.se>

* Remove code quotes.

* Rename stacklet to myairflow

* Typo

* Example with HDFS

* Update modules/concepts/pages/operations/cluster_operations.adoc

Co-authored-by: Sebastian Bernauer <sebastian.bernauer@stackable.de>

* Update modules/concepts/pages/operations/cluster_operations.adoc

Co-authored-by: Sebastian Bernauer <sebastian.bernauer@stackable.de>

* Update modules/concepts/pages/operations/cluster_operations.adoc

Co-authored-by: Sebastian Bernauer <sebastian.bernauer@stackable.de>

---------

Co-authored-by: Natalie <nat@nullable.se>
Co-authored-by: Sebastian Bernauer <sebastian.bernauer@stackable.de>

Author: 3 people

modules/concepts/pages/operations/cluster_operations.adoc

@@ -6,6 +6,8 @@ Stackable operators offer different cluster operations to control the reconcilia
 * `reconciliationPaused` - Stop the operator from reconciling the cluster spec. The status will still be updated.
 * `stopped` - Stop all running pods but keep updating all deployed resources like `ConfigMaps`, `Services` and the cluster status.
 
+If not specified, `clusterOperation.reconciliationPaused` and `clusterOperation.stopped` default to `false`.
+
 == Example
 
 [source,yaml]
@@ -15,8 +17,54 @@ include::example$cluster-operations.yaml[]
 <1> The `clusterOperation.reconciliationPaused` flag set to `true` stops the operator from reconciling any changes to the cluster spec. The cluster status is still updated.
 <2> The `clusterOperation.stopped` flag set to `true` stops all pods in the cluster. This is done by setting all deployed `StatefulSet` replicas to 0.
 
-== Notes
-
-If not specified, `clusterOperation.reconciliationPaused` and `clusterOperation.stopped` default to `false`.
 
 IMPORTANT: When setting `clusterOperation.reconciliationPaused` and `clusterOperation.stopped` to true in the same step, `clusterOperation.reconciliationPaused` will take precedence. This means the cluster will stop reconciling immediately and the `stopped` field is ignored. To avoid this, the cluster should first be stopped and then paused.
+
+== Service Restarts
+
+=== Manual Restarts
+
+Sometimes it is necessary to restart services deployed in Kubernetes. A service restart should induce as little disruption as possible, ideally none.
+
+Most operators create StatefulSet objects for the products they manage and Kubernetes offers a rollout mechanism to restart them. You can use `kubectl rollout restart statefulset` to restart a StatefulSet previously created by an operator.
+
+To illustrate how to use the command line to restart one or more Pods, we will assume you used the Stackable HDFS Operator to deploy an HDFS stacklet called `dumbo`.
+
+This stacklet will consist, among other things, of three StatefulSets created for each HDFS role: `namenode`, `datanode` and `journalnode`. Let's list them:
+
+[source,shell]
+----
+❯ kubectl get statefulset -l app.kubernetes.io/instance=dumbo
+NAME                        READY   AGE
+dumbo-datanode-default      2/2     4m41s
+dumbo-journalnode-default   1/1     4m41s
+dumbo-namenode-default      2/2     4m41s
+----
+
+To restart the HDFS DataNode Pods, run:
+
+[source,shell]
+----
+❯ kubectl rollout restart statefulset dumbo-datanode-default 
+statefulset.apps/dumbo-datanode-default restarted
+----
+
+Sometimes you want to restart all Pods of a stacklet and not just individual roles. This can be achieved in a similar manner by using labels instead of StatefulSet names. Continuing with the example above, to restart all HDFS Pods you would have to run:
+
+[source,shell]
+----
+❯ kubectl rollout restart statefulset --selector app.kubernetes.io/instance=dumbo
+----
+
+To wait for all Pods to be running again:
+
+[source,shell]
+----
+❯ kubectl rollout status statefulset --selector app.kubernetes.io/instance=dumbo
+----
+
+Here we used the label `app.kubernetes.io/instance=dumbo` to select all Pods that belong to a specific HDFS stacklet. This label is created by the operator and `dumbo` is the name of the HDFS stacklet as specified in the custom resource. You can add more labels to make finer grained restarts.
+
+== Automatic Restarts
+
+The Commons Operator of the Stackable Platform may restart Pods automatically, for purposes such as ensuring that TLS certificates are up-to-date. For details, see the xref:commons-operator:index.adoc[Commons Operator documentation].