diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 09608a20b..9511f6235 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -13,6 +13,7 @@
     - [Building your own imge](custom-mysqld.md)
     - [Customize system container](customize-system-container.md)
     - [Change volumeClaimTemplates](change-pvc-template.md)
+    - [Rollout strategy](rolling-update-strategy.md)
 - [Known issues](known_issues.md)
 
 # References
diff --git a/docs/metrics.md b/docs/metrics.md
index fa56c3896..8da864de1 100644
--- a/docs/metrics.md
+++ b/docs/metrics.md
@@ -26,6 +26,9 @@ All these metrics are prefixed with `moco_cluster_` and have `name` and `namespa
 | `failover_total`                    | The number of times MOCO changed the failed primary instance           | Counter   |
 | `replicas`                          | The number of mysqld instances in the cluster                          | Gauge     |
 | `ready_replicas`                    | The number of ready mysqld Pods in the cluster                         | Gauge     |
+| `current_replicas`                  | The number of current replicas                                         | Gauge     |
+| `updated_replicas`                  | The number of updated replicas                                         | Gauge     |
+| `last_partition_updated`            | The timestamp of the last successful partition update                  | Gauge     |
 | `clustering_stopped`                | 1 if the cluster is clustering stopped, 0 otherwise                    | Gauge     |
 | `reconciliation_stopped`            | 1 if the cluster is reconciliation stopped, 0 otherwise                | Gauge     |
 | `errant_replicas`                   | The number of mysqld instances that have [errant transactions][errant] | Gauge     |
diff --git a/docs/rolling-update-strategy.md b/docs/rolling-update-strategy.md
new file mode 100644
index 000000000..c18890014
--- /dev/null
+++ b/docs/rolling-update-strategy.md
@@ -0,0 +1,86 @@
+# RollingUpdate strategy
+
+MOCO manages MySQLCluster pods using StatefulSets.
+
+```text
+MySQLCluster/test
+└─StatefulSet/moco-test
+  ├─ControllerRevision/moco-test-554c56f456
+  ├─ControllerRevision/moco-test-5794c57c7c
+  ├─Pod/moco-test-0
+  ├─Pod/moco-test-1
+  └─Pod/moco-test-2
+```
+
+By default, StatefulSet's standard rolling update does not consider whether MySQLCluster is Healthy during pod updates.
+This can sometimes cause problems, as a rolling update may proceed even if MySQLCluster becomes UnHealthy during the process.
+
+To address this issue, MOCO controls StatefulSet partitions to perform rolling updates. This behavior is enabled by default.
+
+## Partitions
+
+By setting a number in `.spec.updateStrategy.rollingUpdate.partition` of a StatefulSet, you can divide the rolling update into partitions.
+When a partition is specified, pods with a pod number equal to or greater than the partition value are updated.
+Pods with a pod number smaller than the partition value are not updated, and even if those pods are deleted, they will be recreated with the previous version.
+
+## Architecture
+
+### When Creating a StatefulSet
+
+When creating a StatefulSet, MOCO injects a partition into the StatefulSet using a MutatingAdmissionWebhook.
+The partition to be set is the same as the number of replicas of the StatefulSet.
+
+```yaml
+replicas: 3
+...
+updateStrategy:
+  type: RollingUpdate
+  rollingUpdate:
+    partition: 3
+...
+```
+
+### Updating Partitions
+
+MOCO monitors the rollout status of the StatefulSet and the status of MySQLCluster.
+If the update of pods based on the current partition value is completed successfully and the containers are Running, and the status of MySQLCluster is Healthy, MOCO decrements the partition of the StatefulSet by 1.
+This operation is repeated until the partition value reaches 0.
+
+### When Updating a StatefulSet
+
+When a StatefulSet is updated, MOCO determines the contents of the StatefulSet update and controls partitions using AdmissionWebhook.
+
+1. If the StatefulSet update is only the partition number
+    * The MutatingAdmissionWebhook does nothing.
+2. If fields other than the partition of the StatefulSet are updated
+    * The MutatingAdmissionWebhook updates the partition of the StatefulSet to the same value as the replica using MutatingAdmissionWebhook.
+
+### Forcefully Rolling Out
+
+By setting the annotation `moco.cybozu.com/force-rolling-update` to `true`, you can update the StatefulSet without partition control.
+
+```yaml
+apiVersion: moco.cybozu.com/v1beta2
+kind: MySQLCluster
+metadata:
+  namespace: default
+  name: test
+  annotations:
+    moco.cybozu.com/force-rolling-update: "true"
+...
+```
+
+When creating or updating a StatefulSet with the annotation `moco.cybozu.com/force-rolling-update` set, MOCO deletes the partition setting using MutatingAdmissionWebhook.
+
+### Metrics
+
+MOCO outputs the following metrics related to rolling updates:
+
+* `moco_cluster_current_replicas`
+  * The same as `.status.currentReplicas` of the StatefulSet.
+* `moco_cluster_updated_replicas`
+  * The same as `.status.updatedReplicas` of the StatefulSet.
+* `moco_cluster_last_partition_updated`
+  * The time the partition was last updated.
+
+By setting an alert with the condition that `moco_cluster_updated_replicas` is not equal to `moco_cluster_replicas` and a certain amount of time has passed since `moco_cluster_last_partition_updated`, you can detect MySQLClusters where the rolling update is stopped.