Update formatting and wording for StatefulSet Migration Redis demo

Author: pwschuurman

content/en/blog/_posts/2022-12-16-statefulset-migration.md

@@ -2,7 +2,7 @@
 layout: blog
 title: "Kubernetes 1.26: StatefulSet Start Ordinal Simplifies Migration"
 date: 2023-01-03
-slug: statefulset-migration
+slug: statefulset-start-ordinal
 ---
 
 **Author**: Peter Schuurman (Google)
@@ -32,11 +32,12 @@ You lose the self-healing benefit of the StatefulSet controller when your Pods
 fail or are evicted.
 
 Kubernetes v1.26 enables a StatefulSet to be responsible for a range of ordinals
-within a half-open interval `[0, N)` (the ordinals 0, 1, ... N-1).
+within a range {0..N-1} (the ordinals 0, 1, ... up to N-1).
 With it, you can scale down a range
-(`[0, k)`) in a source cluster, and scale up the complementary range (`[k, N)`)
+{0..k-1} in a source cluster, and scale up the complementary range {k..N-1}
 in a destination cluster, while maintaining application availability. This
-enables you to retain *at most one* semantics and
+enables you to retain *at most one* semantics (meaning there is at most one Pod
+with a given identity running in a StatefulSet) and
 [Rolling Update](/docs/tutorials/stateful-application/basic-stateful-set/#rolling-update)
 behavior when orchestrating a migration across clusters.
 
@@ -61,42 +62,50 @@ to a different cluster. There are many reasons why you would need to do this:
 Enable the `StatefulSetStartOrdinal` feature gate on a cluster, and create a
 StatefulSet with a customized `.spec.ordinals.start`.
 
-## Try it for yourself
+## Try it out
 
-In this demo, you'll use the `StatefulSetStartOrdinal` feature to migrate a
-StatefulSet from one Kubernetes cluster to another. For this demo, the
+In this demo, I'll use the new mechanism to migrate a
+StatefulSet from one Kubernetes cluster to another. The
 [redis-cluster](https://github.com/bitnami/charts/tree/main/bitnami/redis-cluster)
-Bitnami Helm chart is used to install Redis.
+Bitnami Helm chart will be used to install Redis.
 
 Tools Required:
  * [yq](https://github.com/mikefarah/yq)
  * [helm](https://helm.sh/docs/helm/helm_install/)
 
-Pre-requisites: Two Kubernetes clusters named `source` and `destination`.
- * `StatefulSetStartOrdinal` feature gate is enabled on both clusters
- * The same default `StorageClass` is installed on both clusters. This
-   `StorageClass` should provision underlying storage that is accessible from
-   both clusters.
- * A flat network topology that allows for pods to be accessible across both
-   Kubernetes clusters. If creating clusters on a cloud provider, this
-   configuration may be called private cloud or private network.
+### Pre-requisites {#demo-pre-requisites}
 
-1. Create a demo namespace on both clusters.
+To do this, I need two Kubernetes clusters that can both access common
+networking and storage; I've named my clusters `source` and `destination`.
+Specifically, I need:
+
+* The `StatefulSetStartOrdinal` feature gate enabled on both clusters.
+* Client configuration for `kubectl` that lets me access both clusters as an
+  administrator.
+* The same `StorageClass` installed on both clusters, and set as the default
+  StorageClass for both clusters. This `StorageClass` should provision
+  underlying storage that is accessible from either or both clusters.
+* A flat network topology that allows for pods to send and receive packets to
+  and from Pods in either clusters. If you are creating clusters on a cloud
+  provider, this configuration may be called private cloud or private network.
+
+1. Create a demo namespace on both clusters:
 
    ```
    kubectl create ns kep-3335
    ```
 
-2. Deploy a Redis cluster on `source`.
+2. Deploy a Redis cluster with six replicas in the source cluster:
 
    ```
    helm repo add bitnami https://charts.bitnami.com/bitnami
    helm install redis --namespace kep-3335 \
      bitnami/redis-cluster \
-     --set persistence.size=1Gi
+     --set persistence.size=1Gi \
+     --set cluster.nodes=6
    ```
 
-3. On `source`, check the replication status.
+3. Check the replication status in the source cluster:
 
    ```
    kubectl exec -it redis-redis-cluster-0 -- /bin/bash -c \
@@ -112,7 +121,7 @@ Pre-requisites: Two Kubernetes clusters named `source` and `destination`.
    2cff613d763b22c180cd40668da8e452edef3fc8 10.104.0.17:6379@16379 master - 0 1669764410000 2 connected 5461-10922
    ```
 
-4. On `destination`, deploy Redis with zero replicas.
+4. Deploy a Redis cluster with zero replicas in the destination cluster:
 
    ```
    helm install redis --namespace kep-3335 \
@@ -123,19 +132,20 @@ Pre-requisites: Two Kubernetes clusters named `source` and `destination`.
      --set existingSecret=redis-redis-cluster
    ```
 
-5. Scale down replica `redis-redis-cluster-5` in the source cluster.
+5. Scale down the `redis-redis-cluster` StatefulSet in the source cluster by 1,
+   to remove the replica `redis-redis-cluster-5`:
 
    ```
    kubectl patch sts redis-redis-cluster -p '{"spec": {"replicas": 5}}'
    ```
 
-6. Migrate dependencies from `source` to `destination`.
+6. Migrate dependencies from the source cluster to the destination cluster:
 
    The following commands copy resources from `source` to `destionation`. Details
    that are not relevant in `destination` cluster are removed (eg: `uid`,
    `resourceVersion`, `status`).
 
-   #### Source cluster
+   **Steps for the source cluster**
 
    Note: If using a `StorageClass` with `reclaimPolicy: Delete` configured, you
          should patch the PVs in `source` with `reclaimPolicy: Retain` prior to
@@ -149,7 +159,7 @@ Pre-requisites: Two Kubernetes clusters named `source` and `destination`.
    kubectl get secret redis-redis-cluster -o yaml | yq 'del(.metadata.uid, .metadata.resourceVersion)' > /tmp/secret-redis-redis-cluster.yaml
    ```
 
-   #### Destination cluster
+   **Steps for the destination cluster**
 
    Note: For the PV/PVC, this procedure only works if the underlying storage system
          that your PVs use can support being copied into `destination`. Storage
@@ -164,42 +174,49 @@ Pre-requisites: Two Kubernetes clusters named `source` and `destination`.
    kubectl create -f /tmp/secret-redis-redis-cluster.yaml
    ```
 
-7. Scale up replica `redis-redis-cluster-5` in the destination cluster, with a
-   start ordinal of 5:
+7. Scale up the `redis-redis-cluster` StatefulSet in the destination cluster by
+   1, with a start ordinal of 5:
 
    ```
    kubectl patch sts redis-redis-cluster -p '{"spec": {"ordinals": {"start": 5}, "replicas": 1}}'
    ```
 
-8. On the source cluster, check the replication status.
+8. Check the replication status in the destination cluster:
 
    ```
-   kubectl exec -it redis-redis-cluster-0 -- /bin/bash -c \
+   kubectl exec -it redis-redis-cluster-5 -- /bin/bash -c \
      "redis-cli -c -h redis-redis-cluster -a $(kubectl get secret redis-redis-cluster -o jsonpath="{.data.redis-password}" | base64 -d) CLUSTER NODES;"
    ```
 
-   You should see that the new replica's address has joined the Redis cluster.
+   I should see that the new replica (labeled `myself`) has joined the Redis
+   cluster (the IP address belongs to a different CIDR block than the
+   replicas in the source cluster).
 
    ```
-   2cff613d763b22c180cd40668da8e452edef3fc8 10.104.0.17:6379@16379 myself,master - 0 1669766684000 2 connected 5461-10922
-   7136e37d8864db983f334b85d2b094be47c830e5 10.108.0.22:6379@16379 slave 2cff613d763b22c180cd40668da8e452edef3fc8 0 1669766685609 2 connected
+   2cff613d763b22c180cd40668da8e452edef3fc8 10.104.0.17:6379@16379 master - 0 1669766684000 2 connected 5461-10922
+   7136e37d8864db983f334b85d2b094be47c830e5 10.108.0.22:6379@16379 myself,slave 2cff613d763b22c180cd40668da8e452edef3fc8 0 1669766685609 2 connected
    2ce30362c188aabc06f3eee5d92892d95b1da5c3 10.104.0.14:6379@16379 master - 0 1669766684000 3 connected 10923-16383
    961f35e37c4eea507cfe12f96e3bfd694b9c21d4 10.104.0.18:6379@16379 slave a8765caed08f3e185cef22bd09edf409dc2bcc61 0 1669766683600 1 connected
    a8765caed08f3e185cef22bd09edf409dc2bcc61 10.104.0.19:6379@16379 master - 0 1669766685000 1 connected 0-5460
    7743661f60b6b17b5c71d083260419588b4f2451 10.104.0.16:6379@16379 slave 2ce30362c188aabc06f3eee5d92892d95b1da5c3 0 1669766686613 3 connected
    ```
 
+9. Repeat steps #5 to #7 for the remainder of the replicas, until the
+   Redis StatefulSet in the source cluster is scaled to 0, and the Redis
+   StatefulSet in the destination cluster is healthy with 6 total replicas.
+
 ## What's Next?
 
 This feature provides a building block for a StatefulSet to be split up across
 clusters, but does not prescribe the mechanism as to how the StatefulSet should
 be migrated. Migration requires coordination of StatefulSet replicas, along with
 orchestration of the storage and network layer. This is dependent on the storage
 and connectivity requirements of the application installed by the StatefulSet.
-Additionally, many StatefulSets are controlled by Operators, which adds another
+Additionally, many StatefulSets are managed by
+[operators](/docs/concepts/extend-kubernetes/operator/), which adds another
 layer of complexity to migration.
 
-If you're interested in building blocks to make these processes easier, get
-involved with
+If you're interested in building enhancements to make these processes easier,
+get involved with
 [SIG Multicluster](https://github.com/kubernetes/community/blob/master/sig-multicluster)
 to contribute!