Enable a StatefulSet to support persistent storage - Container Service for Kubernetes

You can create a pair of persistent volume (PV) and persistent volume claim (PVC) for each pod by setting the VolumeClaimTemplate in a StatefulSet. If pods are deleted or scaled in, PVs and PVCs of the StatefulSet application are not deleted. This topic describes how to enable a StatefulSet to support persistent storage by setting the VolumeClaimTemplate.

Prerequisites

Create an ACK managed cluster

Scenarios

When to use a StatefulSet:

Predefined deployment order: Pods are deployed or scaled out from 0 to N-1 in sequence. The system must wait until all of the preceding pods reach the Running or Ready state before it can deploy another pod.
Predefined scale-in order: Pods are scaled in or deleted from N-1 to 0 in sequence. The system must wait until all of the preceding pods are deleted before it can delete another pod.
Consistent network identifiers: After a pod is rescheduled, its PodName and HostName values remain unchanged.
Stable data persistence: After a pod is rescheduled, the pod can still access the same persisted data.

Create a StatefulSet

PVCs and PVs can be automatically created based on VolumeClaimTemplates.

Note volumeClaimTemplates represents a type of template that the system uses to create PVCs. The number of PVCs equals the number of replicas that are deployed for the StatefulSet application. The configurations of these PVCs are the same except for the PVC names.

Use the following template to create a file named statefulset.yaml.

Deploy a Service and a StatefulSet, and provision two pods for the StatefulSet.

apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx
  serviceName: "nginx"
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: disk-ssd
          mountPath: /data
  volumeClaimTemplates:
  - metadata:
      name: disk-ssd
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: "alicloud-disk-ssd"
      resources:
        requests:
          storage: 20Gi

replicas: the parameter is set to 2 in this example. This indicates that two pods are created.
mountPath: the path where you want to mount the disk in the container.
accessModes: the access mode of the StatefulSet.
storageClassName: the parameter is set to alicloud-disk-ssd in this example. This indicates that an Alibaba Cloud standard SSD is used.
storage: specifies the storage that is required by the application.

Run the following command to create a StatefulSet:
```
kubectl create -f statefulset.yaml
```

Run the following command to query the deployed pods:

kubectl get pod

Expected output:

NAME                          READY   STATUS    RESTARTS   AGE
web-0                         1/1     Running   0          6m
web-1                         1/1     Running   0          6m

Run the following command to query the PVCs:

kubectl get pvc

Expected output:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS        AGE
disk-ssd-web-0   Bound    d-2zegw7et6xc96nbojuoo   20Gi       RWO            alicloud-disk-ssd   7m
disk-ssd-web-1   Bound    d-2zefbrqggvkd10xb523h   20Gi       RWO            alicloud-disk-ssd   6m

Verify that the PVCs are scaled out together with the StatefulSet application

Run the following command to scale out the StatefulSet application to three pods:
```
kubectl scale sts web --replicas=3
```
Expected output:
```
statefulset.apps/web scaled
```

Run the following command to view the pods after the StatefulSet application is scaled out:

kubectl get pod

Expected output:

NAME                          READY   STATUS    RESTARTS   AGE
web-0                         1/1     Running   0          34m
web-1                         1/1     Running   0          33m
web-2                         1/1     Running   0          26m

Run the following command to view the PVCs after the StatefulSet application is scaled out:

kubectl get pvc

Expected output:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS        AGE
disk-ssd-web-0   Bound    d-2zegw7et6xc96nbojuoo   20Gi       RWO            alicloud-disk-ssd   35m
disk-ssd-web-1   Bound    d-2zefbrqggvkd10xb523h   20Gi       RWO            alicloud-disk-ssd   34m
disk-ssd-web-2   Bound    d-2ze4jx1zymn4n9j3pic2   20Gi       RWO            alicloud-disk-ssd   27m

The output indicates that three PVCs are provisioned for the StatefulSet application after the StatefulSet application is scaled to three pods.

Verify that the PVCs remain unchanged after the StatefulSet application is scaled in.

Run the following command to scale the StatefulSet application to two pods:
```
kubectl scale sts web --replicas=2
```
Expected output:
```
statefulset.apps/web scaled
```

Run the following command to view the pods after the StatefulSet application is scaled in:

kubectl get pod

Expected output:

NAME                          READY   STATUS    RESTARTS   AGE
web-0                         1/1     Running   0          38m
web-1                         1/1     Running   0          38m

Only two pods are deployed for the StatefulSet application.

Run the following command to view the PVCs after the StatefulSet application is scaled in:

kubectl get pvc

Expected output:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS        AGE
disk-ssd-web-0   Bound    d-2zegw7et6xc96nbojuoo   20Gi       RWO            alicloud-disk-ssd   39m
disk-ssd-web-1   Bound    d-2zefbrqggvkd10xb523h   20Gi       RWO            alicloud-disk-ssd   39m
disk-ssd-web-2   Bound    d-2ze4jx1zymn4n9j3pic2   20Gi       RWO            alicloud-disk-ssd   31m

After the StatefulSet application is scaled to two pods, the StatefulSet application still has three PVCs. This indicates that the PVCs are not scaled in together with the StatefulSet application.

Verify that the PVCs remain unchanged when the StatefulSet application is scaled out again

When the StatefulSet is scaled out again, verify that the PVCs remain unchanged.

Run the following command to scale out the StatefulSet application to three pods:
```
kubectl scale sts web --replicas=3
```
Expected output:
```
statefulset.apps/web scaled
```

Run the following command to view the pods after the StatefulSet application is scaled out:

kubectl get pod

Expected output:

NAME                          READY   STATUS    RESTARTS   AGE
web-0                         1/1     Running   0          1h
web-1                         1/1     Running   0          1h
web-2                         1/1     Running   0          8s

Run the following command to view the PVCs after the StatefulSet application is scaled out:

kubectl get pvc

Expected output:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS        AGE
disk-ssd-web-0   Bound    d-2zegw7et6xc96nbojuoo   20Gi       RWO            alicloud-disk-ssd   1h
disk-ssd-web-1   Bound    d-2zefbrqggvkd10xb523h   20Gi       RWO            alicloud-disk-ssd   1h
disk-ssd-web-2   Bound    d-2ze4jx1zymn4n9j3pic2   20Gi       RWO            alicloud-disk-ssd   1h

The newly created pod uses an existing PVC.

Verify that the PVCs remain unchanged when the pod of the StatefulSet application is deleted

Run the following command to view the PVC that is used by the pod named web-1:
```
kubectl describe pod web-1 | grep ClaimName
```
Expected output:
```
ClaimName:  disk-ssd-web-1
```
Run the following command to delete the pod named web-1:
```
kubectl delete pod web-1
```
Expected output:
```
pod "web-1" deleted
```

Run the following command to view the pod:

kubectl get pod

Expected output:

NAME                          READY   STATUS    RESTARTS   AGE
web-0                         1/1     Running   0          1h
web-1                         1/1     Running   0          25s
web-2                         1/1     Running   0          9m

The recreated pod uses the same name as the deleted pod.

Run the following command to view the PVCs:

kubectl get pvc

Expected output:

NAME             STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS        AGE
disk-ssd-web-0   Bound    d-2zegw7et6xc96nbojuoo   20Gi       RWO            alicloud-disk-ssd   1h
disk-ssd-web-1   Bound    d-2zefbrqggvkd10xb523h   20Gi       RWO            alicloud-disk-ssd   1h
disk-ssd-web-2   Bound    d-2ze4jx1zymn4n9j3pic2   20Gi       RWO            alicloud-disk-ssd   1h

The recreated pod uses an existing PVC.

Verify that the StatefulSet application supports persistent storage

Run the following command to view the file in the /data path:
```
kubectl exec web-1 -- ls /data
```
Expected output:
```
lost+found
```
Run the following command to create a file named statefulset in the /data path:
```
kubectl exec web-1 -- touch /data/statefulset
```
Run the following command to view the file in the /data path:
```
kubectl exec web-1 -- ls /data
```
Expected output:
```
lost+found
statefulset
```
Run the following command to delete the pod named web-1:
```
kubectl delete pod web-1
```
Expected output:
```
pod "web-1" deleted
```
Run the following command to view the file in the /data path:
```
kubectl exec web-1 -- ls /data
```
Expected output:
```
lost+found
statefulset
```
The statefulset file still exists in the /data path. This indicates that data is persisted to the disk.