#StatefulSets

#Understanding StatefulSets

Refer to the official Kubernetes documentation: StatefulSets

StatefulSet is a Kubernetes workload API object designed to manage stateful applications by providing:

• Stable network identity: DNS hostname <statefulset-name>-<ordinal>.<service-name>.ns.svc.cluster.local.
• Stable persistent storage: via volumeClaimTemplates.
• Ordered deployment/scaling: sequential Pod creation/deletion: Pod-0 → Pod-1 → Pod-N.
• Ordered rolling updates: reverse-ordinal Pod updates: Pod-N → Pod-0.

In distributed systems, multiple StatefulSets can be deployed as discrete components to deliver specialized stateful services (e.g., Kafka brokers, MongoDB shards).

#Creating StatefulSets

#Creating a StatefulSet by using CLI

#Prerequisites

• Ensure you have kubectl configured and connected to your cluster.

#YAML file example

# example-statefulset.yaml
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx # has to match .spec.template.metadata.labels
  serviceName: "nginx" # this headless Service is responsible for the network identity of the Pods
  replicas: 3 # defines the desired number of Pod replicas (default: 1)
  minReadySeconds: 10 # by default is 0
  template: # defines the Pod template for the StatefulSet
    metadata:
      labels:
        app: nginx # has to match .spec.selector.matchLabels
    spec:
      terminationGracePeriodSeconds: 10
      containers:
      - name: nginx
        image: registry.k8s.io/nginx-slim:0.24
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates: # defines PersistentVolumeClaim (PVC) templates. Each Pod gets a unique PersistentVolume (PV) dynamically provisioned based on these templates.
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: "my-storage-class"
      resources:
        requests:
          storage: 1Gi
---
# example-service.yaml
apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx

#Creating a StatefulSet via YAML

# Step 1: To create the StatefulSet defined in *example-statefulset.yaml*, execute the following command
kubectl apply -f example-statefulset.yaml

# Step 2: To verify the creation and status of your StatefulSet and its associated Pods and PVCs:
kubectl get statefulset web # View StatefulSet
kubectl get pods -l app=nginx # Check Pods managed by this StatefulSet
kubectl get pvc -l app=nginx # Check PVCs created by volumeClaimTemplates

#Creating a StatefulSet by using web console

#Prerequisites

Obtain the image address. The source of the images can be from the image repository integrated by the platform administrator through the toolchain or from third-party platforms' image repositories.

• For the former, the Administrator typically assigns the image repository to your project, and you can use the images within it. If the required image repository is not found, please contact the Administrator for allocation.

• If it is a third-party platform's image repository, ensure that images can be pulled directly from it in the current cluster.

#Procedure - Configure Basic Info

1. Container Platform, navigate to Workloads > StatefulSets in the left sidebar.

2. Click Create StatefulSet.

3. Select or Input an image, and click Confirm.

In the Basic Info section, configure declarative parameters for StatefulSet workloads:

#Procedure - Configure Pod

Pod section, please refer to Deployment - Configure Pod

#Procedure - Configure Containers

Containers section, please refer to Deployment - Configure Containers

#Procedure - Create

Click Create.

#Heath Checks

• Health checks YAML file example
• Health checks configuration parameters in web console

#Managing StatefulSets

#Managing a StatefulSet by using CLI

#Viewing a StatefulSet

You can view a StatefulSet to get information of your application.

• Check the StatefulSet was created.

 kubectl get statefulsets

• Get details of your StatefulSet.

kubectl describe statefulsets

#Scaling a StatefulSet

• To change the number of replicas for an existing StatefulSet:

kubectl scale statefulset <statefulset-name> --replicas=<new-replica-count>

• Example:

kubectl scale statefulset web --replicas=5

#Updating a StatefulSet (Rolling Update)

When you modify the Pod template of a StatefulSet (e.g., changing the container image), Kubernetes performs a rolling update by default (if updateStrategy is set to RollingUpdate, which is the default).

• First, edit the YAML file (e.g., example-statefulset.yaml) with the desired changes, then apply it:

kubectl apply -f example-statefulset.yaml

• Then, you can monitor the progress of the rolling update:

kubectl rollout status statefulset/<statefulset-name>

#Deleting a StatefulSet

To delete a StatefulSet and its associated Pods:

kubectl delete statefulset <statefulset-name>

By default, deleting a StatefulSet does not delete its associated PersistentVolumeClaims (PVCs) or PersistentVolumes (PVs) to prevent data loss. To also delete the PVCs, you must do so explicitly:

kubectl delete pvc -l app=<label-selector-for-your-statefulset> # Example: kubectl delete pvc -l app=nginx

Alternatively, if your volumeClaimTemplates use a StorageClass with a reclaimPolicy of Delete, the PVs and underlying storage will be deleted automatically when the PVCs are deleted.

#Managing a StatefulSet by using web console

#Viewing a StatefulSet

1. Container Platform, and navigate to Workloads > StatefulSets.
2. Locate the StatefulSet you wish to view.
3. Click the statefulSet name to see the Details, Topology, Logs, Events, Monitoring, etc.

#Updating a StatefulSet

1. Container Platform, and navigate to Workloads > StatefulSets.
2. Locate the StatefulSet you wish to update.
3. In the Actions drop-down menu, select Update to view the Edit StatefulSet page, you can update Replicas, image, updateStrategy, etc.

#Deleting a StatefulSet

1. Container Platform, and navigate to Workloads > StatefulSets.
2. Locate the StatefulSet you wish to delete.
3. In the Actions drop-down menu, Click the Delete button in the operations column and confirm.