Part 1: Using StatefulSets

This is Part 1 of a multi-part, self-paced quick start exercise.

Note
This exercise requires MicroK8s and uses alias kubectl='microk8s kubectl'. If you do not have these already installed and running, see " Prerequisites".

What Will You Do

In part 1, you will:

• Use StatefulSets on your cluster.
• Scale up a StatefulSet.
• Scale down a StatefulSet.

Estimated Time
Estimated time for this exercise is 20 minutes. Watch a video of this exercise.

Kubernetes 301-StatefulSets

Using StatefulSets

Kubernetes supports multiple virtual clusters backed by the same physical cluster. These virtual clusters are called Namespaces. A namespace isolates and virtualizes system resources. Processes are restricted to a namespace and can only interact with processes and resources in the same namespace.

Namespaces are useful for larger teams or projects to share a cluster without impacting each other's work. Namespaces also help when access to a namespace needs to be restricted, like creating a namespace for your production site and restricting access to the operations team.

Note: For information about Deployments, see the Deployments exercise.

Create a StatefulSet

You can also create a namespace using a YAML file, which is a configuration file. You could create a YAML file from the command line, but for this exercise, you can just use a text editor. Using a YAML file to create namespaces can be useful if you create multiple clusters that have the same namespaces, like development and production.

For macOS, use touch namespace.yaml to create a YAML file. You can then edit the YAML file and copy/paste the YAML file contents.

1. Open the Terminal.
2. Make sure that Storage is enabled for MicroK8s.

    microk8s status
    ```

- If this add-on is not enabled, run the following command.

 microk8s enable storage
 ```

3. Navigate to the Downloads folder.

    cd ./Downloads
    ```

4. Use the following command to open an empty YAML file in the nano text editor.

nano statefulset.yaml
```

5. Copy and paste the configuration below into the text editor.
6. Press Cmd + X, then type Y and press Return to save the namespace.yaml file.

Add a StatefulSet

1. Add the StatefulSet to your environment using a YAML file.

    kubectl create -f statefulset.yaml
    ```

2. List the Services. The NGINX service has been added.

kubectl get service
```

3. List the StatefulSet. The Web StatefulSet has been added.

    kubectl get statefulset
    ```

4. List the Pods. Two Web Pods have been added (web-0 and web-1).

kubectl get pods
```

5. List the PersistentVolumeClaims (PVC). Two PVCs have been added (www-web-0 and www-web-1).

    kubectl get pvc
    ```

6. Open a second Terminal to watch the StatefulSet Pods. **Note**: Using the second Terminal requires including `microk8s` or adding the alias.

microk8s kubectl get pod -w -l app=nginx
```

7. In the first Terminal, delete the StatefulSet Pods. The second Terminal should show the Pods being deleted.

    kubectl delete pod -l app=nginx
    ```

8. In the first Terminal, manually scale up the Replicas to 5.

kubectl scale sts web --replicas=5
```

9. In the first Terminal, manually scale down the Replicas to 3.

    kubectl scale sts web --replicas=3
    ```

---

### statefulset.yaml

apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: www
spec:
  capacity:
    storage: 1Gi
  volumeMode: Filesystem
  accessModes:
  - ReadWriteOnce
  storageClassName: slow
  claimRef:
    name: web
  hostPath:
    path: /var/lib/pv
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  serviceName: "nginx"
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: k8s.gcr.io/nginx-slim:0.8
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi
```