How to deploy StatefulSets in Kubernetes (K8s)?

StatefulSets

StatefulSets will represent the set of pods with different (unique), persistent identities, and elastic hostnames (stable). It makes you assure about the ordering of scaling and deployments

StatefulSets are valuable for applications that require one or more of the following:

1. Stable, unique network identifiers
2. Stable, persistent storage
3. Ordered, graceful deployment and scaling
4. Ordered, graceful deletion and termination

If an application doesn’t require any stable identifiers or ordered deployment, deletion, or scaling, you should deploy your application with a controller such as Deployments or ReplicaSets that provides a set of stateless replicas.

StatefulSet Components

1. A Headless Service
2. A StatefulSet
3. A PersistentVolume

Below are manifests of a Service, StatefulSet, and Persistent volume:

---
apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx

2

---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx
  serviceName: "nginx"
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
      volumes:
      - name: www
        persistentVolumeClaim:
          claimName: myclaim

3

---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: myclaim
spec:
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 8Gi

Create and manage StatefulSets

Create a stateful set

kubectl create -f statefulset.yaml
service "nginx" created
statefulset.apps "web" created

It will create three Pods named web-0,web-1,web-2

kubectl get pods
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          1m
web-1     1/1       Running   0          46s
web-2     1/1       Running   0          18s

Check services in Kubernetes

kubectl get svc nginx
NAME      TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)   AGE
nginx     ClusterIP   None         < none >      80/TCP    2m

List your stateful sets:

kubectl get statefulsets

Get details of a stateful set:

kubectl describe statefulset web

Edit a stateful set:

kubectl edit statefulset web

Scaling a stateful set:

Scaling a StatefulSet refers to increasing or decreasing the number of replicas.

Scale-up a stateful set:

kubectl scale statefulset web --replicas=5 
statefulset.apps "web" scaled

Check and get pods

kubectl get pods -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          11m
web-1     1/1       Running   0          10m
web-2     1/1       Running   0          10m
web-3     1/1       Running   0          33s
web-4     1/1       Running   0          19s

Scale down a stateful set:

kubectl scale statefulset web --replicas=2 
statefulset.apps "web" scaled

Check the scaling down of pods

kubectl get pods -w -l app=nginx
NAME      READY     STATUS        RESTARTS   AGE
web-0     1/1       Running       0          13m
web-1     1/1       Running       0          12m
web-2     0/1       Terminating   0          12m
web-3     0/1       Terminating   0          2m
web-4     0/1       Terminating   0          1m

Check again :

kubectl get pods -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          13m
web-1     1/1       Running   0          12m

Delete a stateful set

kubectl delete statefulset web 
statefulset.apps "web" deleted

You must delete the Service manually.

kubectl delete service nginx 
service "nginx" deleted

Limitations:

1. StatefulSet was a beta resource before 1.9 and not available in any Kubernetes release before 1.5.
2. The storage for a given Pod must either be provisioned by a PersistentVolume Provisioner based on the requested storage class or pre-provisioned by an admin.
3. Deleting and/or scaling a StatefulSet down will not delete the volumes associated with the StatefulSet. This is done to ensure data safety, which is generally more valuable than an automatic purge of all related StatefulSet resources.
4. StatefulSets currently require a Headless Service to be responsible for the network identity of the Pods. You are responsible for creating this Service.
5. StatefulSets do not provide any guarantees on the termination of pods when a StatefulSet is deleted. To achieve ordered and graceful termination of the pods in the StatefulSet, it is possible to scale the StatefulSet down to 0 before deletion.

For More, Recommendations are

StatefulSets

Kubernetes StatefulSets 101 - State Of The Pods

Deploying a stateful application | Kubernetes Engine Documentation

Kubernetes: move from deployment to statefulset - set env from secrets

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