StatefulSet was described in Chapter 3, Playing with Containers. It assigns a pod name with a sequence number—for example, my-nginx-0, my-nginx-1, my-nginx-2. StatefulSet also uses these pod names to assign a DNS entry instead of IP addresses. Because it uses Kubernetes Service, FQDN appear as follows: <StatefulSet name>-<sequence number>.<Service name>.<Namespace name>.svc.cluster.local.

Let's create StatefulSet to examine how DNS works in StatefulSet:

1. Prepare StatefulSet and normal service YAML configurations as follows:

$ cat nginx-sts.yaml 
apiVersion: v1
kind: Service
metadata:
  name: nginx-sts-svc
  labels:
    app: nginx-sts
spec:
  ports:
  - port: 80
  selector:
    app: nginx-sts
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
  name: nginx-sts
spec:
  serviceName: "nginx-sts-svc"
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx-sts
    spec:
      containers:
        - name: nginx-sts
          image: nginx
          ports:
          - containerPort: 80
      restartPolicy: Always

2. Create StatefulSet on chap8-domain2:

$ kubectl create -f nginx-sts.yaml --namespace=chap8-domain2
service "nginx-sts-svc" created
statefulset "nginx-sts" created

3. Use the kubectl command to check the status of the pod and service creation:

//check StatefulSet (in short sts)
$ kubectl get sts --namespace=chap8-domain2
NAME        DESIRED   CURRENT   AGE
nginx-sts   3         3         46s


//check Service (in short svc)
$ kubectl get svc nginx-sts-svc --namespace=chap8-domain2
NAME            TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)   AGE
nginx-sts-svc   ClusterIP   10.104.63.124   <none>        80/TCP    8m 


//check Pod with "-o wide" to show an IP address
$ kubectl get pods --namespace=chap8-domain2 -o wide
NAME                         READY     STATUS    RESTARTS   AGE       IP            NODE
my-apache-55fb679f49-z9gsr   1/1       Running   1          22h       172.17.0.4    minikube
nginx-sts-0                  1/1       Running   0          2m        172.17.0.2    minikube
nginx-sts-1                  1/1       Running   0          2m        172.17.0.9    minikube
nginx-sts-2                  1/1       Running   0          1m        172.17.0.10   minikube

4. Launch the busybox pod in the foreground:

$ kubectl run -it busybox --restart=Never --image=busybox 

5. Use the nslookup command to query the service's IP address:

# nslookup nginx-sts-svc.chap8-domain2.svc.cluster.local
Server:    10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local

Name:      nginx-sts-svc.chap8-domain2.svc.cluster.local
Address 1: 10.104.63.124 nginx-sts-svc.chap8-domain2.svc.cluster.local

6. Use the nslookup command to query the individual pod's IP address:

# nslookup nginx-sts-0.nginx-sts-svc.chap8-domain2.svc.cluster.local
Server:    10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
Name:      nginx-sts-0.nginx-sts-svc.chap8-domain2.svc.cluster.local
Address 1: 172.17.0.2 nginx-sts-0.nginx-sts-svc.chap8-domain2.svc.cluster.local


# nslookup nginx-sts-1.nginx-sts-svc.chap8-domain2.svc.cluster.local
Server:    10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
Name:      nginx-sts-1.nginx-sts-svc.chap8-domain2.svc.cluster.local
Address 1: 172.17.0.9 nginx-sts-1.nginx-sts-svc.chap8-domain2.svc.cluster.local


# nslookup nginx-sts-2.nginx-sts-svc.chap8-domain2.svc.cluster.local
Server:    10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
Name:      nginx-sts-2.nginx-sts-svc.chap8-domain2.svc.cluster.local
Address 1: 172.17.0.10 nginx-sts-2.nginx-sts-svc.chap8-domain2.svc.cluster.local

7. Clean up the busybox pod:

# exit
$ kubectl delete pod busybox
pod "busybox" deleted