README.md

StatefulSet

If you have a stateless app you want to use a deployment. However, for a stateful app you might want to use a StatefulSet. Unlike a deployment, the StatefulSet provides certain guarantees about the identity of the pods it is managing (that is, predictable names) and about the startup order. Two more things that are different compared to a deployment: for network communication you need to create a headless services and for persistency the StatefulSet manages a persistent volume per pod.

In order to see how this all plays together, we will be using an educational Kubernetes-native NoSQL datastore.

$ kubectl apply -f statefulset.yaml      
statefulset.apps/mehdb created

After a few minutes, let's verify if everything went smootlhy so far.

$ kubectl get pods -o wide
NAME                               READY   STATUS    RESTARTS   AGE     IP           NODE     NOMINATED NODE   READINESS GATES
mehdb-0                            1/1     Running   0          2m49s   10.244.1.2   node1    <none>           <none>
mehdb-1                            1/1     Running   0          89s     10.244.2.2   node2    <none>           <none>
nfs-provisioner-77bb4bd457-whlbj   1/1     Running   0          23m     10.244.0.6   master   <none>           <none>

We now have the PV provisioner pod and two pods created by the StatefulSet. Please note that the latters have a more predictable name ending with - and a sequential number, compared to if they were created by a Deployment object.

Also, every mehdb pod has been scheduled on different node.

If you take a look of PVs, you can see that every mehdb pod claimed its own instead of sharing one, and theire staus is Bound (thus PV provisioner pod worked as expected).

$ kubectl get pv
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                  STORAGECLASS   REASON   AGE
pvc-60309b5b-039c-44ae-96a1-37782ba2e26f   1Gi        RWX            Delete           Bound    default/data-mehdb-1   nfs-dynamic             9m17s
pvc-d2d553fd-fec0-4f02-b97b-c4c7bf76dc45   1Gi        RWX            Delete           Bound    default/data-mehdb-0   nfs-dynamic             10m

This behaviour is caused by the StatefulSet's volumeClaimTemplates element:

...
  volumeClaimTemplates:
  - metadata:
      name: data
    spec:
      accessModes: [ "ReadWriteMany" ]
      storageClassName: "nfs-dynamic"
      resources:
        requests:
          storage: 1Gi

storageClassName nfs-dynamic must match the one configured in ../12-StorageClass/nfs-storageclass.yaml so as to link the correct provisioner.

Before testing our stateful application, we need to create the corresponding headless Service.

A headless Service is a service without a cluster IP so instead of load-balancing it will return the IPs of the associated Pods. This allows us to interact directly with the pods instead of the cluster IP.

To create an headless Service you need to specify None as a value for .spec.clusterIP.

Let's create the headless Service with the following command:

$ kubectl apply -f headless-service.yaml
service/mehdb created

To see if it worked:

$ kubectl get svc
NAME              TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)                              AGE
...
mehdb             ClusterIP   None             <none>        9876/TCP                             42s
...

Note the None value under ClusterIP header.

Now we can check if the stateful app is working properly. To do this, we use the /status endpoint of the headless service mehdb:9876 and since we haven’t put any data yet into the datastore, we’d expect that 0 keys are reported:

$ kubectl run -i --rm --tty busybox --image=busybox --restart=Never -- wget -qO- "mehdb:9876/status?level=full"
0
pod "busybox" deleted

And indeed we see 0 keys being available, reported above.

Don't forget to clean up after you:

$ kubectl delete -f .
service "mehdb" deleted
statefulset.apps "mehdb" deleted

$ kubectl delete -f ../12-StorageClass/. 
service "nfs-provisioner" deleted
serviceaccount "nfs-provisioner" deleted
deployment.apps "nfs-provisioner" deleted
storageclass.storage.k8s.io "nfs-dynamic" deleted
persistentvolumeclaim "nfs" deleted
clusterrole.rbac.authorization.k8s.io "nfs-provisioner-runner" deleted
clusterrolebinding.rbac.authorization.k8s.io "run-nfs-provisioner" deleted
role.rbac.authorization.k8s.io "leader-locking-nfs-provisioner" deleted
rolebinding.rbac.authorization.k8s.io "leader-locking-nfs-provisioner" deleted