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StatefulSet Basics

This tutorial provides an introduction to manage applications with StatefulSets. It demonstrates how to create, delete, scale, and update the container image of a StatefulSet.

Objectives

StatefulSets are intended to be used with stateful applications and distributed systems. However, the administration of stateful applications and distributed systems on Kubernetes is a broad, complex topic. In order to demonstrate the basic features of a StatefulSet, and to not conflate the former topic with the latter, you will deploy a simple web application using StatefulSets.

After this tutorial, you will be familiar with the following.

Before you begin

Before you begin this tutorial, you should familiarize yourself with the following Kubernetes concepts.

This tutorial assumes that your cluster is configured to dynamically provision PersistentVolumes. If your cluster is not configured to do so, you will have to manually provision five 1 GiB volumes prior to starting this tutorial.

Creating a StatefulSet

Begin by creating a StatefulSet using the example below. It is similar to the example presented in the StatefulSets concept. It creates a Headless Service, nginx, to control the domain of the StatefulSet, web.

web.yaml
---
apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
  name: web
spec:
  serviceName: "nginx"
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: gcr.io/google_containers/nginx-slim:0.8
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
      annotations:
        volume.alpha.kubernetes.io/storage-class: anything
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi

Download the example above, and save it to a file named web.yaml

You will need to use two terminal windows. In the first terminal, use kubectl get to watch the creation of the StatefulSet’s Pods.

kubectl get pods -w -l app=nginx

In the second terminal, use kubectl create to create the Headless Service and StatefulSet defined in web.yaml.

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

The command above creates two Pods, each running an NGINX webserver. Get the nginx Service and the web StatefulSet to verify that they were created successfully.

kubectl get service nginx
NAME      CLUSTER-IP   EXTERNAL-IP   PORT(S)   AGE
nginx     None         <none>        80/TCP    12s

kubectl get statefulset web
NAME      DESIRED   CURRENT   AGE
web       2         1         20s

Ordered Pod Creation

For a StatefulSet with N replicas, when Pods are being deployed, they are created sequentially, in order from {0..N-1}. Examine the output of the kubectl get command in the first terminal. Eventually, the output will look like the example below.

kubectl get pods -w -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
NAME      READY     STATUS    RESTARTS   AGE
web-0     0/1       Pending   0          0s
web-0     0/1       Pending   0         0s
web-0     0/1       ContainerCreating   0         0s
web-0     1/1       Running   0         19s
web-1     0/1       Pending   0         0s
web-1     0/1       Pending   0         0s
web-1     0/1       ContainerCreating   0         0s
web-1     1/1       Running   0         18s

Notice that the web-0 Pod is launched and set to Pending prior to launching web-1. In fact, web-1 is not launched until web-0 is Running and Ready.

Pods in a StatefulSet

Unlike Pods in other controllers, the Pods in a StatefulSet have a unique ordinal index and a stable network identity.

Examining the Pod’s Ordinal Index

Get the StatefulSet’s Pods.

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

As mentioned in the StatefulSets concept, the Pods in a StatefulSet have a sticky, unique identity. This identity is based on a unique ordinal index that is assigned to each Pod by the Stateful Set controller. The Pods’ names take the form <statefulset name>-<ordinal index>. Since the web StatefulSet has two replicas, it creates two Pods, web-0 and web-1.

Using Stable Network Identities

Each Pod has a stable hostname based on its ordinal index. Use kubectl exec to execute the hostname command in each Pod.

for i in 0 1; do kubectl exec web-$i -- sh -c 'hostname'; done
web-0
web-1

Use kubectl run to execute a container that provides the nslookup command from the dnsutils package. Using nslookup on the Pods’ hostnames, you can examine their in-cluster DNS addresses.

kubectl run -i --tty --image busybox dns-test --restart=Never --rm /bin/sh 
nslookup web-0.nginx
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      web-0.nginx
Address 1: 10.244.1.6

nslookup web-1.nginx
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      web-1.nginx
Address 1: 10.244.2.6

The CNAME of the headless service points to SRV records (one for each Pod that is Running and Ready). The SRV records point to A record entries that contain the Pods’ IP addresses.

In one terminal, watch the StatefulSet’s Pods.

kubectl get pod -w -l app=nginx

In a second terminal, use kubectl delete to delete all the Pods in the StatefulSet.

kubectl delete pod -l app=nginx
pod "web-0" deleted
pod "web-1" deleted

Wait for the StatefulSet to restart them, and for both Pods to transition to Running and Ready.

kubectl get pod -w -l app=nginx
NAME      READY     STATUS              RESTARTS   AGE
web-0     0/1       ContainerCreating   0          0s
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          2s
web-1     0/1       Pending   0         0s
web-1     0/1       Pending   0         0s
web-1     0/1       ContainerCreating   0         0s
web-1     1/1       Running   0         34s

Use kubectl exec and kubectl run to view the Pods hostnames and in-cluster DNS entries.

for i in 0 1; do kubectl exec web-$i -- sh -c 'hostname'; done
web-0
web-1

kubectl run -i --tty --image busybox dns-test --restart=Never --rm /bin/sh 
nslookup web-0.nginx
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      web-0.nginx
Address 1: 10.244.1.7

nslookup web-1.nginx
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      web-1.nginx
Address 1: 10.244.2.8

The Pods’ ordinals, hostnames, SRV records, and A record names have not changed, but the IP addresses associated with the Pods may have changed. In the cluster used for this tutorial, they have. This is why it is important not to configure other applications to connect to Pods in a StatefulSet by IP address.

If you need to find and connect to the active members of a StatefulSet, you should query the CNAME of the Headless Service (nginx.default.svc.cluster.local). The SRV records associated with the CNAME will contain only the Pods in the StatefulSet that are Running and Ready.

If your application already implements connection logic that tests for liveness and readiness, you can use the SRV records of the Pods ( web-0.nginx.default.svc.cluster.local, web-1.nginx.default.svc.cluster.local), as they are stable, and your application will be able to discover the Pods’ addresses when they transition to Running and Ready.

Writing to Stable Storage

Get the PersistentVolumeClaims for web-0 and web-1.

kubectl get pvc -l app=nginx
NAME        STATUS    VOLUME                                     CAPACITY   ACCESSMODES   AGE
www-web-0   Bound     pvc-15c268c7-b507-11e6-932f-42010a800002   1Gi        RWO           48s
www-web-1   Bound     pvc-15c79307-b507-11e6-932f-42010a800002   1Gi        RWO           48s

The StatefulSet controller created two PersistentVolumeClaims that are bound to two PersistentVolumes. As the cluster used in this tutorial is configured to dynamically provision PersistentVolumes, the PersistentVolumes were created and bound automatically.

The NGINX webservers, by default, will serve an index file at /usr/share/nginx/html/index.html. The volumeMounts field in the StatefulSets spec ensures that the /usr/share/nginx/html directory is backed by a PersistentVolume.

Write the Pods’ hostnames to their index.html files and verify that the NGINX webservers serve the hostnames.

for i in 0 1; do kubectl exec web-$i -- sh -c 'echo $(hostname) > /usr/share/nginx/html/index.html'; done

for i in 0 1; do kubectl exec -it web-$i -- curl localhost; done
web-0
web-1

Note, if you instead see 403 Forbidden responses for the above curl command, you will need to fix the permissions of the directory mounted by the volumeMounts (due to a bug when using hostPath volumes) with:

for i in 0 1; do kubectl exec web-$i -- chmod 755 /usr/share/nginx/html; done

before retrying the curl command above.

In one terminal, watch the StatefulSet’s Pods.

kubectl get pod -w -l app=nginx

In a second terminal, delete all of the StatefulSet’s Pods.

kubectl delete pod -l app=nginx
pod "web-0" deleted
pod "web-1" deleted

Examine the output of the kubectl get command in the first terminal, and wait for all of the Pods to transition to Running and Ready.

kubectl get pod -w -l app=nginx
NAME      READY     STATUS              RESTARTS   AGE
web-0     0/1       ContainerCreating   0          0s
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          2s
web-1     0/1       Pending   0         0s
web-1     0/1       Pending   0         0s
web-1     0/1       ContainerCreating   0         0s
web-1     1/1       Running   0         34s

Verify the web servers continue to serve their hostnames.

for i in 0 1; do kubectl exec -it web-$i -- curl localhost; done
web-0
web-1

Event though web-0 and web-1 were rescheduled, they continue to serve their hostnames because the PersistentVolumes associated with their Persistent Volume Claims are remounted to their volumeMounts. No matter what node web-0 and web-1 are scheduled on, their PersistentVolumes will be mounted to the appropriate mount points.

Scaling a StatefulSet

Scaling a StatefulSet refers to increasing or decreasing the number of replicas. This is accomplished by updating the replicas field. You can use either kubectl scale or kubectl patch to scale a Stateful Set.

Scaling Up

In one terminal window, watch the Pods in the StatefulSet.

kubectl get pods -w -l app=nginx

In another terminal window, use kubectl scale to scale the number of replicas to 5.

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

Examine the output of the kubectl get command in the first terminal, and wait for the three additional Pods to transition to Running and Ready.

kubectl get pods -w -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          2h
web-1     1/1       Running   0          2h
NAME      READY     STATUS    RESTARTS   AGE
web-2     0/1       Pending   0          0s
web-2     0/1       Pending   0         0s
web-2     0/1       ContainerCreating   0         0s
web-2     1/1       Running   0         19s
web-3     0/1       Pending   0         0s
web-3     0/1       Pending   0         0s
web-3     0/1       ContainerCreating   0         0s
web-3     1/1       Running   0         18s
web-4     0/1       Pending   0         0s
web-4     0/1       Pending   0         0s
web-4     0/1       ContainerCreating   0         0s
web-4     1/1       Running   0         19s

The StatefulSet controller scaled the number of replicas. As with StatefulSet creation, the StatefulSet controller created each Pod sequentially with respect to its ordinal index, and it waited for each Pod’s predecessor to be Running and Ready before launching the subsequent Pod.

Scaling Down

In one terminal, watch the StatefulSet’s Pods.

kubectl get pods -w -l app=nginx

In another terminal, use kubectl patch to scale the StatefulSet back down to 3 replicas.

kubectl patch statefulset web -p '{"spec":{"replicas":3}}'
"web" patched

Wait for web-4 and web-3 to transition to Terminating.

kubectl get pods -w -l app=nginx
NAME      READY     STATUS              RESTARTS   AGE
web-0     1/1       Running             0          3h
web-1     1/1       Running             0          3h
web-2     1/1       Running             0          55s
web-3     1/1       Running             0          36s
web-4     0/1       ContainerCreating   0          18s
NAME      READY     STATUS    RESTARTS   AGE
web-4     1/1       Running   0          19s
web-4     1/1       Terminating   0         24s
web-4     1/1       Terminating   0         24s
web-3     1/1       Terminating   0         42s
web-3     1/1       Terminating   0         42s

Ordered Pod Termination

The controller deleted one Pod at a time, with respect to its ordinal index, in reverse order, and it waited for each to be completely shutdown before deleting the next.

Get the StatefulSet’s PersistentVolumeClaims.

kubectl get pvc -l app=nginx
NAME        STATUS    VOLUME                                     CAPACITY   ACCESSMODES   AGE
www-web-0   Bound     pvc-15c268c7-b507-11e6-932f-42010a800002   1Gi        RWO           13h
www-web-1   Bound     pvc-15c79307-b507-11e6-932f-42010a800002   1Gi        RWO           13h
www-web-2   Bound     pvc-e1125b27-b508-11e6-932f-42010a800002   1Gi        RWO           13h
www-web-3   Bound     pvc-e1176df6-b508-11e6-932f-42010a800002   1Gi        RWO           13h
www-web-4   Bound     pvc-e11bb5f8-b508-11e6-932f-42010a800002   1Gi        RWO           13h

There are still five PersistentVolumeClaims and five PersistentVolumes. When exploring a Pod’s stable storage, we saw that the PersistentVolumes mounted to the Pods of a StatefulSet are not deleted when the StatefulSet’s Pods are deleted. This is still true when Pod deletion is caused by scaling the StatefulSet down. This feature can be used to facilitate upgrading the container images of Pods in a StatefulSet.

Updating Containers

As demonstrated in the Scaling a StatefulSet section, the replicas field of a StatefulSet is mutable. The only other field of a StatefulSet that can be updated is the spec.template.containers field.

StatefulSet currently does not support automated image upgrade. However, you can update the image field of any container in the podTemplate and delete StatefulSet’s Pods one by one, the StatefulSet controller will recreate each Pod with the new image.

Patch the container image for the web StatefulSet.

kubectl patch statefulset web --type='json' -p='[{"op": "replace", "path": "/spec/template/spec/containers/0/image", "value":"gcr.io/google_containers/nginx-slim:0.7"}]'
"web" patched

Delete the web-0 Pod.

kubectl delete pod web-0
pod "web-0" deleted

Watch web-0, and wait for the Pod to transition to Running and Ready.

kubectl get pod web-0 -w
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          54s
web-0     1/1       Terminating   0         1m
web-0     0/1       Terminating   0         1m
web-0     0/1       Terminating   0         1m
web-0     0/1       Terminating   0         1m
web-0     0/1       Pending   0         0s
web-0     0/1       Pending   0         0s
web-0     0/1       ContainerCreating   0         0s
web-0     1/1       Running   0         3s

Get the Pods to view their container images.

for p in 0 1 2; do kubectl get po web-$p --template '{{range $i, $c := .spec.containers}}{{$c.image}}{{end}}'; echo; done
gcr.io/google_containers/nginx-slim:0.7
gcr.io/google_containers/nginx-slim:0.8
gcr.io/google_containers/nginx-slim:0.8

web-0 has had its image updated. Complete the update by deleting the remaining Pods.

kubectl delete pod web-1 web-2
pod "web-1" deleted
pod "web-2" deleted

Watch the Pods, and wait for all of them to transition to Running and Ready.

kubectl get pods -w -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          8m
web-1     1/1       Running   0          4h
web-2     1/1       Running   0          23m
NAME      READY     STATUS        RESTARTS   AGE
web-1     1/1       Terminating   0          4h
web-1     1/1       Terminating   0         4h
web-1     0/1       Pending   0         0s
web-1     0/1       Pending   0         0s
web-1     0/1       ContainerCreating   0         0s
web-2     1/1       Terminating   0         23m
web-2     1/1       Terminating   0         23m
web-1     1/1       Running   0         4s
web-2     0/1       Pending   0         0s
web-2     0/1       Pending   0         0s
web-2     0/1       ContainerCreating   0         0s
web-2     1/1       Running   0         36s

Get the Pods to view their container images.

for p in 0 1 2; do kubectl get po web-$p --template '{{range $i, $c := .spec.containers}}{{$c.image}}{{end}}'; echo; done
gcr.io/google_containers/nginx-slim:0.7
gcr.io/google_containers/nginx-slim:0.7
gcr.io/google_containers/nginx-slim:0.7

All the Pods in the StatefulSet are now running a new container image.

Deleting StatefulSets

StatefulSet supports both Non-Cascading and Cascading deletion. In a Non-Cascading Delete, the StatefulSet’s Pods are not deleted when the Stateful Set is deleted. In a Cascading Delete, both the StatefulSet and its Pods are deleted.

Non-Cascading Delete

In one terminal window, watch the Pods in the StatefulSet.

kubectl get pods -w -l app=nginx

Use kubectl delete to delete the StatefulSet. Make sure to supply the --cascade=false parameter to the command. This parameter tells Kubernetes to only delete the StatefulSet, and to not delete any of its Pods.

kubectl delete statefulset web --cascade=false
statefulset "web" deleted

Get the Pods to examine their status.

kubectl get pods -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          6m
web-1     1/1       Running   0          7m
web-2     1/1       Running   0          5m

Even though web has been deleted, all of the Pods are still Running and Ready. Delete web-0.

kubectl delete pod web-0
pod "web-0" deleted

Get the StatefulSet’s Pods.

kubectl get pods -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-1     1/1       Running   0          10m
web-2     1/1       Running   0          7m

As the web StatefulSet has been deleted, web-0 has not been relaunched.

In one terminal, watch the StatefulSet’s Pods.

kubectl get pods -w -l app=nginx

In a second terminal, recreate the StatefulSet. Note that, unless you deleted the nginx Service ( which you should not have ), you will see an error indicating that the Service already exists.

kubectl create -f web.yaml 
statefulset "web" created
Error from server (AlreadyExists): error when creating "web.yaml": services "nginx" already exists

Ignore the error. It only indicates that an attempt was made to create the nginx Headless Service even though that Service already exists.

Examine the output of the kubectl get command running in the first terminal.

kubectl get pods -w -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-1     1/1       Running   0          16m
web-2     1/1       Running   0          2m
NAME      READY     STATUS    RESTARTS   AGE
web-0     0/1       Pending   0          0s
web-0     0/1       Pending   0         0s
web-0     0/1       ContainerCreating   0         0s
web-0     1/1       Running   0         18s
web-2     1/1       Terminating   0         3m
web-2     0/1       Terminating   0         3m
web-2     0/1       Terminating   0         3m
web-2     0/1       Terminating   0         3m

When the web StatefulSet was recreated, it first relaunched web-0. Since web-1 was already Running and Ready, when web-0 transitioned to Running and Ready, it simply adopted this Pod. Since you recreated the StatefulSet with replicas equal to 2, once web-0 had been recreated, and once web-1 had been determined to already be Running and Ready, web-2 was terminated.

Let’s take another look at the contents of the index.html file served by the Pods’ webservers.

for i in 0 1; do kubectl exec -it web-$i -- curl localhost; done
web-0
web-1

Even though you deleted both the StatefulSet and the web-0 Pod, it still serves the hostname originally entered into its index.html file. This is because the StatefulSet never deletes the PersistentVolumes associated with a Pod. When you recreated the StatefulSet and it relaunched web-0, its original PersistentVolume was remounted.

Cascading Delete

In one terminal window, watch the Pods in the StatefulSet.

kubectl get pods -w -l app=nginx

In another terminal, delete the StatefulSet again. This time, omit the --cascade=false parameter.

kubectl delete statefulset web
statefulset "web" deleted

Examine the output of the kubectl get command running in the first terminal, and wait for all of the Pods to transition to Terminating.

kubectl get pods -w -l app=nginx
NAME      READY     STATUS    RESTARTS   AGE
web-0     1/1       Running   0          11m
web-1     1/1       Running   0          27m
NAME      READY     STATUS        RESTARTS   AGE
web-0     1/1       Terminating   0          12m
web-1     1/1       Terminating   0         29m
web-0     0/1       Terminating   0         12m
web-0     0/1       Terminating   0         12m
web-0     0/1       Terminating   0         12m
web-1     0/1       Terminating   0         29m
web-1     0/1       Terminating   0         29m
web-1     0/1       Terminating   0         29m

As you saw in the Scaling Down section, the Pods are terminated one at a time, with respect to the reverse order of their ordinal indices. Before terminating a Pod, the StatefulSet controller waits for the Pod’s successor to be completely terminated.

Note that, while a cascading delete will delete the StatefulSet and its Pods, it will not delete the Headless Service associated with the StatefulSet. You must delete the nginx Service manually.

kubectl delete service nginx
service "nginx" deleted

Recreate the StatefulSet and Headless Service one more time.

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

When all of the StatefulSet’s Pods transition to Running and Ready, retrieve the contents of their index.html files.

for i in 0 1; do kubectl exec -it web-$i -- curl localhost; done
web-0
web-1

Even though you completely deleted the StatefulSet, and all of its Pods, the Pods are recreated with their PersistentVolumes mounted, and web-0 and web-1 will still serve their hostnames.

Finally delete the web StatefulSet and the nginx service.

kubectl delete service nginx
service "nginx" deleted

kubectl delete statefulset web
statefulset "web" deleted

Cleaning up

You will need to delete the persistent storage media for the PersistentVolumes used in this tutorial. Follow the necessary steps, based on your environment, storage configuration, and provisioning method, to ensure that all storage is reclaimed.

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