This tutorial provides an introduction to managing applications with StatefulSets. It demonstrates how to create, delete, scale, and update the Pods of StatefulSets.
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 not to conflate the former topic with the latter, you will deploy a simple web application using a StatefulSet.
After this tutorial, you will be familiar with the following.
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 two 1 GiB volumes prior to starting this tutorial.
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 publish the IP addresses of Pods in the StatefulSet, web
.
application/web/web.yaml
|
---|
|
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.apps/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 TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
nginx ClusterIP None <none> 80/TCP 12s
kubectl get statefulset web
NAME DESIRED CURRENT AGE
web 2 1 20s
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
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-1
Pod is not launched until the web-0
Pod is
Running and Ready.
Pods in a StatefulSet have a unique ordinal index and a stable network identity.
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
StatefulSet 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
.
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-$i -- sh -c 'hostname'; done
web-0
web-1
Use kubectl run
to execute
a container that provides the nslookup
command from the dnsutils
package.
Using nslookup
on the Pods’ hostnames, you can examine their in-cluster DNS
addresses.
kubectl run -i --tty --image busybox:1.28 dns-test --restart=Never --rm
nslookup web-0.nginx
Server: 10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-0.nginx
Address 1: 10.244.1.6
nslookup web-1.nginx
Server: 10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-1.nginx
Address 1: 10.244.2.6
The CNAME of the headless service points to SRV records (one for each Pod that is Running and Ready). The SRV records point to A record entries that contain the Pods’ IP addresses.
In one terminal, watch the StatefulSet’s Pods.
kubectl get pod -w -l app=nginx
In a second terminal, use
kubectl delete
to delete all
the Pods in the StatefulSet.
kubectl delete pod -l app=nginx
pod "web-0" deleted
pod "web-1" deleted
Wait for the StatefulSet to restart them, and for both Pods to transition to Running and Ready.
kubectl get pod -w -l app=nginx
NAME READY STATUS RESTARTS AGE
web-0 0/1 ContainerCreating 0 0s
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 2s
web-1 0/1 Pending 0 0s
web-1 0/1 Pending 0 0s
web-1 0/1 ContainerCreating 0 0s
web-1 1/1 Running 0 34s
Use kubectl exec
and kubectl run
to view the Pods hostnames and in-cluster
DNS entries.
for i in 0 1; do kubectl exec web-$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.
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
Even though web-0
and web-1
were rescheduled, they continue to serve their
hostnames because the PersistentVolumes associated with their
PersistentVolumeClaims 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 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 StatefulSet.
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 sts web --replicas=5
statefulset.apps/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.
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
three replicas.
kubectl patch sts web -p '{"spec":{"replicas":3}}'
statefulset.apps/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
The controller deleted one Pod at a time, in reverse order with respect to its ordinal index, 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.
In Kubernetes 1.7 and later, the StatefulSet controller supports automated updates. The
strategy used is determined by the spec.updateStrategy
field of the
StatefulSet API Object. This feature can be used to upgrade the container
images, resource requests and/or limits, labels, and annotations of the Pods in a
StatefulSet. There are two valid update strategies, RollingUpdate
and
OnDelete
.
RollingUpdate
update strategy is the default for StatefulSets.
The RollingUpdate
update strategy will update all Pods in a StatefulSet, in
reverse ordinal order, while respecting the StatefulSet guarantees.
Patch the web
StatefulSet to apply the RollingUpdate
update strategy.
kubectl patch statefulset web -p '{"spec":{"updateStrategy":{"type":"RollingUpdate"}}}'
statefulset.apps/web patched
In one terminal window, patch the web
StatefulSet to change the container
image again.
kubectl patch statefulset web --type='json' -p='[{"op": "replace", "path": "/spec/template/spec/containers/0/image", "value":"gcr.io/google_containers/nginx-slim:0.8"}]'
statefulset.apps/web patched
In another terminal, watch the Pods in the StatefulSet.
kubectl get po -l app=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 7m
web-1 1/1 Running 0 7m
web-2 1/1 Running 0 8m
web-2 1/1 Terminating 0 8m
web-2 1/1 Terminating 0 8m
web-2 0/1 Terminating 0 8m
web-2 0/1 Terminating 0 8m
web-2 0/1 Terminating 0 8m
web-2 0/1 Terminating 0 8m
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-1 1/1 Terminating 0 8m
web-1 0/1 Terminating 0 8m
web-1 0/1 Terminating 0 8m
web-1 0/1 Terminating 0 8m
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 6s
web-0 1/1 Terminating 0 7m
web-0 1/1 Terminating 0 7m
web-0 0/1 Terminating 0 7m
web-0 0/1 Terminating 0 7m
web-0 0/1 Terminating 0 7m
web-0 0/1 Terminating 0 7m
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 10s
The Pods in the StatefulSet are updated in reverse ordinal order. The StatefulSet controller terminates each Pod, and waits for it to transition to Running and Ready prior to updating the next Pod. Note that, even though the StatefulSet controller will not proceed to update the next Pod until its ordinal successor is Running and Ready, it will restore any Pod that fails during the update to its current version. Pods that have already received the update will be restored to the updated version, and Pods that have not yet received the update will be restored to the previous version. In this way, the controller attempts to continue to keep the application healthy and the update consistent in the presence of intermittent failures.
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
k8s.gcr.io/nginx-slim:0.8
k8s.gcr.io/nginx-slim:0.8
k8s.gcr.io/nginx-slim:0.8
All the Pods in the StatefulSet are now running the previous container image.
Tip You can also use kubectl rollout status sts/<name>
to view
the status of a rolling update.
You can stage an update to a StatefulSet by using the partition
parameter of
the RollingUpdate
update strategy. A staged update will keep all of the Pods
in the StatefulSet at the current version while allowing mutations to the
StatefulSet’s .spec.template
.
Patch the web
StatefulSet to add a partition to the updateStrategy
field.
kubectl patch statefulset web -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":3}}}}'
statefulset.apps/web patched
Patch the StatefulSet again to change the container’s image.
kubectl patch statefulset web --type='json' -p='[{"op": "replace", "path": "/spec/template/spec/containers/0/image", "value":"k8s.gcr.io/nginx-slim:0.7"}]'
statefulset.apps/web patched
Delete a Pod in the StatefulSet.
kubectl delete po web-2
pod "web-2" deleted
Wait for the Pod to be Running and Ready.
kubectl get po -l app=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 4m
web-1 1/1 Running 0 4m
web-2 0/1 ContainerCreating 0 11s
web-2 1/1 Running 0 18s
Get the Pod’s container.
kubectl get po web-2 --template '{{range $i, $c := .spec.containers}}{{$c.image}}{{end}}'
k8s.gcr.io/nginx-slim:0.8
Notice that, even though the update strategy is RollingUpdate
the StatefulSet
controller restored the Pod with its original container. This is because the
ordinal of the Pod is less than the partition
specified by the
updateStrategy
.
You can roll out a canary to test a modification by decrementing the partition
you specified above.
Patch the StatefulSet to decrement the partition.
kubectl patch statefulset web -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":2}}}}'
statefulset.apps/web patched
Wait for web-2
to be Running and Ready.
kubectl get po -l app=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 4m
web-1 1/1 Running 0 4m
web-2 0/1 ContainerCreating 0 11s
web-2 1/1 Running 0 18s
Get the Pod’s container.
kubectl get po web-2 --template '{{range $i, $c := .spec.containers}}{{$c.image}}{{end}}'
k8s.gcr.io/nginx-slim:0.7
When you changed the partition
, the StatefulSet controller automatically
updated the web-2
Pod because the Pod’s ordinal was greater than or equal to
the partition
.
Delete the web-1
Pod.
kubectl delete po web-1
pod "web-1" deleted
Wait for the web-1
Pod to be Running and Ready.
kubectl get po -l app=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 6m
web-1 0/1 Terminating 0 6m
web-2 1/1 Running 0 2m
web-1 0/1 Terminating 0 6m
web-1 0/1 Terminating 0 6m
web-1 0/1 Terminating 0 6m
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
Get the web-1
Pods container.
kubectl get po web-1 --template '{{range $i, $c := .spec.containers}}{{$c.image}}{{end}}'
k8s.gcr.io/nginx-slim:0.8
web-1
was restored to its original configuration because the Pod’s ordinal
was less than the partition. When a partition is specified, all Pods with an
ordinal that is greater than or equal to the partition will be updated when the
StatefulSet’s .spec.template
is updated. If a Pod that has an ordinal less
than the partition is deleted or otherwise terminated, it will be restored to
its original configuration.
You can perform a phased roll out (e.g. a linear, geometric, or exponential
roll out) using a partitioned rolling update in a similar manner to how you
rolled out a canary. To perform a phased roll out, set
the partition
to the ordinal at which you want the controller to pause the
update.
The partition is currently set to 2
. Set the partition to 0
.
kubectl patch statefulset web -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":0}}}}'
statefulset.apps/web patched
Wait for all of the Pods in the StatefulSet to become Running and Ready.
kubectl get po -l app=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 3m
web-1 0/1 ContainerCreating 0 11s
web-2 1/1 Running 0 2m
web-1 1/1 Running 0 18s
web-0 1/1 Terminating 0 3m
web-0 1/1 Terminating 0 3m
web-0 0/1 Terminating 0 3m
web-0 0/1 Terminating 0 3m
web-0 0/1 Terminating 0 3m
web-0 0/1 Terminating 0 3m
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 Pod’s containers.
for p in 0 1 2; do kubectl get po web-$p --template '{{range $i, $c := .spec.containers}}{{$c.image}}{{end}}'; echo; done
k8s.gcr.io/nginx-slim:0.7
k8s.gcr.io/nginx-slim:0.7
k8s.gcr.io/nginx-slim:0.7
By moving the partition
to 0
, you allowed the StatefulSet controller to
continue the update process.
The OnDelete
update strategy implements the legacy (1.6 and prior) behavior,
When you select this update strategy, the StatefulSet controller will not
automatically update Pods when a modification is made to the StatefulSet’s
.spec.template
field. This strategy can be selected by setting the
.spec.template.updateStrategy.type
to OnDelete
.
StatefulSet supports both Non-Cascading and Cascading deletion. In a Non-Cascading Delete, the StatefulSet’s Pods are not deleted when the StatefulSet is deleted. In a Cascading Delete, both the StatefulSet and its Pods are deleted.
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.apps "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.apps/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-$i -- curl localhost; done
web-0
web-1
Even though you deleted both the StatefulSet and the web-0
Pod, it still
serves the hostname originally entered into its index.html
file. This is
because the StatefulSet never deletes the PersistentVolumes associated with a
Pod. When you recreated the StatefulSet and it relaunched web-0
, its original
PersistentVolume was remounted.
In one terminal window, watch the Pods in the StatefulSet.
kubectl get pods -w -l app=nginx
In another terminal, delete the StatefulSet again. This time, omit the
--cascade=false
parameter.
kubectl delete statefulset web
statefulset.apps "web" deleted
Examine the output of the kubectl get
command running in the first terminal,
and wait for all of the Pods to transition to Terminating.
kubectl get pods -w -l app=nginx
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 11m
web-1 1/1 Running 0 27m
NAME READY STATUS RESTARTS AGE
web-0 1/1 Terminating 0 12m
web-1 1/1 Terminating 0 29m
web-0 0/1 Terminating 0 12m
web-0 0/1 Terminating 0 12m
web-0 0/1 Terminating 0 12m
web-1 0/1 Terminating 0 29m
web-1 0/1 Terminating 0 29m
web-1 0/1 Terminating 0 29m
As you saw in the Scaling Down section, the Pods are terminated one at a time, with respect to the reverse order of their ordinal indices. Before terminating a Pod, the StatefulSet controller waits for the Pod’s successor to be completely terminated.
Note that, while a cascading delete will delete the StatefulSet and its Pods,
it will not delete the Headless Service associated with the StatefulSet. You
must delete the nginx
Service manually.
kubectl delete service nginx
service "nginx" deleted
Recreate the StatefulSet and Headless Service one more time.
kubectl create -f web.yaml
service/nginx created
statefulset.apps/web created
When all of the StatefulSet’s Pods transition to Running and Ready, retrieve
the contents of their index.html
files.
for i in 0 1; do kubectl exec -it web-$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
For some distributed systems, the StatefulSet ordering guarantees are
unnecessary and/or undesirable. These systems require only uniqueness and
identity. To address this, in Kubernetes 1.7, we introduced
.spec.podManagementPolicy
to the StatefulSet API Object.
OrderedReady
pod management is the default for StatefulSets. It tells the
StatefulSet controller to respect the ordering guarantees demonstrated
above.
Parallel
pod management tells the StatefulSet controller to launch or
terminate all Pods in parallel, and not to wait for Pods to become Running
and Ready or completely terminated prior to launching or terminating another
Pod.
application/web/web-parallel.yaml
|
---|
|
Download the example above, and save it to a file named web-parallel.yaml
This manifest is identical to the one you downloaded above except that the .spec.podManagementPolicy
of the web
StatefulSet is set to Parallel
.
In one terminal, watch the Pods in the StatefulSet.
kubectl get po -l app=nginx -w
In another terminal, create the StatefulSet and Service in the manifest.
kubectl create -f web-parallel.yaml
service/nginx created
statefulset.apps/web created
Examine the output of the kubectl get
command that you executed in the first terminal.
kubectl get po -l app=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 0/1 Pending 0 0s
web-0 0/1 Pending 0 0s
web-1 0/1 Pending 0 0s
web-1 0/1 Pending 0 0s
web-0 0/1 ContainerCreating 0 0s
web-1 0/1 ContainerCreating 0 0s
web-0 1/1 Running 0 10s
web-1 1/1 Running 0 10s
The StatefulSet controller launched both web-0
and web-1
at the same time.
Keep the second terminal open, and, in another terminal window scale the StatefulSet.
kubectl scale statefulset/web --replicas=4
statefulset.apps/web scaled
Examine the output of the terminal where the kubectl get
command is running.
web-3 0/1 Pending 0 0s
web-3 0/1 Pending 0 0s
web-3 0/1 Pending 0 7s
web-3 0/1 ContainerCreating 0 7s
web-2 1/1 Running 0 10s
web-3 1/1 Running 0 26s
The StatefulSet controller launched two new Pods, and it did not wait for the first to become Running and Ready prior to launching the second.
Keep this terminal open, and in another terminal delete the web
StatefulSet.
kubectl delete sts web
Again, examine the output of the kubectl get
command running in the other terminal.
web-3 1/1 Terminating 0 9m
web-2 1/1 Terminating 0 9m
web-3 1/1 Terminating 0 9m
web-2 1/1 Terminating 0 9m
web-1 1/1 Terminating 0 44m
web-0 1/1 Terminating 0 44m
web-0 0/1 Terminating 0 44m
web-3 0/1 Terminating 0 9m
web-2 0/1 Terminating 0 9m
web-1 0/1 Terminating 0 44m
web-0 0/1 Terminating 0 44m
web-2 0/1 Terminating 0 9m
web-2 0/1 Terminating 0 9m
web-2 0/1 Terminating 0 9m
web-1 0/1 Terminating 0 44m
web-1 0/1 Terminating 0 44m
web-1 0/1 Terminating 0 44m
web-0 0/1 Terminating 0 44m
web-0 0/1 Terminating 0 44m
web-0 0/1 Terminating 0 44m
web-3 0/1 Terminating 0 9m
web-3 0/1 Terminating 0 9m
web-3 0/1 Terminating 0 9m
The StatefulSet controller deletes all Pods concurrently, it does not wait for a Pod’s ordinal successor to terminate prior to deleting that Pod.
Close the terminal where the kubectl get
command is running and delete the nginx
Service.
kubectl delete svc nginx
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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