StatefulSet is the workload API object used to manage stateful applications.
Manages the deployment and scaling of a set of PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. , and provides guarantees about the ordering and uniqueness of these Pods.
Like a DeploymentAn API object that manages a replicated application. , a StatefulSet manages Pods that are based on an identical container spec. Unlike a Deployment, a StatefulSet maintains a sticky identity for each of their Pods. These pods are created from the same spec, but are not interchangeable: each has a persistent identifier that it maintains across any rescheduling.
StatefulSets are valuable for applications that require one or more of the following.
In the above, stable is synonymous with persistence across Pod (re)scheduling. If an application doesn’t require any stable identifiers or ordered deployment, deletion, or scaling, you should deploy your application using a workload object that provides a set of stateless replicas. Deployment or ReplicaSet may be better suited to your stateless needs.
storage class, or pre-provisioned by an admin.
OrderedReady), it’s possible to get into a broken state that requires manual intervention to repair.
The example below demonstrates the components of a StatefulSet.
apiVersion: v1 kind: Service metadata: name: nginx labels: app: nginx spec: ports: - port: 80 name: web clusterIP: None selector: app: nginx --- apiVersion: apps/v1 kind: StatefulSet metadata: name: web spec: selector: matchLabels: app: nginx # has to match .spec.template.metadata.labels serviceName: "nginx" replicas: 3 # by default is 1 template: metadata: labels: app: nginx # has to match .spec.selector.matchLabels spec: terminationGracePeriodSeconds: 10 containers: - name: nginx image: k8s.gcr.io/nginx-slim:0.8 ports: - containerPort: 80 name: web volumeMounts: - name: www mountPath: /usr/share/nginx/html volumeClaimTemplates: - metadata: name: www spec: accessModes: [ "ReadWriteOnce" ] storageClassName: "my-storage-class" resources: requests: storage: 1Gi
In the above example:
nginx, is used to control the network domain.
web, has a Spec that indicates that 3 replicas of the nginx container will be launched in unique Pods.
volumeClaimTemplateswill provide stable storage using PersistentVolumes provisioned by a PersistentVolume Provisioner.
You must set the
.spec.selector field of a StatefulSet to match the labels of its
.spec.template.metadata.labels. Prior to Kubernetes 1.8, the
.spec.selector field was defaulted when omitted. In 1.8 and later versions, failing to specify a matching Pod Selector will result in a validation error during StatefulSet creation.
StatefulSet Pods have a unique identity that is comprised of an ordinal, a stable network identity, and stable storage. The identity sticks to the Pod, regardless of which node it’s (re)scheduled on.
For a StatefulSet with N replicas, each Pod in the StatefulSet will be assigned an integer ordinal, from 0 up through N-1, that is unique over the Set.
Each Pod in a StatefulSet derives its hostname from the name of the StatefulSet
and the ordinal of the Pod. The pattern for the constructed hostname
$(statefulset name)-$(ordinal). The example above will create three Pods
A StatefulSet can use a Headless Service
to control the domain of its Pods. The domain managed by this Service takes the form:
$(service name).$(namespace).svc.cluster.local, where “cluster.local” is the
As each Pod is created, it gets a matching DNS subdomain, taking the form:
$(podname).$(governing service domain), where the governing service is defined
serviceName field on the StatefulSet.
Here are some examples of choices for Cluster Domain, Service name, StatefulSet name, and how that affects the DNS names for the StatefulSet’s Pods.
|Cluster Domain||Service (ns/name)||StatefulSet (ns/name)||StatefulSet Domain||Pod DNS||Pod Hostname|
Note: Cluster Domain will be set to
cluster.localunless otherwise configured.
Kubernetes creates one PersistentVolume for each
VolumeClaimTemplate. In the nginx example above, each Pod will receive a single PersistentVolume
with a StorageClass of
my-storage-class and 1 Gib of provisioned storage. If no StorageClass
is specified, then the default StorageClass will be used. When a Pod is (re)scheduled
onto a node, its
volumeMounts mount the PersistentVolumes associated with its
PersistentVolume Claims. Note that, the PersistentVolumes associated with the
Pods’ PersistentVolume Claims are not deleted when the Pods, or StatefulSet are deleted.
This must be done manually.
When the StatefulSet ControllerA control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state. creates a Pod,
it adds a label,
statefulset.kubernetes.io/pod-name, that is set to the name of
the Pod. This label allows you to attach a Service to a specific Pod in
The StatefulSet should not specify a
pod.Spec.TerminationGracePeriodSeconds of 0. This practice is unsafe and strongly discouraged. For further explanation, please refer to force deleting StatefulSet Pods.
When the nginx example above is created, three Pods will be deployed in the order web-0, web-1, web-2. web-1 will not be deployed before web-0 is Running and Ready, and web-2 will not be deployed until web-1 is Running and Ready. If web-0 should fail, after web-1 is Running and Ready, but before web-2 is launched, web-2 will not be launched until web-0 is successfully relaunched and becomes Running and Ready.
If a user were to scale the deployed example by patching the StatefulSet such that
replicas=1, web-2 would be terminated first. web-1 would not be terminated until web-2
is fully shutdown and deleted. If web-0 were to fail after web-2 has been terminated and
is completely shutdown, but prior to web-1’s termination, web-1 would not be terminated
until web-0 is Running and Ready.
In Kubernetes 1.7 and later, StatefulSet allows you to relax its ordering guarantees while
preserving its uniqueness and identity guarantees via its
OrderedReady pod management is the default for StatefulSets. It implements the behavior
Parallel pod management tells the StatefulSet controller to launch or
terminate all Pods in parallel, and to not wait for Pods to become Running
and Ready or completely terminated prior to launching or terminating another
Pod. This option only affects the behavior for scaling operations. Updates are not
In Kubernetes 1.7 and later, StatefulSet’s
.spec.updateStrategy field allows you to configure
and disable automated rolling updates for containers, labels, resource request/limits, and
annotations for the Pods in a StatefulSet.
OnDelete update strategy implements the legacy (1.6 and prior) behavior. When a StatefulSet’s
.spec.updateStrategy.type is set to
OnDelete, the StatefulSet controller will not automatically
update the Pods in a StatefulSet. Users must manually delete Pods to cause the controller to
create new Pods that reflect modifications made to a StatefulSet’s
RollingUpdate update strategy implements automated, rolling update for the Pods in a
StatefulSet. It is the default strategy when
.spec.updateStrategy is left unspecified. When a StatefulSet’s
.spec.updateStrategy.type is set to
StatefulSet controller will delete and recreate each Pod in the StatefulSet. It will proceed
in the same order as Pod termination (from the largest ordinal to the smallest), updating
each Pod one at a time. It will wait until an updated Pod is Running and Ready prior to
updating its predecessor.
RollingUpdate update strategy can be partitioned, by specifying a
.spec.updateStrategy.rollingUpdate.partition. If 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. All Pods with an ordinal that is less than the partition will not
be updated, and, even if they are deleted, they will be recreated at the previous version. If a
.spec.updateStrategy.rollingUpdate.partition is greater than its
updates to its
.spec.template will not be propagated to its Pods.
In most cases you will not need to use a partition, but they are useful if you want to stage an
update, roll out a canary, or perform a phased roll out.
If you update the Pod template to a configuration that never becomes Running and Ready (for example, due to a bad binary or application-level configuration error), StatefulSet will stop the rollout and wait.
In this state, it’s not enough to revert the Pod template to a good configuration. Due to a known issue, StatefulSet will continue to wait for the broken Pod to become Ready (which never happens) before it will attempt to revert it back to the working configuration.
After reverting the template, you must also delete any Pods that StatefulSet had already attempted to run with the bad configuration. StatefulSet will then begin to recreate the Pods using the reverted template.
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