rootsongjc-pr-20170815
This commit is contained in:
Jimmy Song
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---
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approvers:
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- enisoc
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- erictune
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- foxish
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- janetkuo
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- kow3ns
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- smarterclayton
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title: StatefulSets
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---
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{% capture overview %}
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**StatefulSets are a beta feature in 1.7. This feature replaces the
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PetSets feature from 1.4. Users of PetSets are referred to the 1.5
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[Upgrade Guide](/docs/tasks/manage-stateful-set/upgrade-pet-set-to-stateful-set/)
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for further information on how to upgrade existing PetSets to StatefulSets.**
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{% include templates/glossary/snippet.md term="statefulset" length="long" %}
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{% endcapture %}
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{% capture body %}
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## Using StatefulSets
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StatefulSets are valuable for applications that require one or more of the
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following.
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* Stable, unique network identifiers.
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* Stable, persistent storage.
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* Ordered, graceful deployment and scaling.
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* Ordered, graceful deletion and termination.
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* Ordered, automated rolling updates.
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In the above, stable is synonymous with persistence across Pod (re)scheduling.
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If an application doesn't require any stable identifiers or ordered deployment,
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deletion, or scaling, you should deploy your application with a controller that
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provides a set of stateless replicas. Controllers such as
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[Deployment](/docs/concepts/workloads/controllers/deployment/) or
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[ReplicaSet](/docs/concepts/workloads/controllers/replicaset/) may be better suited to your stateless needs.
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## Limitations
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* StatefulSet is a beta resource, not available in any Kubernetes release prior to 1.5.
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* As with all alpha/beta resources, you can disable StatefulSet through the `--runtime-config` option passed to the apiserver.
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* The storage for a given Pod must either be provisioned by a [PersistentVolume Provisioner](http://releases.k8s.io/{{page.githubbranch}}/examples/persistent-volume-provisioning/README.md) based on the requested `storage class`, or pre-provisioned by an admin.
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* Deleting and/or scaling a StatefulSet down will *not* delete the volumes associated with the StatefulSet. This is done to ensure data safety, which is generally more valuable than an automatic purge of all related StatefulSet resources.
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* StatefulSets currently require a [Headless Service](/docs/concepts/services-networking/service/#headless-services) to be responsible for the network identity of the Pods. You are responsible for creating this Service.
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## Components
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The example below demonstrates the components of a StatefulSet.
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* A Headless Service, named nginx, is used to control the network domain.
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* The StatefulSet, named web, has a Spec that indicates that 3 replicas of the nginx container will be launched in unique Pods.
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* The volumeClaimTemplates will provide stable storage using [PersistentVolumes](/docs/concepts/storage/volumes/) provisioned by a
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PersistentVolume Provisioner.
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```yaml
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apiVersion: v1
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kind: Service
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metadata:
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name: nginx
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labels:
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app: nginx
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spec:
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ports:
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- port: 80
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name: web
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clusterIP: None
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selector:
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app: nginx
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---
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apiVersion: apps/v1beta1
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kind: StatefulSet
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metadata:
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name: web
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spec:
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serviceName: "nginx"
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replicas: 3
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template:
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metadata:
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labels:
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app: nginx
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spec:
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terminationGracePeriodSeconds: 10
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containers:
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- name: nginx
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image: gcr.io/google_containers/nginx-slim:0.8
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ports:
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- containerPort: 80
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name: web
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volumeMounts:
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- name: www
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mountPath: /usr/share/nginx/html
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volumeClaimTemplates:
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- metadata:
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name: www
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spec:
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accessModes: [ "ReadWriteOnce" ]
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storageClassName: my-storage-class
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resources:
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requests:
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storage: 1Gi
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```
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## Pod Identity
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StatefulSet Pods have a unique identity that is comprised of an ordinal, a
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stable network identity, and stable storage. The identity sticks to the Pod,
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regardless of which node it's (re)scheduled on.
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### Ordinal Index
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For a StatefulSet with N replicas, each Pod in the StatefulSet will be
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assigned an integer ordinal, in the range [0,N), that is unique over the Set.
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### Stable Network ID
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Each Pod in a StatefulSet derives its hostname from the name of the StatefulSet
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and the ordinal of the Pod. The pattern for the constructed hostname
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is `$(statefulset name)-$(ordinal)`. The example above will create three Pods
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named `web-0,web-1,web-2`.
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A StatefulSet can use a [Headless Service](/docs/concepts/services-networking/service/#headless-services)
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to control the domain of its Pods. The domain managed by this Service takes the form:
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`$(service name).$(namespace).svc.cluster.local`, where "cluster.local"
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is the [cluster domain](http://releases.k8s.io/{{page.githubbranch}}/cluster/addons/dns/README.md).
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As each Pod is created, it gets a matching DNS subdomain, taking the form:
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`$(podname).$(governing service domain)`, where the governing service is defined
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by the `serviceName` field on the StatefulSet.
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Here are some examples of choices for Cluster Domain, Service name,
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StatefulSet name, and how that affects the DNS names for the StatefulSet's Pods.
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Cluster Domain | Service (ns/name) | StatefulSet (ns/name) | StatefulSet Domain | Pod DNS | Pod Hostname |
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-------------- | ----------------- | ----------------- | -------------- | ------- | ------------ |
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cluster.local | default/nginx | default/web | nginx.default.svc.cluster.local | web-{0..N-1}.nginx.default.svc.cluster.local | web-{0..N-1} |
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cluster.local | foo/nginx | foo/web | nginx.foo.svc.cluster.local | web-{0..N-1}.nginx.foo.svc.cluster.local | web-{0..N-1} |
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kube.local | foo/nginx | foo/web | nginx.foo.svc.kube.local | web-{0..N-1}.nginx.foo.svc.kube.local | web-{0..N-1} |
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Note that Cluster Domain will be set to `cluster.local` unless
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[otherwise configured](http://releases.k8s.io/{{page.githubbranch}}/cluster/addons/dns/README.md).
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### Stable Storage
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Kubernetes creates one [PersistentVolume](/docs/concepts/storage/volumes/) for each
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VolumeClaimTemplate. In the nginx example above, each Pod will receive a single PersistentVolume
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with a StorageClass of `my-storage-class` and 1 Gib of provisioned storage. If no StorageClass
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is specified, then the default StorageClass will be used. When a Pod is (re)scheduled
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onto a node, its `volumeMounts` mount the PersistentVolumes associated with its
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PersistentVolume Claims. Note that, the PersistentVolumes associated with the
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Pods' PersistentVolume Claims are not deleted when the Pods, or StatefulSet are deleted.
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This must be done manually.
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## Deployment and Scaling Guarantees
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* For a StatefulSet with N replicas, when Pods are being deployed, they are created sequentially, in order from {0..N-1}.
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* When Pods are being deleted, they are terminated in reverse order, from {N-1..0}.
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* Before a scaling operation is applied to a Pod, all of its predecessors must be Running and Ready.
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* Before a Pod is terminated, all of its successors must be completely shutdown.
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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](/docs/tasks/run-application/force-delete-stateful-set-pod/).
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When the nginx example above is created, three Pods will be deployed in the order
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web-0, web-1, web-2. web-1 will not be deployed before web-0 is
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[Running and Ready](/docs/user-guide/pod-states), and web-2 will not be deployed until
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web-1 is Running and Ready. If web-0 should fail, after web-1 is Running and Ready, but before
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web-2 is launched, web-2 will not be launched until web-0 is successfully relaunched and
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becomes Running and Ready.
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If a user were to scale the deployed example by patching the StatefulSet such that
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`replicas=1`, web-2 would be terminated first. web-1 would not be terminated until web-2
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is fully shutdown and deleted. If web-0 were to fail after web-2 has been terminated and
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is completely shutdown, but prior to web-1's termination, web-1 would not be terminated
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until web-0 is Running and Ready.
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### Pod Management Policies
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In Kubernetes 1.7 and later, StatefulSet allows you to relax its ordering guarantees while
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preserving its uniqueness and identity guarantees via its `.spec.podManagementPolicy` field.
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#### OrderedReady Pod Management
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`OrderedReady` pod management is the default for StatefulSets. It implements the behavior
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described [above](#deployment-and-scaling-guarantees).
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#### Parallel Pod Management
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`Parallel` pod management tells the StatefulSet controller to launch or
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terminate all Pods in parallel, and to not wait for Pods to become Running
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and Ready or completely terminated prior to launching or terminating another
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Pod.
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## Update Strategies
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In Kubernetes 1.7 and later, StatefulSet's `.spec.updateStrategy` field allows you to configure
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and disable automated rolling updates for containers, labels, resource request/limits, and
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annotations for the Pods in a StatefulSet.
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### On Delete
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The `OnDelete` update strategy implements the legacy (1.6 and prior) behavior. It is the default
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strategy when `spec.updateStrategy` is left unspecified. When a StatefulSet's
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`.spec.updateStrategy.type` is set to `OnDelete`, the StatefulSet controller will not automatically
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update the Pods in a StatefulSet. Users must manually delete Pods to cause the controller to
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create new Pods that reflect modifications made to a StatefulSet's `.spec.template`.
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### Rolling Updates
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The `RollingUpdate` update strategy implements automated, rolling update for the Pods in a
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StatefulSet. When a StatefulSet's `.spec.updateStrategy.type` is set to `RollingUpdate`, the
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StatefulSet controller will delete and recreate each Pod in the StatefulSet. It will proceed
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in the same order as Pod termination (from the largest ordinal to the smallest), updating
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each Pod one at a time. It will wait until an updated Pod is Running and Ready prior to
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updating its predecessor.
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#### Partitions
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The `RollingUpdate` update strategy can be partitioned, by specifying a
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`.spec.updateStrategy.rollingUpdate.partition`. If a partition is specified, all Pods with an
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ordinal that is greater than or equal to the partition will be updated when the StatefulSet's
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`.spec.template` is updated. All Pods with an ordinal that is less than the partition will not
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be updated, and, even if they are deleted, they will be recreated at the previous version. If a
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StatefulSet's `.spec.updateStrategy.rollingUpdate.partition` is greater than its `.spec.replicas`,
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updates to its `.spec.template` will not be propagated to its Pods.
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In most cases you will not need to use a partition, but they are useful if you want to stage an
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update, roll out a canary, or perform a phased roll out.
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{% endcapture %}
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{% capture whatsnext %}
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* Follow an example of [deploying a stateful application](/docs/tutorials/stateful-application/basic-stateful-set).
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{% endcapture %}
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{% include templates/concept.md %}
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