Initial commit of statefulsets concept.
Kenneth Owens
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@ -5,7 +5,9 @@ The Concepts section of the Kubernetes documentation is a work in progress.
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#### Object Metadata
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[Annotations](/docs/concepts/object-metadata/annotations/)
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* [StatefulSets](/docs/concepts/object-metadata/statefulsets/)
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* [Annotations](/docs/concepts/object-metadata/annotations/)
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### What's next
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@ -0,0 +1,164 @@
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---
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---
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{% capture overview %}
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**StatefulSets are a beta feature in 1.5. This feature replaces the deprecated
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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/task/upgrade-to-statefulset)
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for further information on how to upgrade existing PetSets to StatefulSets.**
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A StatefulSet is a Controller that ensures that, at most, a given number of
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replicas of a Pod are running at a time. Pods in a Stateful Set have an ordinal
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(a unique integer index in the StatefulSet), a stable, unique network id that is
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avialable in DNS, and stable, persistent storage.
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For a StatefulSet with N replicas, when Pods are being deployed, they are
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created sequentially, in order from {0..N-1}. Before a new Pod is deployed, all
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of its predecessors must be [Running and Ready](/docs/user-guide/pod-states).
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When Pods are being deleted, they are terminated in reverse order, from {N-1..0},
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and no pod is terminated until its successors have been terminated and are
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completely shutdown or its [Termination Grace Period](/docs/user-guide/pods/index#termination-of-pods))
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has elapsed.
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The exmpale below demonstrates the components of a StatefulSet.
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* A [Headless Service](/docs/user-guide/services/#headless-services), 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 [Persistent Volumes](/docs/user-guide/volumes/) provisioned by a
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[Persistent Volume Provisioner](http://releases.k8s.io/{{page.githubbranch}}/examples/experimental/persistent-volume-provisioning/README.md).
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```yaml
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---
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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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annotations:
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pod.alpha.kubernetes.io/initialized: "true"
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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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annotations:
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volume.alpha.kubernetes.io/storage-class: anything
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spec:
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accessModes: [ "ReadWriteOnce" ]
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resources:
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requests:
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storage: 1Gi
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```
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{% endcapture %}
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{% capture body %}
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### When to Use a Stateful Set
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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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As it is generally easier to manage, if an application doesn't require any of
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the above garuantees, and if it is feasible to do so, it should be deployed as
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a set of stateless replicas.
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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, it can be disabled through the `--runtime-config` option passed to the apiserver.
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* The only updatable field on a StatefulSet is `replicas`
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* The storage for a given pet must either be provisioned by a [Persistent Volume Provisioner](http://releases.k8s.io/{{page.githubbranch}}/examples/experimental/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 safety first, your data is more valuable than an auto purge of all related PetSet resources. **Deleting the Persistent Volume Claims will result in a deletion of the associated volumes**.
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* All StatefulSets currently require a [Headless Service](/docs/user-guide/services/#headless-services) to be responsible for the network identity of the pets. The user is responsible for this Service.
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* Updating an existing StatefulSet is currently a manual process, meaning you either need to deploy a new StatefulSet with the new image version, or orphan Pets one by one, update their image, and join them back to the cluster.
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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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assinged a integer ordinal, in the range [0,N), that is unique over the Set.
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__Stable Network Id__
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The hostname of a Pod in a StatefulSet is derived from the name of the
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StatefulSet 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 StatelefulSet can use a [Headless Service](/docs/user-guide/services/#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}}/build/kube-dns/README.md#how-do-i-configure-it).
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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) | PetSet (ns/name) | PetSet Domain | Pet DNS | Pet 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 [otherwise configured](http://releases.k8s.io/{{page.githubbranch}}/build/kube-dns/README.md#how-do-i-configure-it).
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__Stable Storage__
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[Persistent Volumes](/docs/user-guide/volumes/), one for each VolumeClaimTemplate,
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are created based on the `volumeClaimTemplates` field of the StatefulSet. In the
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example above, each Pod will recieve a single persistent volume with a storage
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class of anything and 1 Gib of provisioned storage. When a Pod is (re)scheculed,
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its volume(s) are avialable on the node on which it is launched. Note that, the
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volumes associated with the Pods' Persistent Volume Claims are not deleted when
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the Pods, or Stateful Set are deleted. This must be done manually.
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### Deployment and Scaling Garuantees
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{% endcapture %}
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When the exmample 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 until
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web-1 is Running and Ready.
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{% capture whatsnext %}
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* Learn more about [this](...).
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* See this [related task](...).
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{% endcapture %}
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{% include templates/concept.md %}
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