---
assignees:
- bprashanth
- enisoc
- erictune
- foxish
- janetkuo
- kow3ns
- smarterclayton
---

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**Stateful Sets are a beta feature in 1.5. This feature replaces the 
Pet Sets feature from 1.4. Users of Pet Sets are referred to the 1.5 
[Upgrade Guide](/docs/tasks/stateful-set/upgrade-from-petsets-to-stateful-sets/)
for further information on how to upgrade existing Pet Sets to Stateful Sets.**

A Stateful Set is a Controller that provides a unique identity to its Pods. It provides
guarantees about the ordering of deployment and scaling.
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### When to Use a Stateful Set
Stateful Sets are valuable for applications that require one or more of the 
following.

* Stable, unique network identifiers.
* Stable, persistent storage.
* Ordered, graceful deployment and scaling.
* Ordered, graceful deletion and termination.

In the above, by stable, we mean persistent across Pod (re) schedulings.
As it is generally easier to manage, if an application doesn't require any of 
the above guarantees, and if it is feasible to do so, it should be deployed as 
a set of stateless replicas.

### Limitations
* Stateful Set is a beta resource, not available in any Kubernetes release prior to 1.5.
* As with all alpha/beta resources, it can be disabled through the `--runtime-config` option passed to the apiserver.
* The storage for a given Pod 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.
* Deleting and/or scaling a Stateful Set down will *not* delete the volumes associated with the Stateful Set. This is done to ensure safety first, your data is more valuable than an auto purge of all related Stateful Set resources. 
* Stateful Sets currently require a [Headless Service](/docs/user-guide/services/#headless-services) to be responsible for the network identity of the Pods. The user is responsible for this Service.
* Updating an existing Stateful Set is currently a manual process, meaning you either need to deploy a new Stateful Set with the new image version, or orphan Pods one by one, update their image, and join them back to the cluster.

### Components
The example below demonstrates the components of a Stateful Set. 

* A [Headless Service](/docs/user-guide/services/#headless-services), named nginx, is used to control the network domain. 
* The Stateful Set, named web, has a Spec that indicates that 3 replicas of the nginx container will be launched in unique Pods.
* The volumeClaimTemplates, will provide stable storage using [Persistent Volumes](/docs/user-guide/volumes/) provisioned by a 
 [Persistent Volume Provisioner](http://releases.k8s.io/{{page.githubbranch}}/examples/experimental/persistent-volume-provisioning/README.md).

```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: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      terminationGracePeriodSeconds: 10
      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
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi
```

### Pod Identity
Stateful Set 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.

__Ordinal Index__

For a Stateful Set with N replicas, each Pod in the Stateful Set will be 
assigned an integer ordinal, in the range [0,N), that is unique over the Set. 

__Stable Network ID__

The hostname of a Pod in a Stateful Set is derived from the name of the 
Stateful Set and the ordinal of the Pod. The pattern for the constructed hostname 
is `$(statefulset name)-$(ordinal)`. The example above will create three Pods 
named `web-0,web-1,web-2`.
A Stateful Set can use a [Headless Service](/docs/user-guide/services/#headless-services)
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 [cluster domain](http://releases.k8s.io/{{page.githubbranch}}/build/kube-dns/README.md#how-do-i-configure-it). 
As each Pod is created, it gets a matching DNS subdomain, taking the form: 
`$(podname).$(governing service domain)`, where the governing service is defined 
by the `serviceName` field on the Stateful Set.

Here are some examples of choices for Cluster Domain, Service name, 
Stateful Set name, and how that affects the DNS names for the Stateful Set's Pods.

Cluster Domain | Service (ns/name) | Stateful Set (ns/name)  | Stateful Set Domain  | Pod DNS | Pod Hostname |
-------------- | ----------------- | ----------------- | -------------- | ------- | ------------ |
 cluster.local | default/nginx     | default/web       | nginx.default.svc.cluster.local | web-{0..N-1}.nginx.default.svc.cluster.local | web-{0..N-1} |
 cluster.local | foo/nginx         | foo/web           | nginx.foo.svc.cluster.local     | web-{0..N-1}.nginx.foo.svc.cluster.local     | web-{0..N-1} |
 kube.local    | foo/nginx         | foo/web           | nginx.foo.svc.kube.local        | web-{0..N-1}.nginx.foo.svc.kube.local        | web-{0..N-1} |

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).

__Stable Storage__

[Persistent Volumes](/docs/user-guide/volumes/), one for each Volume Claim Template, 
are created based on the `volumeClaimTemplates` field of the Stateful Set. In the 
example above, each Pod will receive a single persistent volume with a storage 
class of `anything` and 1 Gib of provisioned storage. When a Pod is (re) scheduled onto
a node, its `volumeMounts` mount the Persistent Volumes associated with its 
Persistent Volume Claims. Note that, the Persistent Volumes associated with the 
Pods' Persistent Volume  Claims are not deleted when the Pods, or Stateful Set are deleted. 
This must be done manually.

### Deployment and Scaling Guarantee

* For a Stateful Set with N replicas, when Pods are being deployed, they are created sequentially, in order from {0..N-1}. 
* When Pods are being deleted, they are terminated in reverse order, from {N-1..0}.
* Before a scaling operation is applied to a Pod, all of its predecessors must be Running and Ready. 
* Before a Pod is terminated, all of its successors must be completely shutdown.

When the web 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](/docs/user-guide/pod-states), 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 Stateful Set 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.
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