StatefulSets
StatefulSet is the workload API object used to manage stateful applications.
Note: StatefulSets are stable (GA) in 1.9.
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.
A StatefulSet operates under the same pattern as any other Controller. You define your desired state in a StatefulSet object, and the StatefulSet controller makes any necessary updates to get there from the current state.
- Using StatefulSets
- Limitations
- Components
- Pod Selector
- Pod Identity
- Deployment and Scaling Guarantees
- Update Strategies
- What's next
Using StatefulSets
StatefulSets 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.
- Ordered, automated rolling updates.
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 with a controller that provides a set of stateless replicas. Controllers such as Deployment or ReplicaSet may be better suited to your stateless needs.
Limitations
- StatefulSet was a beta resource prior to 1.9 and not available in any Kubernetes release prior to 1.5.
- As with all alpha/beta resources, you can disable StatefulSet through the
--runtime-configoption passed to the apiserver. - The storage for a given Pod must either be provisioned by a PersistentVolume Provisioner based on the requested
storage class, or pre-provisioned by an admin. - 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.
- StatefulSets currently require a Headless Service to be responsible for the network identity of the Pods. You are responsible for creating this Service.
Components
The example below demonstrates the components of a StatefulSet.
- A Headless Service, named nginx, is used to control the network domain.
- The StatefulSet, 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 PersistentVolumes provisioned by a PersistentVolume Provisioner.
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: 1GiPod Selector
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.
Pod Identity
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.
Ordinal Index
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.
Stable Network ID
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
is $(statefulset name)-$(ordinal). The example above will create three Pods
named web-0,web-1,web-2.
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
cluster domain.
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 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 |
|---|---|---|---|---|---|
| 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.
Stable Storage
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.
Pod Name Label
When the StatefulSet controller 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.
Deployment and Scaling Guarantees
- For a StatefulSet 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.
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.
Pod Management Policies
In Kubernetes 1.7 and later, StatefulSet allows you to relax its ordering guarantees while
preserving its uniqueness and identity guarantees via its .spec.podManagementPolicy field.
OrderedReady Pod Management
OrderedReady pod management is the default for StatefulSets. It implements the behavior
described above.
Parallel Pod Management
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.
Update Strategies
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.
On Delete
The 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 .spec.template.
Rolling Updates
The 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 RollingUpdate, the
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.
Partitions
The 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
StatefulSet’s .spec.updateStrategy.rollingUpdate.partition is greater than its .spec.replicas,
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.
What's next
- Follow an example of deploying a stateful application.
- Follow an example of deploying Cassandra with Stateful Sets.