StatefulSets
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.
- 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, 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 using a workload object that provides a set of stateless replicas. Deployment or ReplicaSet may be better suited to your stateless needs.
Limitations
- 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.
- StatefulSets do not provide any guarantees on the termination of pods when a StatefulSet is deleted. To achieve ordered and graceful termination of the pods in the StatefulSet, it is possible to scale the StatefulSet down to 0 prior to deletion.
- When using Rolling Updates with the default
Pod Management Policy (
OrderedReady
), it’s possible to get into a broken state that requires manual intervention to repair.
Components
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:
- 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.
The name of a StatefulSet object must be a valid DNS subdomain name.
Pod 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.
As mentioned in the limitations section, you are responsible for creating the Headless Service responsible for the network identity of the pods.
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: Cluster Domain will be set tocluster.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 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.
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. This option only affects the behavior for scaling operations. Updates are not
affected.
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.
Forced Rollback
When using Rolling Updates with the default
Pod Management Policy (OrderedReady
),
it’s possible to get into a broken state that requires manual intervention to repair.
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.
What's next
- Follow an example of deploying a stateful application.
- Follow an example of deploying Cassandra with Stateful Sets.
- Follow an example of running a replicated stateful application.
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