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Blog: Kubernetes 1.27: StatefulSet Start Ordinal Simplifies Migration

Author : Peter Schuurman (Google)

Kubernetes v1.26 introduced a new, alpha-level feature forStatefulSets that controls the ordinal numbering of Pod replicas. As of Kubernetes v1.27, this feature is now beta. Ordinals can start from arbitrary non-negative numbers. This blog post will discuss how this feature can be used.


StatefulSets ordinals provide sequential identities for pod replicas. When usingOrderedReady Pod managementPods are created from ordinal index 0 up to N-1.

With Kubernetes today, orchestrating a StatefulSet migration across clusters is challenging. Backup and restore solutions exist, but these require the application to be scaled down to zero replicas prior to migration. In today's fully connected world, even planned application downtime may not allow you to meet your business goals. You could useCascading DeleteorOn Deleteto migrate individual pods, however this is error prone and tedious to manage. You lose the self-healing benefit of the StatefulSet controller when your Pods fail or are evicted.

Kubernetes v1.26 enables a StatefulSet to be responsible for a range of ordinals within a range {0..N-1} (the ordinals 0, 1, ... up to N-1). With it, you can scale down a range {0..k-1} in a source cluster, and scale up the complementary range {k..N-1} in a destination cluster, while maintaining application availability. This enables you to retain at most one semantics (meaning there is at most one Pod with a given identity running in a StatefulSet) andRolling Updatebehavior when orchestrating a migration across clusters.

Why would I want to use this feature?

Say you're running your StatefulSet in one cluster, and need to migrate it out to a different cluster. There are many reasons why you would need to do this:

  • Scalability : Your StatefulSet has scaled too large for your cluster, and has started to disrupt the quality of service for other workloads in your cluster.
  • Isolation : You're running a StatefulSet in a cluster that is accessed by multiple users, and namespace isolation isn't sufficient.
  • Cluster Configuration : You want to move your StatefulSet to a different cluster to use some environment that is not available on your current cluster.
  • Control Plane Upgrades : You want to move your StatefulSet to a cluster running an upgraded control plane, and can't handle the risk or downtime of in-place control plane upgrades.

How do I use it?

Enable the StatefulSetStartOrdinal feature gate on a cluster, and create a StatefulSet with a customized .spec.ordinals.start.

Try it out

In this demo, I'll use the new mechanism to migrate a StatefulSet from one Kubernetes cluster to another. Theredis-clusterBitnami Helm chart will be used to install Redis.

Tools Required:


To do this, I need two Kubernetes clusters that can both access common networking and storage; I've named my clusters source and destination. Specifically, I need:

  • The StatefulSetStartOrdinal feature gate enabled on both clusters.
  • Client configuration for kubectl that lets me access both clusters as an administrator.
  • The same StorageClass installed on both clusters, and set as the default StorageClass for both clusters. This StorageClass should provision underlying storage that is accessible from either or both clusters.
  • A flat network topology that allows for pods to send and receive packets to and from Pods in either clusters. If you are creating clusters on a cloud provider, this configuration may be called private cloud or private network.
  1. Create a demo namespace on both clusters:

  2. Deploy a Redis cluster with six replicas in the source cluster:

  3. Check the replication status in the source cluster:

  4. Deploy a Redis cluster with zero replicas in the destination cluster:

  5. Scale down the redis-redis-cluster StatefulSet in the source cluster by 1, to remove the replica redis-redis-cluster-5:

  6. Migrate dependencies from the source cluster to the destination cluster:

  7. Scale up the redis-redis-cluster StatefulSet in the destination cluster by 1, with a start ordinal of 5:

  8. Check the replication status in the destination cluster:

  9. Repeat steps #5 to #7 for the remainder of the replicas, until the Redis StatefulSet in the source cluster is scaled to 0, and the Redis StatefulSet in the destination cluster is healthy with 6 total replicas.

What's Next?

This feature provides a building block for a StatefulSet to be split up across clusters, but does not prescribe the mechanism as to how the StatefulSet should be migrated. Migration requires coordination of StatefulSet replicas, along with orchestration of the storage and network layer. This is dependent on the storage and connectivity requirements of the application installed by the StatefulSet. Additionally, many StatefulSets are managed byoperators, which adds another layer of complexity to migration.

If you're interested in building enhancements to make these processes easier, get involved withSIG Multiclusterto contribute!

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