Minimum Ready Seconds for StatefulSets

This blog describes the notion of Availability for StatefulSet workloads, and a new alpha feature in Kubernetes 1.22 which adds minReadySeconds configuration for StatefulSets.

What problems does this solve?

Prior to Kubernetes 1.22 release, once a StatefulSet Pod is in the Ready state it is considered Available to receive traffic. For some of the StatefulSet workloads, it may not be the case. For example, a workload like Prometheus with multiple instances of Alertmanager, it should be considered Available only when Alertmanager's state transfer is complete, not when the Pod is in Ready state. Since minReadySeconds adds buffer, the state transfer may be complete before the Pod becomes Available. While this is not a fool proof way of identifying if the state transfer is complete or not, it gives a way to the end user to express their intention of waiting for sometime before the Pod is considered Available and it is ready to serve requests.

Another case, where minReadySeconds helps is when using LoadBalancer Services with cloud providers. Since minReadySeconds adds latency after a Pod is Ready, it provides buffer time to prevent killing pods in rotation before new pods show up. Imagine a load balancer in unhappy path taking 10-15s to propagate. If you have 2 replicas then, you'd kill the second replica only after the first one is up but in reality, first replica cannot be seen because it is not yet ready to serve requests.

So, in general, the notion of Availability in StatefulSets is pretty useful and this feature helps in solving the above problems. This is a feature that already exists for Deployments and DaemonSets and we now have them for StatefulSets too to give users consistent workload experience.

How does it work?

The statefulSet controller watches for both StatefulSets and the Pods associated with them. When the feature gate associated with this feature is enabled, the statefulSet controller identifies how long a particular Pod associated with a StatefulSet has been in the Running state.

If this value is greater than or equal to the time specified by the end user in .spec.minReadySeconds field, the statefulSet controller updates a field called availableReplicas in the StatefulSet's status subresource to include this Pod. The status.availableReplicas in StatefulSet's status is an integer field which tracks the number of pods that are Available.

How do I use it?

You are required to prepare the following things in order to try out the feature:

• Download and install a kubectl greater than v1.22.0 version
• Switch on the feature gate with the command line flag --feature-gates=StatefulSetMinReadySeconds=true on kube-apiserver and kube-controller-manager

After successfully starting kube-apiserver and kube-controller-manager, you will see AvailableReplicas in the status and minReadySeconds of spec (with a default value of 0).

Specify a value for minReadySeconds for any StatefulSet and you can check if Pods are available or not by checking AvailableReplicas field using: kubectl get statefulset/<name_of_the_statefulset> -o yaml

How can I learn more?

• Read the KEP: minReadySeconds for StatefulSets
• Read the documentation: Minimum ready seconds for StatefulSet
• Review the API definition for StatefulSet

How do I get involved?

Please reach out to us in the #sig-apps channel on Slack (visit https://slack.k8s.io/ for an invitation if you need one), or on the SIG Apps mailing list: kubernetes-sig-apps@googlegroups.com

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