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Kubernetes 1.25: Two Features for Apps Rollouts Graduate to Stable

Authors: Ravi Gudimetla (Apple), Filip Křepinský (Red Hat), Maciej Szulik (Red Hat)

This blog describes the two features namely minReadySeconds for StatefulSets and maxSurge for DaemonSets that SIG Apps is happy to graduate to stable in Kubernetes 1.25.

Specifying minReadySeconds slows down a rollout of a StatefulSet, when using a RollingUpdate value in .spec.updateStrategy field, by waiting for each pod for a desired time. This time can be used for initializing the pod (e.g. warming up the cache) or as a delay before acknowledging the pod.

maxSurge allows a DaemonSet workload to run multiple instances of the same pod on a node during a rollout when using a RollingUpdate value in .spec.updateStrategy field. This helps to minimize the downtime of the DaemonSet for consumers.

These features were already available in a Deployment and other workloads. This graduation helps to align this functionality across the workloads.

What problems do these features solve?

minReadySeconds for StatefulSets

minReadySeconds ensures that the StatefulSet workload is Ready for the given number of seconds before reporting the pod as Available. The notion of being Ready and Available is quite important for workloads. For example, some workloads, like Prometheus with multiple instances of Alertmanager, should be considered Available only when the Alertmanager's state transfer is complete. minReadySeconds also helps when using loadbalancers with cloud providers. Since the pod should be Ready for the given number of seconds, it provides buffer time to prevent killing pods in rotation before new pods show up.

maxSurge for DaemonSets

Kubernetes system-level components like CNI, CSI are typically run as DaemonSets. These components can have impact on the availability of the workloads if those DaemonSets go down momentarily during the upgrades. The feature allows DaemonSet pods to temporarily increase their number, thereby ensuring zero-downtime for the DaemonSets.

Please note that the usage of hostPort in conjunction with maxSurge in DaemonSets is not allowed as DaemonSet pods are tied to a single node and two active pods cannot share the same port on the same node.

How does it work?

minReadySeconds for StatefulSets

The StatefulSet controller watches for the StatefulSet pods and counts how long a particular pod has been in the Running state, if this value is greater than or equal to the time specified in .spec.minReadySeconds field of the StatefulSet, the StatefulSet controller updates the AvailableReplicas field in the StatefulSet's status.

maxSurge for DaemonSets

The DaemonSet controller creates the additional pods (above the desired number resulting from DaemonSet spec) based on the value given in .spec.strategy.rollingUpdate.maxSurge. The additional pods would run on the same node where the old DaemonSet pod is running till the old pod gets killed.

  • The default value is 0.
  • The value cannot be 0 when MaxUnavailable is 0.
  • The value can be specified either as an absolute number of pods, or a percentage (rounded up) of desired pods.

How do I use it?

minReadySeconds for StatefulSets

Specify a value for minReadySeconds for any StatefulSet and check if pods are available or not by inspecting AvailableReplicas field using:

kubectl get statefulset/<name_of_the_statefulset> -o yaml

Please note that the default value of minReadySeconds is 0.

maxSurge for DaemonSets

Specify a value for .spec.updateStrategy.rollingUpdate.maxSurge and set .spec.updateStrategy.rollingUpdate.maxUnavailable to 0.

Then observe a faster rollout and higher number of pods running at the same time in the next rollout.

kubectl rollout restart daemonset <name_of_the_daemonset>
kubectl get pods -w

How can I learn more?

minReadySeconds for StatefulSets

maxSurge for DaemonSets

How do I get involved?

Please reach out to us on #sig-apps channel on Slack, or through the SIG Apps mailing list kubernetes-sig-apps@googlegroups.com.