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Minimum Ready Seconds for StatefulSets
Authors: Ravi Gudimetla (Red Hat), Maciej Szulik (Red Hat)
This blog describes the notion of Availability for
StatefulSet workloads, and a new alpha feature in Kubernetes 1.22 which adds
minReadySeconds configuration for
What problems does this solve?
Prior to Kubernetes 1.22 release, once a
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
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
Services with cloud providers. Since
minReadySeconds adds latency after a
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
StatefulSets is pretty useful and this feature helps in solving the above problems. This is a feature that already exists for
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
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
StatefulSet's status is an integer field which tracks the number of pods that are
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
After successfully starting
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