In addition to Kubernetes core components like api-server, scheduler, controller-manager running on a master machine there are a number of add-ons which, for various reasons, must run on a regular cluster node (rather than the Kubernetes master). Some of these add-ons are critical to a fully functional cluster, such as metrics-server, DNS, and UI. A cluster may stop working properly if a critical add-on is evicted (either manually or as a side effect of another operation like upgrade) and becomes pending (for example when the cluster is highly utilized and either there are other pending pods that schedule into the space vacated by the evicted critical add-on pod or the amount of resources available on the node changed for some other reason).
To be considered critical, the pod has to run in the
kube-system namespace (configurable via flag) and
scheduler.alpha.kubernetes.io/critical-podas key and empty string as value to your pod, but this annotation is deprecated as of version 1.13 and will be removed in 1.14.
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