Certified Kubernetes Administrator Study Guide – Scheduling – Understanding the role of Daemonsets
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Daemonsets are used to ensure a copy of a pod runs on each node within a cluster
Nodes can be included/excluded from the scope using node selectors
They automatically deploy a copy of the pod to any nodes that are added to the cluster
Garbage collection removes the pod when the nodes are removed
Deleting a Daemonset deletes all the associated pods on the nodes
When labels are changed on cluster nodes the Daemonset will automatically add/remove pods from the nodes to ensure each node with a matching label has a running pod, and that the pod doesn’t run on any nodes that are no longer labelled
It is not possible to update or change the pods within a Daemonset without deleting and recreating
Usecases include running a logging pod on each node such as fluentd
Multiple Daemonsets can be created for different usecases
Pods created by Daemonsets are not controlled by the Kubernetes scheduler, instead they are controlled by the DaemonSet controller
The DaemonSet controller can create pods even when the Kubernetes scheduler is unavailable
The DaemonSet controller will ensure pods are restarted based on the RestartPolicy within the pod spec being set to Always, which is a requirement for DaemonSets
The spec used to create Daemonsets has to specify:
The metadata for the Daemonset
The selector spec
The pod template spec to use for the pods (basic spec as per the pod specification without the apiVersion and kind values)
