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KodeCloud CKA Daemon Sets

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Daemon Sets

  • A Daemonset is another controller that manages pods like Deployments, ReplicaSets, and StatefulSets.
  • DaemonSets are used in Kubernetes when you need to run one pod on all (or a subset of) the nodes in a cluster.
    • This means you CANNOT scale daemonset pods in a node.
    • Whenever a new node is added to our cluster a replica of the pod is added to that node and when the node is removed the pod is removed.
    • DaemonSets can be deployed to specific nodes either by the nodes’ user-defined labels or using values provided by Kubernetes like the node hostname.
    • Diagram:
      • attachments/Pasted image 20230519081733.jpg
Daemon set ensures that at least one copy of the pod is always present in all nodes in the cluster.

It does this using the Node Affinity rules and the default scheduler.

  • The typical use case for a DaemonSet is logging and monitoring for the hosts.

    • For example, a node needs a service (daemon - fluentd) that collects health or log data and pushes them to a central system or database (like ELK stack).
      • attachments/Pasted image 20221010224414.jpg
    • kube-proxy is another good use case of daemon sets.
    • Before daemonsets if we wanted to log something we would have used a cronjob or created a shell script and run it as a service.

  • Manifest file for a daemon set is exactly like the replica set except for kind.

  • Get all the daemonsets: k get daemonsets
  • Daemon sets best practices
    • DaemonSet pods must have Restart Policy set to Always or unspecified
    • Separate each DaemonSet into its own namespace to ensure clear isolation and easier resource management.
    • DaemonSet must have a priority. It is not advised for DaemonSet Pods to be evicted from cluster nodes for accommodating low priority pods.

Restrict DaemonSets To Run On Specific Nodes

  • By default, a DaemonSet schedules its pods on all the cluster nodes.
  • But we can restrict daemonset pods on specific nodes using nodeSelector, nodeAffinity, Taints, and Tolerations.
    • For example, nodes that host database pods need different monitoring or logging rules.
    • DaemonSets allow you to select which nodes you want to run the pods on.
      • You can do this by using nodeSelector.
      • With nodeSelector, you can select nodes by their labels the same way you do with pods.
    • However, Kubernetes also allows you to select nodes based on some already-defined node properties.
      • For example, kubernetes.io/hostname matches the node name.
    • So, our example cluster has two nodes. Modified the DaemonSet definition that runs only on the first node.
      • attachments/Pasted image 20230519080007.jpg

References

Last updated: 2023-05-19