• The kubelet functions as one of the many Control Plane components in Kubernetes.
  • The kubelet relies on the kubeapi server for which instructions to run on the node.
    • The decision was made by the kubeScheduler, which is then stored in the etc data store.
  • What if you removed the ControlPlane node entirely from the cluster and only has the Worker Node and its kubelet.
    • The kubelet or “captain” of the node can manage the node independently.
    • On the Worker Node we have the kubelet and Docker to run the containers.
    • A kubeapi server does not exist.
  • The one action the kubelet knows to do is to create pods.
    • The one problem is there is no kubeapi server available to provide pod details.
    • How do you provide a pod definition file to the kubelet without a kubeapi server?
      • You can configure the kubelet to read the pod definition file from a set location (/etc/kubernetes/manifests instead).
        • Place the pod definition files there.
        • The kubelet periodically checks this directory and reads the files there.
        • It creates the pod and ensures that it stays alive (if the application crashes, the pod is also restarted).
        • If any of the pod definition files are changed in the directory, the kubelet recreates the pods.
        • If a file is removed from the directory, the pod is deleted automatically.
        • This setup is now known as a Static Pod. You cannot create Replicasets, Deployments or anything similar. Only Pods.
  • The kubelet only works at a Pod level.
  • The directory can be created anywhere on the host.
    • It is a passed in as an object whilst the kubelet is running the service.
  • Under kubelet.service, the option to specify the directory where the pod manifest files are is: 
    --pod-manifest-path=/etc/Kubernetes/manifest
  • The above line is under ExecStart=/usr/local/bin/kubelet
  • Another way aside from mentioning the option specifically in the kubelet.service file, is by creating a separate yaml file and then linking to that yaml file in the kubelet.service file: 
    ExecStart=/usr/local/bin/kubelet
--config=kubeconfig.yaml
  • Then in kubeconfig.yaml you have to set the staticPodPath like so: 
    staticPodPath: /etc/kubernetes/manifest
  • When the Static Pods are created, you can view them via the docker ps command.
    • For cri-o, we use crictl ps.
    • For containerd, you can use nerdctl ps
      • We are not able to use kubectl here, as we do not have the kube api server available.
  • The kubelet works by taking in requests from different inputs.
    • These can be from the pod definition files from the Static Pods directory as mentioned above.
    • The second way is from an HTTP API endpoint - that is how the kube-apiserver provides input to the kubelet.
  • kubelet can create Static Pods and those from the kube-apiserver at the same time.
    • The kube-apiserver is also aware of the pods being created by the kubelet.
  • kubectl get pods will show the Static Pods as well.
    • If the Worker Node’s kubelet is part of the cluster, it creates a mirror object in the kube-apiserver. The kube-apiserver only has a read-only mirror of the pod.
      • Can receive details of the pods from the kube-apiserver, but you cannot edit or delete them.
        • The pods can only be deleted from the Worker Node’s manifest directory.
      • The name of the pod is automatically appended with the node name static-web-node01.
  • Useful uses of Static Pods to deploy ControlPlane components as Pods on a node.
    • Install a kubelet service on all of the Master Nodes.
    • Create pod definition files that use Docker images of the various Control Plane components –> kube-apiserver, etcd, controller-manager and so on.
    • Then place the manifest files in the directory of each Worker Node.
  • It makes it easier not to need to download and setup additional applications.
    • The above method is how the kubeadm tool sets up a Kubernetes cluster.
      • That is why when you list the pods in the kube-system namespace with kubectl get pods -n kube-system, you see each component as a pod.
  • Difference between static pods and DaemonSets –>
    • Static Pods:
      • Created by kubelet.
      • Deploys Control Plane components as Static Pods.
    • DaemonSets:
      • Created by the kube-apiserver.
      • Deploys Monitoring Agents, Logging Agents on Nodes.
  • Both Static Pods and DaemonSets are ignored by the kube-scheduler.

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