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Kubernetes: 14. Static Pods

Kubelet Service

  • Kubelet service is resposible for creating pods on the nodes
  • Kubelet service gets the request from kube-api server to create the pod
  • Kube-api inturn gets the request from scheduler to create the pod
  • Kube-api server then gets the data from etcd and sends the request to all the kubelet services running on the nodes
Static Pods
  • Kube-api service is not the only service that kubelet service listens to create the pod
  • kubelet service also looks into a specific folder on each of the node
  • In this folder if it finds the pod-definition yaml file, it will create the pods based on it
  • Note that Kubelet only creates a pod, if you have other objects like replicaSet, deployments etc it wont create those services
  • Services other than pods still have to come through kube-api service
  • Pods created by kubelet service looking into this configuration folder are called Static pods
  • They are not requested by kube-api service
  • But when you execute kubectl get pods, the output will show the static pods as well, even though they didnt originate from kube-api service
  • These pods shown from kubectl get pods output are read only pods, no changes can be made to them using kubectl edit or by deleting them
  • To make any changes to the static pods, update the definition file in the configuration folder and kubelet applies those changes
  • To delete the static pods, delete the definition file from the configuration folder and the kubelet deletes these pods
  • From the below kubelet service we can see the property pod-manifest-path=/opt/kubernetes/runconf that kubelet looks for creating static pods
  • Sometimes instead of pod-manifest-path, there will be a config property set in kubelet config.yaml file. staticPodPath property is set in this yaml file
  • kubelet will be running all the pod definition files under pod-manifest-path as static pods
  • Static pods will run in kube-system namespace
  • Static pods name will end with the hostname 
Static Pods vs DaemonSets
  • Static pods are created by kubelet service directly
  • DaemonSets are created by kube-api server
  • Static pods are used to create the control plane component like kube-api server, etcd, kube-scheduler, kube-controller
  • DaemonSets are used to create services that has to run on every node in the kubernetes cluster like kube-proxy, monitoring agents, logging agents etc
  • Unlike daemonSets staticPods cannot be managed with kubectl or other kubernetes API clients. Static Pods do not depend on the API server, making them useful in cluster bootstrapping cases.

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