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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