add notes for k8s cluster

kubernetes cluster.md

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+kubernetes cluster
+
+### Control Plane Components
+
+- control plane : manage the overall state of the cluster
+  kube-api server:  The core component server that exposes kubernetes HTTP API
+  etcd: Consistent and highly-avaliable key value store for all API server data
+  kube-scheduler: looks for pods not yet bound to a node, and assigns each pod to a suitable node.
+- kube-controller-manager: Runs controllers to implement kubernetes API behavior.
+  cloud-controller-manager: integrates with underlying cloud providers. (optional)
+
+### Node Components
+
+- kubelet: Ensures that pods are running, including their containers.
+- kube-proxy: Mantains network rules on nodes to implement services. (optional)
+- container runtime: Software responsible for running containers.
+
+### Add Ons
+
+- DNS: For cluster wide DNS resolution
+-  web UI(dashboard): For cluster management via web interface.
+- Container resource monitoring: For collecting and storing container metrics.
+- cluster-level logging: For saving container logs to a central log store.