Kubernetes components

Kubernetes Cluster spans over multiple nodes, the master (Control Plane) should be separated from worker Nodes

kube-apiserver

Master only component. Accepts user request. Stores resources definitions in etcd.

etcd

Master only component. Key-value store that is highly available. Used to store all cluster data.

kube-scheduler

Master only component. Materializes user requests, watches the kube-apiserver, decides where and when schedule PODs. PODs definition may contain some data that affects kube-scheduler: - affinity/anti-affinity - nodeSelector - taints/tolerations - reservations/limits

It is possible to write custom scheduler

kube-controller-manager

Master only component. Controllers execute routine tasks to synchronize desired state (typically called spec) with observed state. Notable mentions: - Node Controller - monitors Node lifecycle, responds when the Node goes down - Replication Controller - manages *-controllers, e.g., deployment-controller - Endpoints Controller - populates Endpoint - Service Account Controller - creates accounts and access tokens for namespaces

Full list of controllers

kubelet

Master/Worker component. Resides on every Node. Connects to the kube-apiserver. Starts the actual containers via the container runtime. Provides health-checks

kube-proxy

Master/Worker component. Main network component, watches the services and materializes their rule on the Nodes (e.g. handles iptables)

References

1. https://kubernetes.io/docs/concepts/overview/components/
2. https://engineering.bitnami.com/articles/a-deep-dive-into-kubernetes-controllers.html