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# How a Kubernetes cluster works
A Kubernetes cluster has a **control plane** and **worker Nodes**. Control-plane machines run the API server and etcd, so their sizing and failure placement protect cluster state. Worker sizing follows workload demand, accelerators and scheduling policy.
The **API server** receives declarations and reports observed state. The scheduler assigns unscheduled Pods. Controllers work toward the declared state, while **etcd** stores cluster state. On each worker Node, **kubelet** asks the **container runtime** to start and stop containers and reports the result to the API. A **Container Network Interface (CNI)** implementation gives Pods their network connectivity. [\[15\]](https://kubernetes.io/docs/concepts/overview/components/)[\[17\]](https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/)
Figure 2. The control plane decides and records desired state. Node components perform the workload operations.
A worker Node contributes infrastructure capacity; a Pod requests workload capacity. The scheduler places Pods according to resources and policy. Adding a Node therefore does not move every Pod automatically. IaaS still provides compute, networking, images, storage and load balancing beneath this lifecycle.
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