Category: summarization
Instruction: Summarize the main components of a Kubernetes cluster control plane from this paragraph
Context: Kubernetes defines a set of building blocks ("primitives") that collectively provide mechanisms that deploy, maintain, and scale applications based on CPU, memory or custom metrics. Kubernetes is loosely coupled and extensible to meet different workloads. The internal components as well as extensions and containers that run on Kubernetes rely on the Kubernetes API. The platform exerts its control over compute and storage resources by defining resources as Objects, which can then be managed as such.

Kubernetes follows the primary/replica architecture. The components of Kubernetes can be divided into those that manage an individual node and those that are part of the control plane.

Control plane
The Kubernetes master node handles the Kubernetes control plane of the cluster, managing its workload and directing communication across the system. The Kubernetes control plane consists of various components, each its own process, that can run both on a single master node or on multiple masters supporting high-availability clusters. The various components of the Kubernetes control plane are as follows:

etcd is a persistent, lightweight, distributed, key-value data store that CoreOS has developed. It reliably stores the configuration data of the cluster, representing the overall state of the cluster at any given point of time. etcd favors consistency over availability in the event of a network partition (see CAP theorem). The consistency is crucial for correctly scheduling and operating services.
The API server serves the Kubernetes API using JSON over HTTP, which provides both the internal and external interface to Kubernetes. The API server processes and validates REST requests and updates the state of the API objects in etcd, thereby allowing clients to configure workloads and containers across worker nodes. The API server uses etcd's watch API to monitor the cluster, roll out critical configuration changes, or restore any divergences of the state of the cluster back to what the deployer declared. As an example, the deployer may specify that three instances of a particular "pod" (see below) need to be running. etcd stores this fact. If the Deployment Controller finds that only two instances are running (conflicting with the etcd declaration), it schedules the creation of an additional instance of that pod.
The scheduler is the extensible component that selects on which node an unscheduled pod (the basic entity managed by the scheduler) runs, based on resource availability. The scheduler tracks resource use on each node to ensure that workload is not scheduled in excess of available resources. For this purpose, the scheduler must know the resource requirements, resource availability, and other user-provided constraints or policy directives such as quality-of-service, affinity vs. anti-affinity requirements, and data locality. The scheduler's role is to match resource "supply" to workload "demand".
A controller is a reconciliation loop that drives the actual cluster state toward the desired state, communicating with the API server to create, update, and delete the resources it manages (e.g., pods or service endpoints). One kind of controller is a Replication Controller, which handles replication and scaling by running a specified number of copies of a pod across the cluster. It also handles creating replacement pods if the underlying node fails. Other controllers that are part of the core Kubernetes system include a DaemonSet Controller for running exactly one pod on every machine (or some subset of machines), and a Job Controller for running pods that run to completion (e.g., as part of a batch job). Labels selectors that are part of the controller's definition specify the set of pods that a controller manages.
The controller manager is a process that manages a set of core Kubernetes controllers.
Response: The kubernetes cluster control plane runs on one more more master nodes. The components are:
1. etcd - a key value datastore containing the configuration data of the cluster.
2. The api-server is a server which serves and updates the state of the kubernetes cluster. It watches etcd and keeps the clusters actual state in sync with its desired state.
3. The scheduler is a component which selects on which node an unscheduled pod should run, based on what each pod needs and what the nodes in the cluster can support.
4. The controller manager manages several controllers which keep the cluster in sync with its desired state.