# Architecture and Components

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## Terminology

### **Cluster**

* Multiple Nodes form a Cluster

### **Node**

* A compute instance (physical or virtual server)

### **Pod**

* Consists of one or more Containers

### **Container**

* A lightweight, portable OS (e.g., Linux) generally containing an application with all required components

### **Container Runtime Engine**

* Software that manages and runs containers on the Node e.g., Docker, containerd, cri-o, gVisor, etc.&#x20;

***

## Control Plane Components

* The Control Plane consists of the core components needed to manage a Kubernetes cluster
* These sit on the Master Node (which is abstracted from you in a cloud-managed service like AWS EKS)

### kube-apiserver

* The kube-apiserver is the primary management component of Kubernetes
* This exposes an API that we can authenticate to and perform tasks like monitoring the state of the cluster and components, deploying Pods, grabbing information, and more
* All components of the cluster will go through this API

### etcd

* A key/value store used for storing all Kubernetes cluster data
* The data is immutable, anytime it needs to be updated, the whole structure is re-generated with the new data
* The key/value store always preserves the previous values so this data will grow indefinitely unless it's configured to remove old versions

### kube-scheduler

* Assigns newly created Pods to suitable Nodes in the Cluster based on things like resource requirements, constraints, and policies&#x20;

### kube-controller-manager

* Moniters the state of the cluster through the api-server and makes changes attempting to move the current state towards the desired state
* Many controllers can exist on a Cluster e.g., replication controller, deployment controller, etc.&#x20;

***

## Data Plane Components

### kubelet&#x20;

* An agent running on each Node (also including Master Node) responsible for ensuring Pods are healthy and running
* Receives instructions from the kube-scheduler to create/delete Pods&#x20;

### kube-proxy

* A network proxy which maintains network rules on the Nodes e.g., when a new Service is deployed the kube-proxy ensures a rule maps it to the correct Pods running the service
* This can be replaced with Container Network Interface (CNI) Network plugins such as Cilium, Flannel, Calico, and others


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