# Kubernetes Architecture

## Understanding Kubernetes Components

## 1. Overview

A **Kubernetes cluster** is made up of multiple components that work together to deploy, manage, and scale containerized applications.

At a high level, Kubernetes follows a **master–worker architecture**, where:

- The **Control Plane** manages the cluster
- The **Worker Nodes** run the applications (Pods)

{/* TODO: Add E2E Kubernetes Cluster architecture diagram */}

## 2. High-Level Kubernetes Architecture

A Kubernetes cluster consists of:

- **Control Plane** — Responsible for decision-making and cluster state
- **Worker Nodes** — Responsible for running application workloads

```
Kubernetes Cluster
│
├── Control Plane
│
└── Worker Nodes (1 or more)
```

## 3. Control Plane Components

The **Control Plane** manages the overall state of the Kubernetes cluster.

It makes global decisions such as:

- Scheduling pods
- Maintaining desired state
- Handling failures
- Managing cluster configuration

### 3.1 kube-apiserver

**kube-apiserver** is the **central management component** of Kubernetes.

**Key responsibilities:**

- Exposes the Kubernetes REST/HTTP API
- Acts as the **front-end** of the control plane
- All communication (kubectl, controllers, nodes) goes through it
- Validates and processes API requests

### 3.2 etcd

**etcd** is a **distributed, consistent, and highly available key-value store**.

**Key responsibilities:**

- Stores the **entire cluster state**
- Holds:
  - Pod definitions
  - ConfigMaps
  - Secrets
  - Node information
- Acts as the **single source of truth**

### 3.3 kube-scheduler

The **kube-scheduler** decides **where Pods should run**.

**Key responsibilities:**

- Watches for Pods without assigned nodes
- Selects the best node based on:
  - CPU & memory availability
  - Affinity / anti-affinity rules
  - Taints and tolerations
- Assigns the Pod to a node

### 3.4 kube-controller-manager

The **kube-controller-manager** runs multiple controllers.

Each controller continuously monitors the cluster and ensures the **desired state matches the actual state**.

**Common controllers:**

- Node Controller
- ReplicaSet Controller
- Deployment Controller
- Job Controller
- Endpoint Controller

### 3.5 cloud-controller-manager

This component integrates Kubernetes with **cloud providers**.

**Responsibilities:**

- Manages cloud-specific resources:
  - Load balancers
  - Volumes
  - Node lifecycle
- Allows Kubernetes to run consistently across clouds

## 4. Worker Node Components

**Worker Nodes** are the machines where application workloads run.

Each node contains components required to:

- Run Pods
- Communicate with the control plane
- Handle networking and container execution

### 4.1 kubelet

**kubelet** is the **node agent**.

**Key responsibilities:**

- Communicates with kube-apiserver
- Ensures containers defined in Pods are running
- Reports node and pod status
- Performs health checks

### 4.2 kube-proxy

**kube-proxy** handles **networking for Services**.

**Key responsibilities:**

- Maintains network rules on nodes
- Enables Service-to-Pod communication
- Implements load balancing using:
  - iptables or IPVS

### 4.3 Container Runtime

The **container runtime** is responsible for **running containers**.

**Examples:**

- containerd
- CRI-O

**Responsibilities:**

- Pull container images
- Create and manage containers
- Handle container lifecycle


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