> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Kubernetes Architecture

> This article provides an overview of Kubernetes architecture and core concepts for running and scaling containerized applications.

In this lesson, you’ll get a concise overview of Kubernetes architecture and its core concepts. While Kubernetes is a vast ecosystem—entire courses cover every detail—this guide focuses on the essentials you need to understand how Kubernetes runs and scales containerized applications.

## Docker vs. Kubernetes

Docker and Kubernetes often appear together, but they serve different purposes:

* Docker provides a **container runtime** for packaging and running individual containers.
* Kubernetes is an **orchestration system** that automates deployment, scaling, and management of containerized applications across a cluster.

<Callout icon="lightbulb" color="#1CB2FE">
  Kubernetes supports multiple container runtimes. While Docker is the most common, you can also use CRI-O or containerd via the [Container Runtime Interface (CRI)](https://kubernetes.io/docs/concepts/architecture/cri/).
</Callout>

## 1. Nodes and Cluster

A *node* is a physical or virtual machine that runs containerized workloads. You group multiple nodes into a *cluster* to achieve high availability and fault tolerance. If one node fails, other nodes continue serving your application.

<Frame>
  ![The image shows a diagram of a cluster with three nodes, each containing a Redis icon and a Kubernetes symbol, alongside a person standing to the right.](https://kodekloud.com/kk-media/image/upload/v1752873997/notes-assets/images/Docker-Certified-Associate-Exam-Course-Kubernetes-Architecture/kubernetes-redis-cluster-diagram.jpg)
</Frame>

## 2. Control Plane Components

The control plane (formerly called “master”) runs components that maintain the cluster’s desired state:

<Frame>
  ![The image shows a diagram of Kubernetes components, including API Server, etcd, kubelet, Scheduler, Controller, and Container Runtime, with a person standing on the right side.](https://kodekloud.com/kk-media/image/upload/v1752873998/notes-assets/images/Docker-Certified-Associate-Exam-Course-Kubernetes-Architecture/kubernetes-components-diagram-person.jpg)
</Frame>

| Component             | Role                                                                                   |
| --------------------- | -------------------------------------------------------------------------------------- |
| **API Server**        | The cluster’s front end. All CLI (`kubectl`), UI, and internal requests go through it. |
| **etcd**              | A highly available key-value store for all cluster data and configuration.             |
| **Scheduler**         | Assigns pods to nodes based on resource requirements and policies.                     |
| **Controller**        | Monitors state and takes corrective actions (e.g., launching new pods on failure).     |
| **kubelet**           | Agent on each node ensuring containers described in PodSpecs are running and healthy.  |
| **Container Runtime** | Software that runs containers (e.g., Docker, containerd, CRI-O).                       |

<Callout icon="triangle-alert" color="#FF6B6B">
  Data in **etcd** is critical: back it up regularly. Loss of etcd data can render your cluster unusable.
</Callout>

## 3. Kubernetes CLI (`kubectl`)

`kubectl` is the primary command-line tool to interact with the Kubernetes API. Here are common commands:

| Command                    | Description                                         |
| -------------------------- | --------------------------------------------------- |
| `kubectl run`              | Deploy an application (create a Deployment or Pod). |
| `kubectl get nodes`        | List all nodes in the cluster.                      |
| `kubectl get pods`         | List all pods in the current namespace.             |
| `kubectl cluster-info`     | Display addresses of the control plane.             |
| `kubectl scale deployment` | Adjust the number of replicas in a Deployment.      |
| `kubectl set image`        | Update the image of a Deployment.                   |
| `kubectl rollout undo`     | Roll back to a previous Deployment version.         |

### Example Workflow

```bash theme={null}
# Deploy a simple BusyBox pod with a single replica
kubectl run hello-minikube --image=busybox --replicas=1

# Verify nodes and cluster
kubectl get nodes
kubectl cluster-info

# Scale up your application
kubectl scale deployment hello-minikube --replicas=3

# Perform a rolling update
kubectl set image deployment/hello-minikube hello-minikube=busybox:1.1 --record

# Roll back if needed
kubectl rollout undo deployment/hello-minikube
```

<Callout icon="lightbulb" color="#1CB2FE">
  You can also configure **Horizontal Pod Autoscaler** to automatically adjust replica counts based on CPU or custom metrics. See [Horizontal Pod Autoscaling](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/) for details.
</Callout>

***

Ready to dive deeper? Explore our in-depth Kubernetes courses to master topics like Networking, Storage, Security, and become a certified Kubernetes Administrator (CKA).

## Links and References

* [Kubernetes Official Documentation](https://kubernetes.io/docs/)
* [Docker Hub](https://hub.docker.com/)
* [Terraform Registry](https://registry.terraform.io/)

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