> ## 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.

# Brief Overview on Kubernetes

> Kubernetes is an open-source platform for automating deployment, scaling, and management of containerized applications in cloud-native architectures.

Kubernetes is an open-source container orchestration platform, originally developed by Google and now maintained by the [Cloud Native Computing Foundation (CNCF)](https://cncf.io/). It automates deployment, scaling, and management of containerized applications, making it a cornerstone technology for modern cloud-native architectures.

## Kubernetes Architecture

A Kubernetes cluster is composed of two main node types:

* **Control Plane Nodes**\
  Hosts the core components that manage cluster state and orchestration:
  * **etcd**: Distributed key-value store for all cluster data.
  * **kube-apiserver**: Central API endpoint for administrative operations.
  * **kube-controller-manager**: Runs controllers to reconcile desired vs. actual state.
  * **kube-scheduler**: Assigns Pods to Nodes based on resource requirements.

* **Worker Nodes**\
  Runs application workloads and contains:
  * **kubelet**: Ensures containers in Pods are healthy and running.
  * **kube-proxy**: Configures network routes and load balancing for Services.
  * Container runtime (e.g., [Docker](https://www.docker.com/), [containerd](https://containerd.io/)).

<Frame>
  ![The image is a diagram illustrating the basics of Kubernetes architecture, showing the interaction between developers, admins, and operations with the controller node and worker nodes, including components like etcd, kube apiserver, and pods.](https://kodekloud.com/kk-media/image/upload/v1752875875/notes-assets/images/GitHub-Actions-Certification-Brief-Overview-on-Kubernetes/kubernetes-architecture-diagram-developers-admins.jpg)
</Frame>

## Pods

A **Pod** is the smallest deployable unit in Kubernetes, encapsulating one or more containers that share networking and storage. Containers within a Pod communicate over `localhost` and share volume mounts.

Example Pod manifest:

```yaml theme={null}
apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  containers:
    - name: web
      image: nginx:latest
      ports:
        - containerPort: 80
```

<Callout icon="lightbulb" color="#1CB2FE">
  By default, Pods use `restartPolicy: Always`. While containers will restart on failure, if the Pod object is deleted or its Node fails, Kubernetes will not recreate it unless managed by a higher-level controller (see Deployments).
</Callout>

## Controllers: ReplicaSets & Deployments

Controllers ensure your Pods maintain the desired state and scale automatically.

| Controller | Purpose                                              | Definition Example |
| ---------- | ---------------------------------------------------- | ------------------ |
| ReplicaSet | Maintains a specified number of identical Pods.      | `kind: ReplicaSet` |
| Deployment | Declarative updates, rollbacks, and scaling of Pods. | `kind: Deployment` |

Example Deployment manifest:

```yaml theme={null}
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.21
          ports:
            - containerPort: 80
```

With a Deployment, you declare the desired state—such as replica count and container image—and Kubernetes handles rolling updates, rollbacks, and Pod rescheduling.

## Services

**Services** provide stable network endpoints for Pods, decoupling clients from dynamically assigned Pod IPs. Kubernetes supports several Service types:

| Type         | Description                                               | Use Case                              |
| ------------ | --------------------------------------------------------- | ------------------------------------- |
| ClusterIP    | Internal-only cluster IP (default).                       | In-cluster communication.             |
| NodePort     | Exposes Service on each Node’s IP at a static port.       | Simple external access on known port. |
| LoadBalancer | Provisions cloud provider LB to route traffic externally. | Production-grade external access.     |

Example LoadBalancer Service:

```yaml theme={null}
apiVersion: v1
kind: Service
metadata:
  name: web-service
spec:
  type: LoadBalancer
  selector:
    app: nginx
  ports:
    - port: 80
      targetPort: 80
```

<Callout icon="triangle-alert" color="#FF6B6B">
  LoadBalancer Services may incur additional cloud provider costs. To consolidate routing for multiple hostnames or paths under a single IP, consider using an **Ingress** resource.
</Callout>

## Ingress

**Ingress** manages HTTP/HTTPS routing into the cluster with host- and path-based rules. Unlike a LoadBalancer, Ingress can serve multiple domains or paths on one IP.

Example Ingress manifest:

```yaml theme={null}
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: example-ingress
spec:
  rules:
    - host: example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: web-service
                port:
                  number: 80
```

<Callout icon="lightbulb" color="#1CB2FE">
  Ingress controllers (e.g., [NGINX](https://nginx.org/), [Traefik](https://traefik.io/)) must be installed separately to process Ingress resources. Services used by Ingress typically remain of type `ClusterIP`.
</Callout>

## References

* [Kubernetes Documentation](https://kubernetes.io/docs/)
* [Cloud Native Computing Foundation (CNCF)](https://cncf.io/)
* [Docker Official Site](https://www.docker.com/)
* [containerd Project](https://containerd.io/)

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