# kubernetes

Kubernetes is an open‑source platform that automates the deployment, scaling, and management of containerized applications. It acts as an orchestrator, ensuring your containers run reliably across clusters of machines, handling networking, storage, and updates without downtime.

- [k3s](#k3s)
  - [Install / Setup](#install--setup)
- [Kubernetes DNS](#kubernetes-dns)
  - [Services DNS Name](#services-dns-name)
- [kubectl](#kubectl)
  - [Pod delete](#pod-delete)
  - [OOMKilled](#oomkilled)
  - [Rollout](#rollout)
  - [Custom Resource Definitions](#custom-resource-definitions)
  - [Helper pods](#helper-pods)
    - [network testing](#network-testing)
  - [Set Replicas](#set-replicas)
  - [Resources](#resources)

## k3s

K3s is a lightweight, certified Kubernetes distribution designed to run in resource‑constrained environments such as edge devices, IoT appliances, and small servers. It simplifies installation and operation by packaging Kubernetes into a single small binary, while still being fully compliant with the Kubernetes API.

🌐 What K3s Is

- Definition: K3s is a simplified Kubernetes distribution created by Rancher Labs (now part of SUSE) and maintained under the CNCF.
- Purpose: It’s built for environments where full Kubernetes (K8s) is too heavy — like Raspberry Pis, edge servers, or CI pipelines.
- Size: The entire distribution is packaged into a binary under ~70MB.

### Install / Setup

**Default master installation:**

``` bash
curl -sfL https://get.k3s.io | sh -
```

## Kubernetes DNS

**Automatic DNS Records:** Kubernetes automatically creates DNS entries for Services and Pods. This allows workloads to connect using predictable names instead of IPs, which may change.

### Services DNS Name

```text
<service-name>.<namespace>.svc.<cluster-domain>
```

*Example: [test-services.services.svc.cluster.local](test-services.services.svc.cluster.local).*

## kubectl

kubectl is the command‑line tool used to interact with Kubernetes clusters. Think of it as the “remote control” for Kubernetes: it lets you deploy applications, inspect resources, and manage cluster operations directly from your terminal.

**Create namespace:**

``` bash
 kubectl create namespace tests 
```

### Pod delete

**Restart local Path Provizionizer:**

``` bash
kubectl delete pod -n kube-system -l app=local-path-provisioner
```

### OOMKilled

**list all OOMKilled pods:**

``` bash
kubectl get events --all-namespaces | grep -i "OOMKilled"
```

### Rollout

**rollout coredns:**

``` bash
kubectl rollout restart deployment coredns -n kube-system
```

### Custom Resource Definitions

- **Definition:** A Custom Resource Definition (CRD) is an extension of the Kubernetes API.

- **Purpose:** They allow you to define new resource kinds (e.g., Database, Backup, FooBar) that behave like native Kubernetes objects.

- **Analogy:** By default, Kubernetes understands objects like Pods and Services. With CRDs, you can add your own object types and manage them with kubectl just like built‑in resources

**List traefik CRDS:**

```bash
kubectl get crds | grep traefik
```

### Helper pods

#### network testing

``` bash
kubectl run  -i --tty dns-test --namespace tests --image=busybox --restart=Never -- 
kubectl delete pod dns-test --namespace tests || 0
```

**Example using yaml and hostNetwork:**

- Create Pod

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: dns-test
  namespace: tests
spec:
  hostNetwork: true
  containers:
  - name: dns-test
    image: busybox
    command: ["sh"]
    stdin: true
    tty: true
```

- Attach to Pod

```bash
kubectl attach -it dns-test -n tests
```

- Execute command inside pod.

``` bash
nslookup google.com
```

- Delete pod

```bash
kubectl delete pod dns-test --namespace tests
```

### Set Replicas

**Set deployment replicas to 0:**

```bash
kubectl patch deployment <deployment-name> \
  -n <namespace> \
  -p '{"spec":{"replicas":0}}'
```

**Set statefulset replicas to 0:**

```bash
kubectl patch statefulset zigbee2mqtt \
  -n  mqtt \
  -p '{"spec":{"replicas":1}}'
```

### Resources

**List all resources:**

```bash
kubectl get all -n kube-system | grep traefik
```