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

# 3 Core Concepts

# Kubernetes Core Concepts Guide 🚀

## ETCD 📚

### ETCDCTL Command Interaction

ETCDCTL can interact with the ETCD Server using two API versions: Version 2 and Version 3. By default, it is set to use Version 2. Each version has different sets of commands.

#### ETCDCTL Version 2 Commands:

* etcdctl backup
* etcdctl cluster-health
* etcdctl mk
* etcdctl mkdir
* etcdctl set

#### ETCDCTL Version 3 Commands:

* etcdctl snapshot save
* etcdctl endpoint health
* etcdctl get
* etcdctl put

To set the right version of the API, set the environment variable ETCDCTL\_API:

```bash theme={null}
export ETCDCTL_API=3
```

***

## Replication Controller (RC) 💾

## Replica Set 📊

## Deployments.yml 📄

***

## Imperative CLI Commands 🛠️

### Create an NGINX Pod

```bash theme={null}
kubectl run nginx --image=nginx
```

### Generate POD Manifest YAML file (-o yaml). Don't create it (--dry-run)

```bash theme={null}
kubectl run nginx --image=nginx --dry-run=client -o yaml
```

### Create a Deployment

```bash theme={null}
kubectl create deployment --image=nginx nginx
```

### Generate Deployment YAML file (-o yaml). Don't create it (--dry-run)

```bash theme={null}
kubectl create deployment --image=nginx nginx --dry-run=client -o yaml
```

### Generate Deployment YAML file (-o yaml). Don’t create it (--dry-run) and save it to a file

```bash theme={null}
kubectl create deployment --image=nginx nginx --dry-run=client -o yaml > nginx-deployment.yaml
```

Make necessary changes to the file (e.g., adding more replicas) and then create the deployment:

```bash theme={null}
kubectl create -f nginx-deployment.yaml
```

In Kubernetes version 1.19+, we can specify the `--replicas` option to create a deployment with 4 replicas:

```bash theme={null}
kubectl create deployment --image=nginx nginx --replicas=4 --dry-run=client -o yaml > nginx-deployment.yaml
```

***

## Services 🛠️

### Service Types

#### Cluster IP

#### Load Balancer

***

## Namespaces 🏷️

### Create a Namespace

A namespace is a logical grouping of resources.

### Connecting to Servers in Different Nodes & Namespaces

To connect to the same namespace, we can use a service name of that resource to connect (e.g., `mysql.connect("db-service")`).

#### Pod Definition File

A pod definition file that will auto-create a pod in a specific namespace defined in the YAML label section (e.g., `namespace: dev`).

### Switch to Another Namespace Permanently (from Default)

### Resource Quota for Namespace

Limit the usage of resources in a specific namespace.

***

## Imperative Commands for Services 🛠️

### Create a Service Named `redis-service` of Type `ClusterIP` to Expose Pod `redis` on Port 6379

```bash theme={null}
kubectl expose pod redis --port=6379 --name redis-service --dry-run=client -o yaml
```

(This will automatically use the pod's labels as selectors.)

Or

```bash theme={null}
kubectl create service clusterip redis --tcp=6379:6379 --dry-run=client -o yaml
```

(This will not use the pod's labels as selectors, instead it will assume selectors as `app=redis`. You cannot pass in selectors as an option. Generate the file and modify the selectors before creating the service.)

### Create a Service Named `nginx` of Type `NodePort` to Expose Pod `nginx`'s Port 80 on Port 30080 on the Nodes

```bash theme={null}
kubectl expose pod nginx --type=NodePort --port=80 --name=nginx-service --dry-run=client -o yaml
```

(This will automatically use the pod's labels as selectors, but you cannot specify the node port. Generate a definition file and then add the node port manually before creating the service with the pod.)

Or

```bash theme={null}
kubectl create service nodeport nginx --tcp=80:80 --node-port=30080 --dry-run=client -o yaml
```

(This will not use the pod's labels as selectors.)

Both the above commands have their own challenges. While one cannot accept a selector, the other cannot accept a node port. I recommend going with the `kubectl expose` command. If you need to specify a node port, generate a definition file using the same command and manually input the node port before creating the service.