Kubernetes: POD, YAML and How to use it?

 PODs in Kubernetes

Kubernetes does not deploy containers directly to the worker nodes. Containers are encapsulated in a Kubernetes object as known as pods. A pod is a single instance of an application, it is the smallest object that we can create in Kubernetes. 

In the fact, mostly, a pod has 1 to 1 relationship with a container. its means when we want to scale up, we need to create a new pod with a container. 

Sometimes we may have multiple containers in one pod, and it may do some supporting tasks for the main container such as processing uploaded files, and processing user data... two containers can communicate with each other via a local shared network. the 2nd container will live alongside the main container. 

See Kubernetes overview: https://www.albertpham.me/2022/08/kubernetes-overview_0459759341.html

For more details

https://kubernetes.io/docs/concepts/workloads/pods/

Normally, When we want to deploy a container into the docker host we run the command:

docker run web-application

So with Kubernetes, we run:

kubectl run NAME --image=image

To see the list of pods in our cluster:

kubectl get pods

To check pod status:

kubectl get pods -o wide

To get more information:

kubectl describe pod NAME

To Delete

kubectl delete pod NAME

YAML file, What is that?

I often use 3 formats of configuration: XML,  JSON, and YAML:

A YAML file is used to represent data, in Kubernetes uses it as input for the creation of objects such as PODs, replicas, and deployment services,...

Here are some things to take note of when we use a YAML file:

- Key: Value separated by a colon and a space

Key: Value
Fruit: Apple
Vehicle: car

- List collection starting a dash and a space 

OS:
 - Windows
 - MacOs
 - Linux

- A dictionary starting with content indented

OS:
 Name: Windows
 Version: window 10

- List and dictionary combine:

OS:
 - Name: Windows
   Version: window 10
 - Name: MacOs
   Version: Monterey 12.5.1
 - Name: Linux
   Version: Ubuntu 14.04

- The sequence in a dictionary is not different, but it's different in a list

- The line starting with hash # is a command.

- Careful with a space.

PODs and YAML

How does it work on Kubernetes?

The Kubernetes YAML file always contains 4 top (root)level fields:

apiVersion: => (string) version of kubernetes version used to create object
kind: => (string)type of object we want to create: POD, replica,...
metadata: => (dictionary) data use to identify objects like, name, labels...

spec: =>(dictionary) this is additional infomation for Kubernetes create object

apiVersion and kind values: refer to here 

To create run this command:

kubectl create -f yaml-file.yal

Or

kubectl apply -f yaml-file.yal

To see the pod we just created

kubectl get pods

Example:

I create a YAML file with the name: create-pod.yml

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2 # tells deployment to run 2 pods matching the template
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

And run the command to create the pod

kubectl create -f create-pod.yaml