note

1. create pod yaml file from kubectl
    $ kubectl run reids --image=redis --dry-run -o yaml > pod.yaml

2 .Edit Pods
In any of the practical quizzes if you are asked to edit an existing POD, please note the following:
    2.1 If you are given a pod definition file, edit that file and use it to create a new pod.
    2.2 If you are not given a pod definition file, you may extract the definition to a file using the below command:
        $ kubectl get pod <pod-name> -o yaml > pod-definition.yaml
        Then edit the file to make the necessary changes, delete and re-create the pod.
    2.3 Use the kubectl edit pod <pod-name> command to edit pod properties.

3. create deployment by command
    $ kubectl create deployment httpd-frontend --image=httpd:2.4-alpine --replicas=3 --dry-run -o yaml > my-depoy.yaml

4. scale deployment,replicatset with cmd
    $ kubectl scale --relplicas=3 deployment(replicatset) [deployment(deployment) name]  

5. imperative commands
    5.1 POD
        5.1.1 Create an NGINX Pod
            $ kubectl run nginx --image=nginx
        5.1.2 Generate POD Manifest YAML file (-o yaml). Don't create it(--dry-run)
            $ kubectl run nginx --image=nginx --dry-run=client -o yaml
    5.2 Deployment
        5.2.1 Create a deployment
            $ kubectl create deployment --image=nginx nginx
        5.2.2 Generate Deployment YAML file (-o yaml). Don't create it(--dry-run)
            $ kubectl create deployment --image=nginx nginx --dry-run -o yaml
        5.2.3 Generate Deployment with 4 Replicas
            $ kubectl create deployment nginx --image=nginx --replicas=4
        5.2.4 scale a deployment using the kubectl scale command
            $ kubectl scale deployment nginx --replicas=4
        5.2.5 Another way to do this is to save the YAML definition to a file and modify
            $ kubectl create deployment nginx --image=nginx --dry-run=client -o yaml > nginx-deployment.yaml
    5.3 Service
        5.3.1 Create a Service named redis-service of type ClusterIP to expose pod redis on port 6379
            $ 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
            $ kubectl create service clusterip redis --tcp=6379:6379 --dry-run=client -o yaml
            (This will not use the pods labels as selectors, instead it will assume selectors as app=redis.
            You cannot pass in selectors as an option. So it does not work very well if your pod has a different label set.
            So generate the file and modify the selectors before creating the service))
        5.3.2 Create a Service named nginx of type NodePort to expose pod nginx's port 80 on port 30080 on the nodes
            $ kubectl expose pod nginx --port=80 --name nginx-service --type=NodePort --dry-run=client -o yaml
            (This will automatically use the pod's labels as selectors, but you cannot specify the node port.
            You have to generate a definition file and then add the node port in manually before creating the service
            with the pod.)
            or
            $ kubectl create service nodeport nginx --tcp=80:80 --node-port=30080 --dry-run=client -o yaml
            (This will not use the pods labels as selectors)

            PS: Both the above commands have their own challenges. While one of it cannot accept a selector 
            the other cannot accept a node port. I would 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 nodeport before creating the service.
6. Namespace
    6.1 Refer to service in other Namespace
        <service>.<namespace>.svc.cluster.local
        e.q. mysql.connect("db-service.dev.svc.cluster.local")
    6.2 get/create resource in assign namespace
        add --namespace=<namespace>
        e.q. kubectl get pods --namespace=kube-system
        e.q. kubectl create -f pod-definition.yml --namespce=dev
        (also can add namespace: dev in the metadata section in pod-definition.yml)
    6.3 switch from default namespace to another
        $ kubectl config set-context $(kubectl config current-context) --namespace=dev
    6.4 get resource in all namespaces
        $ kubectl get pods --all-namespaces
    6.5 create namespace
        $ kubectl create ns dev-ns