[CKA] Imperative Command와 Declarative Command

👀 Imperative Command

• 오브젝트의 생성과 수정을 빠르게 도와줌 (YAML파일 수정할 필요X)
• 기능 구현에 한계有
• run once and forgotten (누가 어떻게 만들었는지 트래킹 힘듦)
• 쿠버네티스 내의 definition파일에 저장됨 (로컬파일에 저장X)

Create Objects

$ kubectl run nginx --image=nginx
$ kubectl create deployment nginx --image=nginx
$ kubectl expose deployment nginx --port 80

Update Objects

$ kubectl edit deployment nginx
$ kubectl scale delployment nginx --replicas=5
$ kubectl set image deployment nginx nginx=nginx:1.18

그 외 명령어

# 한 번의 커맨드로 라벨 수정
kubectl run redis --image=redis:alpine --labels="tier=db"

# 한 번의 커맨드로 파드와 서비스 동시 생성
kubectl run httpd --image=htpd:alpine --port=80 --expose=true

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👀 Declarative Command

• 이미 존재하지 않는 파일도 생성 가능
• 경로 지정하여 생성 가능
• 로컬 디렉토리 업데이트

Create Objects

$ kubectl apply -f nginx.yaml
$ kubectl apply -f /path/to/config-files

Update Objects

$ kubectl apply -f nginx.yaml

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--dry-run: By default as soon as the command is run, the resource will be created. If you simply want to test your command , use the --dry-run=client option. This will not create the resource, instead, tell you whether the resource can be created and if your command is right.

-o yaml: This will output the resource definition in YAML format on screen.

Use the above two in combination to generate a resource definition file quickly, that you can then modify and create resources as required, instead of creating the files from scratch.

1) POD

Create an NGINX Pod

kubectl run nginx --image=nginx

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

kubectl run nginx --image=nginx --dry-run=client -o yaml

2) Deployment

Create a deployment

kubectl create deployment --image=nginx nginx

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

kubectl create deployment --image=nginx nginx --dry-run=client -o yaml

Generate Deployment with 4 Replicas

kubectl create deployment nginx --image=nginx --replicas=4

You can also scale a deployment using the kubectl scale command.

kubectl scale deployment nginx --replicas=4

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

You can then update the YAML file with the replicas or any other field before creating the deployment.

3) Service

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)

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 --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. 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)

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.

Reference:
https://kubernetes.io/docs/reference/generated/kubectl/kubectl-commands

https://kubernetes.io/docs/reference/kubectl/conventions/