Imperative vs Declarative -K8s
There are two basic ways to deploy to Kubernetes: imperatively, with the many kubectl commands, or declaratively, by writing manifests and using kubectl apply .
The imperative approach involves using any of the verb based commands, here are some them:
To solve the problem of keeping track of changes to live objects by the system itself (multiple writers problem), one should use a fully declarative mode. In this mode, the creation, deletion and modification of objects is done via a single command.
--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.
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
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.
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.