1.5 imperative and declarative approaches

imperative-declarative

imperative-declarative2

Note : A more effective approach is to first modify the local copy of the object’s configuration file by updating the image name as needed, and then use the kubectl replace command to apply the changes and update the object.

imperative-declarative3

The declarative approach involves using the same object configuration files we've been working with. However, instead of using the create or replace commands, we use kubectl apply to manage the objects.
The kubectl apply command is smart enough to create an object if it doesn’t already exist. When working with multiple configuration files — which is common — you can specify a directory path rather than a single file, allowing all the objects in that directory to be created at once.
To make changes, simply update the relevant configuration file and run kubectl apply again. If the object already exists, apply will detect that and only apply the necessary updates.
This approach avoids errors about objects already existing or updates being invalid — kubectl apply automatically figures out the correct way to update the object based on the changes you've made.

Certification Tips – Imperative Commands with Kubectl

While you will mostly use the declarative approach with definition files, imperative commands are useful for quickly completing one-time tasks or generating YAML templates. This can save significant time during certification exams.

🔧 Key Options

--dry-run=client
Prevents the resource from being created. Useful for testing commands without making changes.
-o yaml
Outputs the object definition in YAML format.

💡 Tip: Use both options together to generate definition files you can edit and apply later.

🐳 POD

Create an NGINX Pod

kubectl run nginx --image=nginx

Generate POD YAML Manifest (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 (dry-run)

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

Create Deployment with 4 Replicas

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

Scale a Deployment

kubectl scale deployment nginx --replicas=4

Generate and Save Deployment YAML to File

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

Then, update the YAML file to add replicas or modify other fields before applying.

🌐 Service

Create ClusterIP Service for Pod `redis` on Port 6379

kubectl expose pod redis --port=6379 --name redis-service --dry-run=client -o yaml

✅ Automatically uses the pod's labels as selectors.

Alternative: Create ClusterIP Service (Manual Selectors Required)

kubectl create service clusterip redis --tcp=6379:6379 --dry-run=client -o yaml

⚠️ Does not use pod labels. Assumes app=redis as selector.

Create NodePort Service for Pod `nginx` Exposing Port 80 on NodePort 30080

kubectl expose pod nginx --type=NodePort --port=80 --name=nginx-service --dry-run=client -o yaml

✅ Uses pod's labels as selectors
⚠️ You cannot specify the nodePort in this command — must edit YAML manually

Alternative: Specify NodePort (Manual Selectors Required)

kubectl create service nodeport nginx --tcp=80:80 --node-port=30080 --dry-run=client -o yaml

⚠️ Does not use pod's labels as selectors

✅ Recommendation

Use kubectl expose to generate the YAML and manually add the nodePort before applying it.