🧷 `kubectl apply`: Declarative Resource Management

The kubectl apply command is a core part of the declarative Kubernetes workflow. It creates or updates resources to match the desired state defined in a file.

✅ Why Use `kubectl apply`?

Idempotent: running it multiple times has the same effect
Only changes fields that differ
Stores last applied configuration for intelligent merging
Ideal for GitOps and CI/CD pipelines

🛠️ Common Usage

kubectl apply -f <file.yaml>

Apply a folder:

kubectl apply -f manifests/

Dry-run to preview changes:

kubectl apply -f deployment.yaml --dry-run=client

🔄 Create vs Apply vs Replace

Command	Purpose	Behavior
`kubectl create`	Create new object	Fails if it exists
`kubectl apply`	Create or update based on diff	Smart merge; stores last config
`kubectl replace`	Replace full object	Deletes and recreates (dangerous for live objects)

🧠 Behind the Scenes: Last Applied Config

Kubernetes stores the last applied configuration as an annotation on the object:

metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: '{...}'

This is used to calculate the diff intelligently when reapplying.

🚫 Gotchas

apply only updates fields it manages — manual changes outside the YAML are retained unless overwritten
Some fields like spec.selector are immutable (change = recreate)
Best not to mix kubectl edit or imperative commands with apply for the same object

🧪 Use `kubectl diff`

Preview changes before applying:

kubectl diff -f my-deployment.yaml

✍️ Apply from stdin

cat pod.yaml | kubectl apply -f -

✅ Summary

Use apply for stable, repeatable infrastructure
Organize YAML into folders for modular apply
Avoid mixing declarative and imperative styles on the same object
Combine with kubectl diff and --dry-run for safe updates

🧷 kubectl apply: Declarative Resource Management

✅ Why Use kubectl apply?