docs: add Kubernetes CKS study notes

kubernetes/level 1/task-12.md

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+# Assignment
+
+An application deployed on the Kubernetes cluster requires an update with new features developed by the Nautilus application development team. The existing setup includes a deployment named nginx-deployment and a service named nginx-service. Below are the necessary changes to be implemented without deleting the deployment and service:
+
+
+1.) Modify the service nodeport from 30008 to 32165
+
+2.) Change the replicas count from 1 to 5
+
+3.) Update the image from nginx:1.19 to nginx:latest
+
+Note: The kubectl utility on the jump-host has been configured to work with the Kubernetes cluster.
+
+# Solution
+
+# Update Deployment + Service (no delete, all patches) — `nginx-deployment` / `nginx-service`
+
+Three in-place changes, each as a surgical `kubectl patch`: replicas `1 → 5`, image
+`nginx:1.19 → nginx:latest`, service nodePort `30008 → 32165`. No object is deleted and no
+full manifest is reproduced.
+
+## Patches
+
+```bash
+# 1) Replicas 1 -> 5  (strategic-merge patch)
+kubectl patch deployment nginx-deployment \
+  -p '{"spec":{"replicas":5}}'
+
+# 2) Image nginx:1.19 -> nginx:latest  (strategic-merge, container matched by name)
+kubectl patch deployment nginx-deployment \
+  -p '{"spec":{"template":{"spec":{"containers":[{"name":"nginx-container","image":"nginx:latest"}]}}}}'
+
+# 3) NodePort 30008 -> 32165  (JSON patch, precise field replace)
+kubectl patch service nginx-service \
+  --type=json \
+  -p='[{"op":"replace","path":"/spec/ports/0/nodePort","value":32165}]'
+
+# Wait for the image change to roll out
+kubectl rollout status deployment/nginx-deployment
+```
+
+## How it works
+
+### Two patch types, chosen per change
+
+`kubectl patch` supports different strategies; each change uses the one that's cleanest for it.
+
+**Strategic-merge patch (default)** — used for replicas and image. It deep-merges the JSON
+fragment into the live object, and it understands Kubernetes list semantics.
+
+- **Replicas** — `{"spec":{"replicas":5}}` merges a single scalar field; nothing else in the
+  spec is touched.
+- **Image** — `{"spec":{"template":{"spec":{"containers":[{"name":"nginx-container","image":"nginx:latest"}]}}}}`.
+  The `containers` list is merged **by the `name` key**, so including `name: nginx-container`
+  tells Kubernetes to patch *that* existing container's image rather than replace the whole
+  list or add a second container. This is why the correct container name matters — a wrong
+  name would append a new container instead of updating the existing one.
+
+**JSON patch (`--type=json`)** — used for the nodePort. It's an ordered list of explicit
+operations (RFC 6902). `replace` on `/spec/ports/0/nodePort` targets exactly one field of the
+first port entry. JSON patch is the right tool for editing **one element of a list** like
+`ports` — a strategic-merge patch on a ports array is ambiguous about how to match entries,
+whereas `ports/0` is unambiguous. `32165` is inside the valid NodePort range
+(`30000–32767`), so the API accepts it.
+
+### Why the image patch triggers a rolling update
+
+Changing the container image mutates the pod template. The Deployment controller detects the
+template change, creates a new ReplicaSet, and rolls `nginx:latest` pods in while retiring the
+`nginx:1.19` pods incrementally — governed by the live `RollingUpdate` strategy
+(`maxSurge/maxUnavailable 25%`). `rollout status` blocks until that completes. The replicas
+patch simply scales the ReplicaSet to 5.
+
+### Nothing gets deleted
+
+All three are `patch` operations that mutate existing objects in place. The service keeps its
+ClusterIP; the deployment keeps its identity and rollout history. "Without deleting" is
+satisfied by construction — a delete + recreate would drop the ClusterIP and history.
+
+### Equivalent shortcuts
+
+The same results are achievable with purpose-built verbs, if you prefer them over raw patches:
+
+```bash
+kubectl scale deployment nginx-deployment --replicas=5
+kubectl set image deployment/nginx-deployment nginx-container=nginx:latest
+# (nodePort still needs a patch/edit — there's no dedicated verb for it)
+```
+
+## Verify
+
+```bash
+kubectl get deployment nginx-deployment
+kubectl get deployment nginx-deployment \
+  -o jsonpath='{.spec.template.spec.containers[0].image}{"\n"}'
+kubectl get service nginx-service \
+  -o jsonpath='{.spec.ports[0].nodePort}{"\n"}'
+```
+
+Expected — deployment `READY 5/5`, image `nginx:latest`, nodePort `32165`.
+
+> If the rollout hangs with new pods in `ImagePullBackOff`, the `nginx:latest` pull failed
+> (node offline / rate-limited); `kubectl describe pod <name>` shows the cause.