docs: add Kubernetes CKS study notes

2026-08-04 23:18:45 +02:00
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+# Assignment
+
+The Nautilus DevOps team is crafting jobs in the Kubernetes cluster. While they're developing actual scripts/commands, they're currently setting up templates and testing jobs with dummy commands. Please create a job template as per details given below:
+
+
+Create a job named countdown-datacenter.
+
+The spec template should be named countdown-datacenter (under metadata), and the container should be named container-countdown-datacenter
+
+Utilize image ubuntu with latest tag (ensure to specify as ubuntu:latest), and set the restart policy to Never.
+
+Execute the command sleep 5
+
+Note: The kubectl utility on the jump-host has been configured to work with the Kubernetes cluster.
+
+# Solution
+
+# Kubernetes Job — `countdown-datacenter`
+
+A one-shot Job that runs `sleep 5` in an `ubuntu:latest` container. Applied inline via a
+heredoc — no manifest file on disk.
+
+## Apply (heredoc → kubectl)
+
+```bash
+kubectl apply -f - <<'EOF'
+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: countdown-datacenter
+spec:
+  template:
+    metadata:
+      name: countdown-datacenter
+    spec:
+      containers:
+        - name: container-countdown-datacenter
+          image: ubuntu:latest
+          command:
+            - sleep
+            - "5"
+      restartPolicy: Never
+EOF
+```
+
+## How it works
+
+### The heredoc apply pattern
+
+- **`kubectl apply -f -`** reads from **stdin**; the heredoc feeds the YAML in, nothing on
+  disk.
+- **`<<'EOF'` (delimiter quoted)** disables shell expansion, keeping the manifest literal —
+  the right default for Kubernetes YAML.
+
+### Job vs. CronJob
+
+A **Job** runs a pod to **completion** once (or a set number of times), unlike a CronJob
+which spawns Jobs on a schedule. So there's no `schedule` or `jobTemplate` here — the pod
+template sits directly under `spec.template`, one level shallower than a CronJob.
+
+### Field by field
+
+- **`apiVersion: batch/v1` / `kind: Job`** — Job is a `batch/v1` object.
+- **`metadata.name: countdown-datacenter`** — the Job's name, exactly as required.
+- **`spec.template.metadata.name: countdown-datacenter`** — the pod template name, as the
+  task explicitly requests. (Note: a template name is optional and normally ignored for
+  Jobs — the actual pod gets an auto-generated name like `countdown-datacenter-abcde` — but
+  the task asks for it, so it's set.)
+- **`spec.template.spec.containers`** — one container:
+  - **`name: container-countdown-datacenter`** — the container name, exactly as required.
+  - **`image: ubuntu:latest`** — tag stated explicitly, as required.
+  - **`command: ["sleep", "5"]`** — the dummy command. The `"5"` is **quoted** so YAML
+    passes it as a string argument; command args must be strings, and an unquoted `5` would
+    be parsed as an integer and rejected.
+- **`restartPolicy: Never`** — required. It sits at the **pod** level (inside
+  `template.spec`), not on the container. Jobs accept only `Never` or `OnFailure` (`Always`
+  is invalid for batch workloads). `Never` means if the pod fails, the Job creates a *new*
+  pod rather than restarting the existing one.
+
+### `restartPolicy: Never` vs. `OnFailure` for Jobs
+
+Both are valid for Jobs, and they differ in failure handling:
+- **`Never`** — a failed pod is left as-is; the Job spins up a fresh pod for the next
+  attempt. You end up with visible failed pods (useful for debugging).
+- **`OnFailure`** — the same pod's container is restarted in place.
+
+The task specifies `Never`, so failed attempts (if any) appear as separate pods.
+
+## Verify
+
+```bash
+# Job created and completing
+kubectl get job countdown-datacenter
+
+# Pod runs then completes
+kubectl get pods -l job-name=countdown-datacenter
+
+# Confirm image and command
+kubectl get job countdown-datacenter \
+  -o jsonpath='{.spec.template.spec.containers[0].image}{"  "}{.spec.template.spec.containers[0].command}{"\n"}'
+```
+
+Expected — `countdown-datacenter` showing `COMPLETIONS 1/1` after ~5 seconds, its pod
+transitioning `Running` → `Completed`, and the jsonpath printing `ubuntu:latest  [sleep 5]`.
+
+> The pod runs `sleep 5` then exits 0, so the Job completes cleanly. If it shows `Error`,
+> check `kubectl logs job/countdown-datacenter`.