ced4568andClaude Sonnet 5
25409d3bd7
root-05-m00-kubernetes-mental-model — a new, full gold-standard-pattern front-door module for ROOT 05: what Kubernetes solves, the desired-state model, the control-plane/worker split, and Pods/Namespaces/labels through kubectl. Lesson, demonstration (watching a Pod get replaced after manual deletion), knowledge check, guided exercise (planning a Namespace layout), a real Lab (namespaces-basics), and a reflection — sequenced ahead of the existing bare kubectl-fluency lab bank. Co-Authored-By: Claude Sonnet 5 <[email protected]>
77 lines
3.2 KiB
Markdown
77 lines
3.2 KiB
Markdown
# Demonstration: Desired State Reconciliation
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Watch what actually happens when reality drifts from desired state — this is the mechanism the
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whole lesson describes, made concrete.
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## Setup
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A Namespace called `demo` exists with a Deployment `web` declaring `replicas: 2`. Right now,
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reality matches desired state:
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```
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$ kubectl get pods -n demo
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NAME READY STATUS RESTARTS AGE
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web-7d8f9c6b5d-abcde 1/1 Running 0 3m
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web-7d8f9c6b5d-fghij 1/1 Running 0 3m
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```
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## Someone (or something) deletes one Pod
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```
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$ kubectl delete pod web-7d8f9c6b5d-abcde -n demo
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pod "web-7d8f9c6b5d-abcde" deleted
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```
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## What happens in the next second or two, without anyone doing anything else
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```
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$ kubectl get pods -n demo
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NAME READY STATUS RESTARTS AGE
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web-7d8f9c6b5d-fghij 1/1 Running 0 3m
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web-7d8f9c6b5d-klmno 0/1 ContainerCreating 0 1s
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```
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A brand new Pod (`...-klmno`, a new random suffix — this is NOT the deleted Pod coming back)
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appeared on its own. Nobody ran `kubectl create` again. The Deployment's controller noticed
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actual state (1 running Pod matching its label selector) no longer matched desired state
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(`replicas: 2`), and created exactly one replacement to close the gap — not two, not zero.
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A few seconds later:
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```
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$ kubectl get pods -n demo
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NAME READY STATUS RESTARTS AGE
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web-7d8f9c6b5d-fghij 1/1 Running 0 3m
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web-7d8f9c6b5d-klmno 1/1 Running 0 8s
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```
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Back to 2/2 — desired state restored, entirely without human intervention.
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## Why this matters beyond the demo
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This exact mechanism is what makes a node crash, a Pod eviction, or a container being
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OOM-killed something Kubernetes recovers from automatically instead of something that pages a
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human at 2am. The controller loop doesn't know or care WHY a Pod disappeared — crash, manual
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delete, node failure — it only ever asks "does reality match desired state right now?" and acts
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if not. Understanding this one loop explains a huge amount of Kubernetes' behavior that would
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otherwise look like unrelated special cases.
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## Instructor Notes
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**Demo script:** Run this live rather than only showing the captured output above — have
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students watch `kubectl get pods -n demo --watch` in one terminal while you delete a Pod from
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another. Watching the new Pod name/age appear in real time lands the "this is a NEW Pod, not a
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restart" point far better than static text.
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**Approximate timing:** 8-10 minutes live; 5 minutes as a self-paced read.
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**Common misconception to correct:** students frequently describe this as "Kubernetes restarted
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the Pod." Correct this explicitly — a deleted Pod is gone permanently; a *new* Pod object was
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created to satisfy the replica count. This distinction matters later when debugging (the new
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Pod has zero history from the old one — no old logs, no old restart count).
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**Optional extension:** ask what would happen if you deleted the Deployment itself instead of
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one Pod (answer: all of its Pods are deleted and NOT replaced, because the thing declaring
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desired state is now gone — a good way to reinforce that the Deployment, not any individual
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Pod, is the actual source of truth).
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