lab15

k8s/STATEFULSET.md

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+# Lab 15
+
+## StatefulSet Overview
+
+- Purpose: manage stateful applications that need stable network IDs and persistent per-pod storage.
+
+### Key differences vs Deployment
+
+- Identity
+  - StatefulSet: each pod gets a stable, unique hostname (ordinal) and predictable DNS name (pod-0, pod-1...).
+  - Deployment: pods are interchangeable; no stable per-pod identity.
+
+- Storage
+  - StatefulSet: supports volumeClaimTemplates so each pod gets its own persistent volume (mapped by ordinal).
+  - Deployment: PVCs must be managed manually; pods don’t get automatic per-pod PVs.
+
+- Ordering & lifecycle
+  - StatefulSet: ordered, graceful pod creation, scaling, rolling updates, and termination (respecting ordinals).
+  - Deployment: parallel, unordered pod creation/termination for speed.
+
+- Network & discovery
+  - StatefulSet: designed to work with a headless Service for stable DNS-based discovery.
+  - Deployment: typically fronted by a ClusterIP/Service load balancer; no per-pod DNS identity.
+
+- Use cases
+  - StatefulSet: databases, distributed systems, apps needing sticky identity.
+  - Deployment: stateless web services, APIs, workers that can be scaled/replaced freely.
+
+
+## Resource Verification
+
+Output of kubectl get po,sts,svc,pvc
+
+```bash
+kubectl get svc
+
+NAME                     TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)        AGE
+kubernetes               ClusterIP   10.96.0.1       <none>        443/TCP        17m
+myapp-mychart-headless   ClusterIP   None            <none>        80/TCP         3m9s
+
+kubectl get statefulset
+
+NAME            READY   AGE
+myapp-mychart   3/3     96s
+
+kubectl get pods
+NAME                             READY   STATUS    RESTARTS   AGE
+myapp-mychart-0  1/1     Running   0          96s
+myapp-mychart-1   1/1     Running   0          96s
+myapp-mychart-2   1/1     Running   0          96s
+
+kubectl get pvc
+
+NAME                   STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   VOLUMEATTRIBUTESCLASS   AGE
+data-myapp-mychart-0   Bound    pvc-24939609-c8ee-4b3d-b75f-c00101172b1f   100Mi      RWO            standard       <unset>                 96s
+data-myapp-mychart-1   Bound    pvc-a5e89bec-01fd-487d-b9a4-9cbc609f3606   100Mi      RWO            standard       <unset>                 83s
+data-myapp-mychart-2   Bound    pvc-d2b4f3b8-9062-4a78-9ca0-20827a61e3f6   100Mi      RWO            standard       <unset>                 70s
+
+```
+
+Network Identity - DNS resolution outputs
+
+```bash
+# Inside pod
+nonroot@myapp-mychart-0:/app$ getent hosts myapp-mychart-1.myapp-mychart-headless.default.svc.cluster.local
+10.244.0.13     myapp-mychart-1.myapp-mychart-headless.default.svc.cluster.local
+```
+
+Per-Pod Storage Evidence - Different visit counts per pod
+
+```bash
+curl localhost:8081/visits
+{"visits":169,"timestamp":"2026-05-07T19:02:56.771901+00:00"}
+
+curl localhost:8082/visits
+{"visits":179,"timestamp":"2026-05-07T19:02:54.913873+00:00"}
+```
+
+Persistence Test - Data survives pod deletion
+
+```bash
+$ kubectl exec myapp-mychart-0 -- cat /data/visits.json
+{
+  "count": 204
+}
+$ kubectl delete pod myapp-mychart-0 
+pod "myapp-mychart-0" deleted from default namespace
+
+$ kubectl exec myapp-mychart-0 -- cat /data/visits.json
+{
+  "count": 205
+}
+```

k8s/mychart/.helmignore

@@ -21,3 +21,9 @@
 .idea/
 *.tmproj
 .vscode/
+
+
+# I want to disable rollouts for this lab
+
+templates/rollout.yaml
+templates/service-preview.yaml

k8s/mychart/templates/service-headless.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: {{ include "mychart.fullname" . }}-headless
+spec:
+  clusterIP: None
+  selector:
+    {{- include "mychart.selectorLabels" . | nindent 4 }}
+  ports:
+    - port: {{ .Values.service.port }}

k8s/mychart/templates/statefulset.yaml

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+apiVersion: apps/v1
+kind: StatefulSet
+metadata:
+  name: {{ include "mychart.fullname" . }}
+spec:
+  serviceName: {{ include "mychart.fullname" . }}-headless
+  replicas: {{ .Values.replicaCount }}
+  selector:
+    matchLabels:
+      {{- include "mychart.selectorLabels" . | nindent 6 }}
+  template:
+    metadata:
+      {{- with .Values.podAnnotations }}
+      annotations:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+      labels:
+        {{- include "mychart.labels" . | nindent 8 }}
+        {{- with .Values.podLabels }}
+        {{- toYaml . | nindent 8 }}
+        {{- end }}
+    spec:
+      {{- with .Values.imagePullSecrets }}
+      imagePullSecrets:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+      {{- with .Values.podSecurityContext }}
+      securityContext:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+      containers:
+        - name: {{ .Chart.Name }}
+          {{- with .Values.securityContext }}
+          securityContext:
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
+          image: "{{ .Values.image.repository }}:{{ .Values.image.tag | default .Chart.AppVersion }}"
+          imagePullPolicy: {{ .Values.image.pullPolicy }}
+          envFrom:
+            - secretRef:
+                name: {{ include "mychart.fullname" . }}-credentials
+            - configMapRef:
+                name: {{ include "mychart.fullname" . }}-env-config
+          ports:
+            - name: http
+              containerPort: {{ .Values.service.targetPort }}
+              protocol: TCP
+          {{- with .Values.livenessProbe }}
+          livenessProbe:
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
+          {{- with .Values.readinessProbe }}
+          readinessProbe:
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
+          {{- with .Values.resources }}
+          resources:
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
+          volumeMounts:
+            - name: config-volume
+              mountPath: /config
+            - name: data
+              mountPath: /data
+            {{- with .Values.volumeMounts }}
+            {{- toYaml . | nindent 12 }}
+            {{- end }}
+      volumes:
+        - name: config-volume
+          configMap:
+            name: {{ include "mychart.fullname" . }}-config
+        {{- with .Values.volumes }}
+        {{- toYaml . | nindent 8 }}
+        {{- end }}
+      {{- with .Values.nodeSelector }}
+      nodeSelector:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+      {{- with .Values.affinity }}
+      affinity:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+      {{- with .Values.tolerations }}
+      tolerations:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+  volumeClaimTemplates:
+    - metadata:
+        name: data
+      spec:
+        accessModes: [ "ReadWriteOnce" ]
+        resources:
+          requests:
+            storage: {{ .Values.persistence.size }}