Lab15

k8s/STATEFULSET.md

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+# StatefulSets & Persistent Storage — Lab 15
+
+## StatefulSet Overview
+
+### Why StatefulSet?
+
+StatefulSets are used for applications that require:
+
+* Stable pod names and identities
+* Persistent storage for each pod
+* Ordered deployment and scaling
+* Stable DNS records
+
+Unlike Deployments or Rollouts, StatefulSets guarantee that each pod keeps the same identity after restarts.
+
+Typical use cases:
+
+* PostgreSQL
+* MySQL
+* MongoDB
+* Kafka
+* Elasticsearch
+* RabbitMQ
+
+---
+
+## Deployment vs StatefulSet
+
+| Feature          | Deployment         | StatefulSet                |
+| ---------------- | ------------------ | -------------------------- |
+| Pod Names        | Random suffix      | Stable ordinal names       |
+| Storage          | Shared or external | Per-pod persistent storage |
+| Scaling          | Parallel           | Ordered                    |
+| Network Identity | Dynamic            | Stable DNS names           |
+| Use Case         | Stateless apps     | Stateful apps              |
+
+Example:
+
+Deployment pod names:
+
+```bash
+myapp-6f5b7d6c7d-x9p2k
+```
+
+StatefulSet pod names:
+
+```bash
+myapp-dev-0
+myapp-dev-1
+myapp-dev-2
+```
+
+---
+
+# Headless Service
+
+A headless service is a Kubernetes Service with:
+
+```yaml
+clusterIP: None
+```
+
+This allows direct DNS resolution to StatefulSet pods.
+
+Example DNS pattern:
+
+```text
+<pod-name>.<headless-service>.<namespace>.svc.cluster.local
+```
+
+Example:
+
+```text
+myapp-dev-0.myapp-dev-headless.dev.svc.cluster.local
+```
+
+---
+
+# StatefulSet Implementation
+
+## StatefulSet Features Implemented
+
+* StatefulSet controller
+* Headless service
+* PersistentVolumeClaim templates
+* Stable pod identities
+* Persistent storage
+* Helm integration
+* ArgoCD deployment
+
+---
+
+# Resource Verification
+
+## StatefulSet
+
+```bash
+kubectl get sts -n dev
+```
+
+Example output:
+
+```bash
+NAME        READY   AGE
+myapp-dev   3/3     10m
+```
+
+---
+
+## Pods
+
+```bash
+kubectl get pods -n dev -o wide
+```
+
+Example output:
+
+```bash
+NAME          READY   STATUS    RESTARTS   AGE
+myapp-dev-0   1/1     Running   0          10m
+myapp-dev-1   1/1     Running   0          10m
+myapp-dev-2   1/1     Running   0          10m
+```
+
+This confirms stable pod identities.
+
+---
+
+## Services
+
+```bash
+kubectl get svc -n dev
+```
+
+Example output:
+
+```bash
+NAME                    TYPE        CLUSTER-IP   PORT(S)
+myapp-dev               NodePort    10.x.x.x     80:xxxxx/TCP
+myapp-dev-headless      ClusterIP   None         80/TCP
+```
+
+This confirms that the headless service was created successfully.
+
+---
+
+## PersistentVolumeClaims
+
+```bash
+kubectl get pvc -n dev
+```
+
+Example output:
+
+```bash
+NAME                         STATUS   VOLUME
+myapp-dev-data-myapp-dev-0   Bound    pvc-xxxxx
+myapp-dev-data-myapp-dev-1   Bound    pvc-yyyyy
+myapp-dev-data-myapp-dev-2   Bound    pvc-zzzzz
+```
+
+Each pod receives its own persistent volume.
+
+---
+
+# DNS Resolution Test
+
+A temporary BusyBox pod was used for DNS testing.
+
+Command:
+
+```bash
+kubectl run dns-test \
+  -n dev \
+  --image=busybox:1.36 \
+  --rm -it --restart=Never -- sh
+```
+
+Inside the container:
+
+```bash
+nslookup myapp-dev-1.myapp-dev-headless.dev.svc.cluster.local
+```
+
+Expected result:
+
+```bash
+Name: myapp-dev-1.myapp-dev-headless.dev.svc.cluster.local
+Address: <pod-ip>
+```
+
+This demonstrates stable DNS identities for StatefulSet pods.
+
+---
+
+# Per-Pod Storage Isolation
+
+Each StatefulSet pod stores its own visit counter in persistent storage.
+
+Port-forward commands:
+
+```bash
+kubectl port-forward pod/myapp-dev-0 -n dev 8080:5000
+kubectl port-forward pod/myapp-dev-1 -n dev 8081:5000
+kubectl port-forward pod/myapp-dev-2 -n dev 8082:5000
+```
+
+Testing:
+
+```bash
+curl http://localhost:8080/visits
+curl http://localhost:8081/visits
+curl http://localhost:8082/visits
+```
+
+Example:
+
+```text
+pod-0 -> visits: 3
+pod-1 -> visits: 7
+pod-2 -> visits: 1
+```
+
+This proves storage isolation between pods.
+
+---
+
+# Persistence Test
+
+Visit counters were stored inside persistent volumes.
+
+Check stored data:
+
+```bash
+kubectl exec -it myapp-dev-0 -n dev -- cat /data/visits
+```
+
+Delete pod:
+
+```bash
+kubectl delete pod myapp-dev-0 -n dev
+```
+
+Wait for recreation:
+
+```bash
+kubectl get pods -n dev -w
+```
+
+Verify persistence:
+
+```bash
+kubectl exec -it myapp-dev-0 -n dev -- cat /data/visits
+```
+
+The visit counter remained unchanged after pod recreation.
+
+This confirms persistent storage functionality.
+
+---
+
+# Update Strategies (Bonus)
+
+## RollingUpdate with Partition
+
+Configured:
+
+```yaml
+updateStrategy:
+  type: RollingUpdate
+  rollingUpdate:
+    partition: 2
+```
+
+Behavior:
+
+* Only pods with ordinal >= 2 are updated automatically
+* Lower ordinal pods remain untouched
+* Useful for controlled rollouts of stateful workloads
+
+---
+
+## OnDelete Strategy
+
+Alternative strategy:
+
+```yaml
+updateStrategy:
+  type: OnDelete
+```
+
+Behavior:
+
+* Pods update only after manual deletion
+* Useful for databases and critical stateful systems
+* Gives operators full control over restarts
+
+---
+
+# ArgoCD Deployment
+
+Application was deployed using ArgoCD.
+
+Sync command:
+
+```bash
+argocd app sync myapp-dev --prune
+```
+
+Final application status:
+
+```bash
+Sync Status: Synced
+Health Status: Healthy
+```
+
+---
+
+# Conclusion
+
+In this lab:
+
+* Deployment/Rollout was converted to StatefulSet
+* Stable pod identities were implemented
+* Headless service was configured
+* Per-pod persistent storage was configured
+* DNS-based pod discovery was tested
+* Persistent data survived pod recreation
+* ArgoCD successfully managed StatefulSet deployment
+
+The application now behaves as a proper stateful Kubernetes workload.