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‎k8s/STATEFULSET.md‎

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# Lab 15 — StatefulSets & Persistent Storage
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**Student**: Selivanov George
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**Date**: May 7, 2026
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## 1) StatefulSet Concepts
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StatefulSet is used when pods need:
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- Stable pod identity (`name-0`, `name-1`, `name-2`)
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- Stable storage per pod (own PVC for each replica)
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- Ordered create/update/delete behavior
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Deployment vs StatefulSet:
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| Feature | Deployment | StatefulSet |
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|---|---|---|
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| Pod identity | Ephemeral/random suffix | Stable ordinal name |
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| Storage | Usually shared/one PVC pattern | Per-pod PVC via template |
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| Scale/update order | Unordered | Ordered by ordinal |
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| Typical workloads | Stateless APIs/web | DBs, queues, clustered systems |
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Headless Service (`clusterIP: None`) is required so each pod gets resolvable DNS:
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- `python-app-devops-python-app-0.python-app-devops-python-app-headless.devops-python-app.svc.cluster.local`
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- `python-app-devops-python-app-1.python-app-devops-python-app-headless.devops-python-app.svc.cluster.local`
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## 2) Implementation (Helm)
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Implemented in chart `k8s/devops-python-app`:
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- Added `templates/statefulset.yaml`
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- Added `templates/service-headless.yaml`
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- Kept normal service for app access
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- Added statefulset configuration and update strategy options used by `volumeClaimTemplates`
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Used values:
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```yaml
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replicaCount: 3
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statefulset:
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enabled: true
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updateStrategy:
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type: RollingUpdate
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rollingUpdate:
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partition: 0
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persistence:
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enabled: true
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size: 100Mi
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storageClass: ""
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accessMode: ReadWriteOnce
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mountPath: /data
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```
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Deploy:
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```bash
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helm dependency update k8s/devops-python-app
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helm upgrade --install python-app k8s/devops-python-app \
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--namespace devops-python-app --create-namespace \
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--set statefulset.enabled=true \
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--set rollout.enabled=false \
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--set image.repository=ge0s1/devops-python-app \
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--set image.tag=lab15 \
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--set image.pullPolicy=IfNotPresent
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kubectl rollout status statefulset/python-app-devops-python-app -n devops-python-app --timeout=240s
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kubectl get po,sts,svc,pvc -n devops-python-app -l app.kubernetes.io/instance=python-app -o wide
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```
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Evidence:
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```text
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NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
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pod/python-app-devops-python-app-0 1/1 Running 0 12s 10.1.0.15 devops-lab-control-plane <none> <none>
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pod/python-app-devops-python-app-1 1/1 Running 0 22s 10.1.0.16 devops-lab-control-plane <none> <none>
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pod/python-app-devops-python-app-2 1/1 Running 0 32s 10.1.0.17 devops-lab-control-plane <none> <none>
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NAME READY AGE CONTAINERS IMAGES
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statefulset.apps/python-app-devops-python-app 3/3 2m59s devops-python-app ge0s1/devops-python-app:lab15
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NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
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service/python-app-devops-python-app-headless ClusterIP None <none> 80/TCP 2m59s app.kubernetes.io/instance=python-app,app.kubernetes.io/name=devops-python-app
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service/python-app-devops-python-app-service NodePort 10.100.50.30 <none> 80:30080/TCP 2m59s app.kubernetes.io/instance=python-app,app.kubernetes.io/name=devops-python-app
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NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS VOLUMEATTRIBUTESCLASS AGE VOLUMEMODE
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persistentvolumeclaim/app-data-python-app-devops-python-app-0 Bound pvc-8a3b7c2d-5501-49e2-9912-fc45e1d7a3b2 100Mi RWO standard <unset> 2m59s Filesystem
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persistentvolumeclaim/app-data-python-app-devops-python-app-1 Bound pvc-6f9e4a1c-7822-4b03-8d55-ba12c8f4e901 100Mi RWO standard <unset> 2m47s Filesystem
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persistentvolumeclaim/app-data-python-app-devops-python-app-2 Bound pvc-3d5b2e7a-9104-41f6-a33c-8790d2e5b8c4 100Mi RWO standard <unset> 2m35s Filesystem
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```
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## 3) Network Identity (Headless DNS)
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Commands:
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```bash
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kubectl exec python-app-devops-python-app-0 -n devops-python-app -- python -c "import socket; print('pod1', socket.gethostbyname('python-app-devops-python-app-1.python-app-devops-python-app-headless.devops-python-app.svc.cluster.local')); print('pod2', socket.gethostbyname('python-app-devops-python-app-2.python-app-devops-python-app-headless.devops-python-app.svc.cluster.local'))"
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```
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Evidence:
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```text
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pod1 10.1.0.16
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pod2 10.1.0.17
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```
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## 4) Per-Pod Storage Isolation
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Test by calling each pod locally from inside the pod:
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```bash
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kubectl exec python-app-devops-python-app-0 -n devops-python-app -- python -c "import urllib.request; [urllib.request.urlopen('http://127.0.0.1:5000/').read() for _ in range(3)]; print(urllib.request.urlopen('http://127.0.0.1:5000/visits').read().decode())"
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kubectl exec python-app-devops-python-app-1 -n devops-python-app -- python -c "import urllib.request; [urllib.request.urlopen('http://127.0.0.1:5000/').read() for _ in range(5)]; print(urllib.request.urlopen('http://127.0.0.1:5000/visits').read().decode())"
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kubectl exec python-app-devops-python-app-2 -n devops-python-app -- python -c "import urllib.request; [urllib.request.urlopen('http://127.0.0.1:5000/').read() for _ in range(2)]; print(urllib.request.urlopen('http://127.0.0.1:5000/visits').read().decode())"
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```
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Evidence:
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```text
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{"visits":3,"visits_file":"/data/visits"}
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{"visits":5,"visits_file":"/data/visits"}
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{"visits":2,"visits_file":"/data/visits"}
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```
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Conclusion: each pod has isolated counter data (separate PVC).
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## 5) Persistence Test
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Commands:
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```bash
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kubectl exec python-app-devops-python-app-0 -n devops-python-app -- cat /data/visits
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kubectl delete pod python-app-devops-python-app-0 -n devops-python-app
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kubectl wait --for=condition=Ready pod/python-app-devops-python-app-0 -n devops-python-app --timeout=180s
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kubectl exec python-app-devops-python-app-0 -n devops-python-app -- cat /data/visits
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```
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Evidence:
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```text
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before:
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3
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pod "python-app-devops-python-app-0" deleted from devops-python-app namespace
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pod/python-app-devops-python-app-0 condition met
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after:
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3
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```
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Conclusion: data persists across pod recreation because PVC is retained and reattached.
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## 6) Bonus — Update Strategies
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### Partitioned rolling update
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```yaml
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updateStrategy:
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type: RollingUpdate
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rollingUpdate:
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partition: 2
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```
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Result:
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- Only pods with ordinal `>= 2` update first.
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- Useful for canarying on highest ordinal replicas.
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```bash
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helm upgrade python-app k8s/devops-python-app \
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--namespace devops-python-app --reuse-values \
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--set image.tag=lab15p \
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--set statefulset.updateStrategy.rollingUpdate.partition=2
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kubectl rollout status statefulset/python-app-devops-python-app -n devops-python-app -w
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```
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Evidence:
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```text
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Waiting for partitioned roll out to finish: 0 out of 1 new pods have been updated...
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partitioned roll out complete: 1 new pods have been updated...
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NAME IMAGE READY
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python-app-devops-python-app-0 ge0s1/devops-python-app:lab15 true
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python-app-devops-python-app-1 ge0s1/devops-python-app:lab15 true
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python-app-devops-python-app-2 ge0s1/devops-python-app:lab15p true
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```
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### OnDelete strategy
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```yaml
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updateStrategy:
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type: OnDelete
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```
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Result:
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- Pods are updated only when manually deleted.
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- Useful for strict maintenance windows and controlled failover.
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```bash
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helm upgrade python-app k8s/devops-python-app \
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--namespace devops-python-app --reuse-values \
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--set image.tag=lab15od \
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--set statefulset.updateStrategy.type=OnDelete
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kubectl get pods -n devops-python-app -l app.kubernetes.io/instance=python-app -o custom-columns=NAME:.metadata.name,IMAGE:.spec.containers[0].image,READY:.status.conditions[?(@.type=='Ready')].status
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kubectl delete pod python-app-devops-python-app-2 -n devops-python-app
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kubectl wait --for=condition=Ready pod/python-app-devops-python-app-2 -n devops-python-app --timeout=180s
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kubectl get pods -n devops-python-app -l app.kubernetes.io/instance=python-app -o custom-columns=NAME:.metadata.name,IMAGE:.spec.containers[0].image,READY:.status.conditions[?(@.type=='Ready')].status
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```
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Evidence:
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```text
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after upgrade (before delete):
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NAME IMAGE READY
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python-app-devops-python-app-0 ge0s1/devops-python-app:lab15 true
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python-app-devops-python-app-1 ge0s1/devops-python-app:lab15 true
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python-app-devops-python-app-2 ge0s1/devops-python-app:lab15p true
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pod "python-app-devops-python-app-2" deleted from devops-python-app namespace
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pod/python-app-devops-python-app-2 condition met
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after manual delete:
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NAME IMAGE READY
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python-app-devops-python-app-0 ge0s1/devops-python-app:lab15 true
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python-app-devops-python-app-1 ge0s1/devops-python-app:lab15 true
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python-app-devops-python-app-2 ge0s1/devops-python-app:lab15od true
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```
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## 7) Useful Commands
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```bash
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kubectl get statefulset,pods,pvc -n devops-python-app
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kubectl describe statefulset python-app-devops-python-app -n devops-python-app
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kubectl get pod python-app-devops-python-app-0 -n devops-python-app -o yaml | grep claimName
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kubectl delete pod python-app-devops-python-app-0 -n devops-python-app
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kubectl rollout status statefulset/python-app-devops-python-app -n devops-python-app
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```

‎k8s/devops-python-app/templates/deployment.yaml‎

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{{- if not .Values.rollout.enabled }}
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{{- if and (not .Values.rollout.enabled) (not .Values.statefulset.enabled) }}
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apiVersion: apps/v1
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kind: Deployment
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metadata:

‎k8s/devops-python-app/templates/pvc.yaml‎

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{{- if .Values.persistence.enabled }}
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{{- if and .Values.persistence.enabled (not .Values.statefulset.enabled) }}
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apiVersion: v1
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kind: PersistentVolumeClaim
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metadata:

‎k8s/devops-python-app/templates/rollout.yaml‎

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{{- if .Values.rollout.enabled }}
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{{- if and .Values.rollout.enabled (not .Values.statefulset.enabled) }}
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apiVersion: argoproj.io/v1alpha1
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kind: Rollout
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metadata:

‎k8s/devops-python-app/templates/service-headless.yaml‎

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{{- if .Values.statefulset.enabled }}
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apiVersion: v1
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kind: Service
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metadata:
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name: {{ include "devops-python-app.fullname" . }}-headless
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labels:
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{{- include "devops-python-app.labels" . | nindent 4 }}
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app.kubernetes.io/component: headless
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spec:
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clusterIP: None
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selector:
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{{- include "devops-python-app.selectorLabels" . | nindent 4 }}
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ports:
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- name: http
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protocol: TCP
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port: {{ .Values.service.port }}
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targetPort: {{ .Values.service.targetPort }}
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{{- end }}

‎k8s/devops-python-app/templates/statefulset.yaml‎

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{{- if .Values.statefulset.enabled }}
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apiVersion: apps/v1
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kind: StatefulSet
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metadata:
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name: {{ include "devops-python-app.fullname" . }}
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labels:
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{{- include "devops-python-app.labels" . | nindent 4 }}
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app.kubernetes.io/component: statefulset
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spec:
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serviceName: {{ include "devops-python-app.fullname" . }}-headless
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replicas: {{ .Values.replicaCount }}
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podManagementPolicy: {{ .Values.statefulset.podManagementPolicy }}
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updateStrategy:
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type: {{ .Values.statefulset.updateStrategy.type }}
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{{- if eq .Values.statefulset.updateStrategy.type "RollingUpdate" }}
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rollingUpdate:
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partition: {{ .Values.statefulset.updateStrategy.rollingUpdate.partition }}
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{{- end }}
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selector:
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matchLabels:
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{{- include "devops-python-app.selectorLabels" . | nindent 6 }}
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template:
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metadata:
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labels:
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{{- include "devops-python-app.selectorLabels" . | nindent 8 }}
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app.kubernetes.io/component: api
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{{- if or .Values.configMap.file.enabled .Values.configMap.env.enabled .Values.vault.enabled }}
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annotations:
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{{- if .Values.configMap.file.enabled }}
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checksum/config-file: {{ .Files.Get "files/config.json" | sha256sum }}
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{{- end }}
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{{- if .Values.configMap.env.enabled }}
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checksum/config-env: {{ printf "%s|%s|%s" .Values.appConfig.appEnv .Values.appConfig.logLevel (toYaml .Values.configMap.env.data) | sha256sum }}
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{{- end }}
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{{- if .Values.vault.enabled }}
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vault.hashicorp.com/agent-inject: "true"
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vault.hashicorp.com/role: {{ .Values.vault.role | quote }}
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{{ printf "vault.hashicorp.com/agent-inject-secret-%s" .Values.vault.fileName | quote }}: {{ .Values.vault.secretPath | quote }}
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{{ printf "vault.hashicorp.com/secret-volume-path-%s" .Values.vault.fileName | quote }}: {{ .Values.vault.mountPath | quote }}
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{{ printf "vault.hashicorp.com/agent-inject-command-%s" .Values.vault.fileName | quote }}: {{ .Values.vault.injectCommand | quote }}
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{{ printf "vault.hashicorp.com/agent-inject-template-%s" .Values.vault.fileName | quote }}: |
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{{ .Values.vault.template | nindent 10 }}
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{{- end }}
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{{- end }}
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spec:
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serviceAccountName: {{ include "devops-python-app.serviceAccountName" . }}
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containers:
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- name: {{ include "devops-python-app.name" . }}
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image: "{{ .Values.image.repository }}:{{ .Values.image.tag | default .Chart.AppVersion }}"
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imagePullPolicy: {{ .Values.image.pullPolicy }}
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ports:
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- name: http
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containerPort: {{ .Values.container.port }}
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protocol: TCP
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env:
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{{- include "devops-python-app.commonEnv" . | nindent 12 }}
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{{- with .Values.extraEnv }}
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{{- toYaml . | nindent 12 }}
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{{- end }}
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{{- if or .Values.secret.enabled .Values.configMap.env.enabled }}
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envFrom:
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{{- if .Values.secret.enabled }}
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- secretRef:
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name: {{ include "devops-python-app.secretName" . }}
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{{- end }}
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{{- if .Values.configMap.env.enabled }}
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- configMapRef:
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name: {{ include "devops-python-app.configEnvConfigMapName" . }}
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{{- end }}
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{{- end }}
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{{- if or .Values.configMap.file.enabled .Values.persistence.enabled }}
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volumeMounts:
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{{- if .Values.configMap.file.enabled }}
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- name: app-config
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mountPath: {{ .Values.configMap.file.mountPath }}
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readOnly: true
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{{- end }}
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{{- if .Values.persistence.enabled }}
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- name: app-data
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mountPath: {{ .Values.persistence.mountPath }}
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{{- end }}
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{{- end }}
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resources:
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{{- toYaml .Values.resources | nindent 12 }}
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readinessProbe:
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{{- toYaml .Values.readinessProbe | nindent 12 }}
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livenessProbe:
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{{- toYaml .Values.livenessProbe | nindent 12 }}
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{{- if .Values.configMap.file.enabled }}
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volumes:
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- name: app-config
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configMap:
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name: {{ include "devops-python-app.configFileConfigMapName" . }}
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{{- end }}
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{{- if .Values.persistence.enabled }}
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volumeClaimTemplates:
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- metadata:
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name: app-data
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labels:
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{{- include "devops-python-app.labels" . | nindent 10 }}
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spec:
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accessModes:
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- {{ .Values.persistence.accessMode }}
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resources:
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requests:
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storage: {{ .Values.persistence.size }}
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{{- if .Values.persistence.storageClass }}
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storageClassName: {{ .Values.persistence.storageClass | quote }}
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{{- end }}
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{{- end }}
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{{- end }}

‎k8s/devops-python-app/values-dev.yaml‎

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replicaCount: 1
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statefulset:
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podManagementPolicy: OrderedReady
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updateStrategy:
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type: RollingUpdate
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rollingUpdate:
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partition: 0
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rollout:
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enabled: false
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strategy: canary
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blueGreen:
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autoPromotionEnabled: false
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image:
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tag: latest

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