Kube-faq

# Deployment-vs-StatefulSet

Taking PostgreSQL as example, ...

What is the main difference:

- in a StatefulSet you declare the PVC's inside your `spec` as `volumeClaimTemplate`

- you don't declare `strategy`. This is a construct for Deployments

- the `template` part is equal in both implementations

Though it's quite easy to convert a Deployment to a StatefulSet in your definition or vice versa.

But keep in mind:

- like in the example down below it is not a good idea to scale up a PostgreSQL Deployment. This will cause some confusion, because you have to pods running on the same persistent after this.

- on the other hand: if you scale up the StatefulSet, the StatefulSet will create an additional PVC and will mount this on the second pod. This may be better, but not what you want. 

  You now have to independent databases and maybe a SVC pointing on both databases ;-).

## StatefulSet

```yaml

kind: StatefulSet

apiVersion: apps/v1

metadata:

  name: postgresql

spec:

  replicas: 1

  serviceName: postgresql

  selector:

    matchLabels:

      app: postgresql

  template:

    metadata:

      creationTimestamp: null

      labels:

        app: postgresql

    spec:

      containers:

        - name: postgresql

          image: docker.io/centos/postgresql-12-centos8:latest

          envFrom:

            - secretRef:

                name: database

          ports:

            - name: postgresql

              containerPort: 5432

              protocol: TCP

          resources:

            requests:

              cpu: "200m"

              memory: "128Mi"

            limits:

              cpu: "400m"

              memory: "256Mi"

          readinessProbe:

            tcpSocket:

              port: 5432

            timeoutSeconds: 1

            periodSeconds: 10

            successThreshold: 1

            failureThreshold: 3

          livenessProbe:

            tcpSocket:

              port: 5432

            timeoutSeconds: 1

            periodSeconds: 10

            successThreshold: 1

            failureThreshold: 3

          volumeMounts:

            - name: postgresql

              mountPath: /var/lib/pgsql/data

          terminationMessagePath: /dev/termination-log

          terminationMessagePolicy: File

          imagePullPolicy: Always

      restartPolicy: Always

      terminationGracePeriodSeconds: 10

      dnsPolicy: ClusterFirst

      securityContext: {}

      schedulerName: default-scheduler

  volumeClaimTemplates:

    - metadata:

        name: postgresql

        labels:

          app: postgresql

      spec:

        accessModes:

          - ReadWriteOnce

        storageClassName: changed-by-upper-kustomize

        volumeMode: Filesystem

        resources:

          requests:

            storage: 500Mi

```

## Deployment

Here we have a split. We have to explicit declare the PVC:

```yaml

---

kind: Deployment

apiVersion: apps/v1

metadata:

  name: postgresql

spec:

  replicas: 1

  selector:

    matchLabels:

      app: postgresql

  template:

    metadata:

      labels:

        app: postgresql

    spec:

      containers:

        - name: postgresql

          image: docker.io/centos/postgresql-12-centos8:latest

          envFrom:

            - secretRef:

                name: database

          resources:

            requests:

              cpu: "200m"

              memory: "128Mi"

            limits:

              cpu: "400m"

              memory: "256Mi"

          readinessProbe:

            tcpSocket:

              port: 5432

            timeoutSeconds: 1

            periodSeconds: 10

            successThreshold: 1

            failureThreshold: 3

          livenessProbe:

            tcpSocket:

              port: 5432

            timeoutSeconds: 1

            periodSeconds: 10

            successThreshold: 1

            failureThreshold: 3

          volumeMounts:

            - name: postgresql

              mountPath: /var/lib/pgsql/data

          terminationMessagePath: /dev/termination-log

          terminationMessagePolicy: File

          imagePullPolicy: Always

      restartPolicy: Always

      terminationGracePeriodSeconds: 10

      dnsPolicy: ClusterFirst

      securityContext: {}

      schedulerName: default-scheduler

      volumes:

      - name: postgresql

        persistentVolumeClaim:

          claimName: postgresql

  strategy:

    type: RollingUpdate

    rollingUpdate:

      maxUnavailable: 25%

      maxSurge: 25%

  revisionHistoryLimit: 10

  progressDeadlineSeconds: 600

---

kind: PersistentVolumeClaim

apiVersion: v1

metadata:

  name: postgresql

spec:

  accessModes:

    - ReadWriteOnce

  resources:

    requests:

      storage: 2Gi

  storageClassName: changed-by-upper-kustomize

  volumeMode: Filesystem

```