<p>I have a cluster with MySQL database which is a <strong>StatefulSet</strong>.</p>
<p>I would like to scale up my database with hpa.</p>
<p>The problem is that the second database that has been created is <strong>empty</strong>.
I don't know how to synchronize the second with the first replica.</p>
<p>Someone told me that I have to create Operators, but I thought that the problem could have been solved with StatefulSets...</p>
<p>This is mysql statefulset code:</p>
<pre><code>apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
spec:
  selector:
    matchLabels:
      app: mysql
  serviceName: mysql
  replicas: 1
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
        - name: mysql
          image: mysql:5.7.21
          imagePullPolicy: Always
          resources:
            requests:
              memory: 50Mi #50
              cpu: 50m
            limits:
                memory: 500Mi #220?
                cpu: 400m #65  
          ports:
          - containerPort: 3306
            name: mysql
          volumeMounts:
            - name: mysql-storage
              mountPath: /var/lib/mysql
              subPath: mysql
          env:
            - name: MYSQL_ROOT_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: mysql-secret # MARK P
                  key: password
            - name: MYSQL_ROOT_HOST
              valueFrom:
                secretKeyRef:
                  name: mysql-secret # MARK P
                  key: host                  
  volumeClaimTemplates:
    - metadata:
        name: mysql-storage
      spec:
        accessModes:
            - ReadWriteOnce
        storageClassName: standard 
        resources:
            requests:
                storage: 5Gi
       
</code></pre>
<p>Does anyone have any idea?</p>

<p>You need to use volumes which mean <code>PersistanceVolume</code> in K8S.</p>
<p>Here is a working sample with all the required resources.</p>
<p><a href="https://github.com/nirgeier/KubernetesLabs/tree/master/Labs/09-StatefulSet" rel="nofollow noreferrer">https://github.com/nirgeier/KubernetesLabs/tree/master/Labs/09-StatefulSet</a></p>
<ul>
<li>Check out the <code>kustomization.yaml</code> to see what is actually required.</li>
<li>You will need the <code>StaefulSet</code> an optional <code>ConfigMap/Secret</code> and a 'Volume' or 'PersistanceVolume<code>&amp;</code>PersistanceVolumeClaim`</li>
</ul>
<h3>How to sync your DB?</h3>
<ul>
<li>In order to &quot;sync&quot; your DB the <code>Staefulset</code> need to share the same volume.</li>
<li>You also need teh secondary instance to be set as <code>super-read-only</code></li>
</ul>
<pre class="lang-yaml prettyprint-override"><code>apiVersion: v1
kind: ConfigMap
metadata:
  name: mysql
  labels:
    app: mysql
data:
  primary.cnf: |
    # Apply this config only on the primary.
    [mysqld]
    log-bin    
  replica.cnf: |
    # Apply this config only on replicas.
    [mysqld]
    super-read-only    
</code></pre>
<pre class="lang-yaml prettyprint-override"><code># Headless service for stable DNS entries of StatefulSet members.
apiVersion: v1
kind: Service
metadata:
  name: mysql
  labels:
    app: mysql
spec:
  ports:
  - name: mysql
    port: 3306
  clusterIP: None
  selector:
    app: mysql
---
# Client service for connecting to any MySQL instance for reads.
# For writes, you must instead connect to the primary: mysql-0.mysql.
apiVersion: v1
kind: Service
metadata:
  name: mysql-read
  labels:
    app: mysql
spec:
  ports:
  - name: mysql
    port: 3306
  selector:
    app: mysql
</code></pre>
<h2>Set up your primary and your replicas</h2>
<p>(Check out the command in the sample)</p>
<pre class="lang-yaml prettyprint-override"><code>apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
spec:
  selector:
    matchLabels:
      app: mysql
  serviceName: mysql
  replicas: 3
  template:
    metadata:
      labels:
        app: mysql
    spec:
      initContainers:
      - name: init-mysql
        image: mysql:5.7
        command:
        - bash
        - &quot;-c&quot;
        - |
          set -ex
          # Generate mysql server-id from pod ordinal index.
          [[ `hostname` =~ -([0-9]+)$ ]] || exit 1
          ordinal=${BASH_REMATCH[1]}
          echo [mysqld] &gt; /mnt/conf.d/server-id.cnf
          # Add an offset to avoid reserved server-id=0 value.
          echo server-id=$((100 + $ordinal)) &gt;&gt; /mnt/conf.d/server-id.cnf
          # Copy appropriate conf.d files from config-map to emptyDir.
          if [[ $ordinal -eq 0 ]]; then
            cp /mnt/config-map/primary.cnf /mnt/conf.d/
          else
            cp /mnt/config-map/replica.cnf /mnt/conf.d/
          fi
        volumeMounts:
        - name: conf
          mountPath: /mnt/conf.d
        - name: config-map
          mountPath: /mnt/config-map
      - name: clone-mysql
        image: gcr.io/google-samples/xtrabackup:1.0
        command:
        - bash
        - &quot;-c&quot;
        - |
          set -ex
          # Skip the clone if data already exists.
          [[ -d /var/lib/mysql/mysql ]] &amp;&amp; exit 0
          # Skip the clone on primary (ordinal index 0).
          [[ `hostname` =~ -([0-9]+)$ ]] || exit 1
          ordinal=${BASH_REMATCH[1]}
          [[ $ordinal -eq 0 ]] &amp;&amp; exit 0
          # Clone data from previous peer.
          ncat --recv-only mysql-$(($ordinal-1)).mysql 3307 | xbstream -x -C /var/lib/mysql
          # Prepare the backup.
          xtrabackup --prepare --target-dir=/var/lib/mysql
        volumeMounts:
        - name: data
          mountPath: /var/lib/mysql
          subPath: mysql
        - name: conf
          mountPath: /etc/mysql/conf.d
      containers:
      - name: mysql
        image: mysql:5.7
        env:
        - name: MYSQL_ALLOW_EMPTY_PASSWORD
          value: &quot;1&quot;
        ports:
        - name: mysql
          containerPort: 3306
        volumeMounts:
        - name: data
          mountPath: /var/lib/mysql
          subPath: mysql
        - name: conf
          mountPath: /etc/mysql/conf.d
        resources:
          requests:
            cpu: 500m
            memory: 1Gi
        livenessProbe:
          exec:
            command: [&quot;mysqladmin&quot;, &quot;ping&quot;]
          initialDelaySeconds: 30
          periodSeconds: 10
          timeoutSeconds: 5
        readinessProbe:
          exec:
            # Check we can execute queries over TCP (skip-networking is off).
            command: [&quot;mysql&quot;, &quot;-h&quot;, &quot;127.0.0.1&quot;, &quot;-e&quot;, &quot;SELECT 1&quot;]
          initialDelaySeconds: 5
          periodSeconds: 2
          timeoutSeconds: 1
      - name: xtrabackup
        image: gcr.io/google-samples/xtrabackup:1.0
        ports:
        - name: xtrabackup
          containerPort: 3307
        command:
        - bash
        - &quot;-c&quot;
        - |
          set -ex
          cd /var/lib/mysql

          # Determine binlog position of cloned data, if any.
          if [[ -f xtrabackup_slave_info &amp;&amp; &quot;x$(&lt;xtrabackup_slave_info)&quot; != &quot;x&quot; ]]; then
            # XtraBackup already generated a partial &quot;CHANGE MASTER TO&quot; query
            # because we're cloning from an existing replica. (Need to remove the tailing semicolon!)
            cat xtrabackup_slave_info | sed -E 's/;$//g' &gt; change_master_to.sql.in
            # Ignore xtrabackup_binlog_info in this case (it's useless).
            rm -f xtrabackup_slave_info xtrabackup_binlog_info
          elif [[ -f xtrabackup_binlog_info ]]; then
            # We're cloning directly from primary. Parse binlog position.
            [[ `cat xtrabackup_binlog_info` =~ ^(.*?)[[:space:]]+(.*?)$ ]] || exit 1
            rm -f xtrabackup_binlog_info xtrabackup_slave_info
            echo &quot;CHANGE MASTER TO MASTER_LOG_FILE='${BASH_REMATCH[1]}',\
                  MASTER_LOG_POS=${BASH_REMATCH[2]}&quot; &gt; change_master_to.sql.in
          fi

          # Check if we need to complete a clone by starting replication.
          if [[ -f change_master_to.sql.in ]]; then
            echo &quot;Waiting for mysqld to be ready (accepting connections)&quot;
            until mysql -h 127.0.0.1 -e &quot;SELECT 1&quot;; do sleep 1; done

            echo &quot;Initializing replication from clone position&quot;
            mysql -h 127.0.0.1 \
                  -e &quot;$(&lt;change_master_to.sql.in), \
                          MASTER_HOST='mysql-0.mysql', \
                          MASTER_USER='root', \
                          MASTER_PASSWORD='', \
                          MASTER_CONNECT_RETRY=10; \
                        START SLAVE;&quot; || exit 1
            # In case of container restart, attempt this at-most-once.
            mv change_master_to.sql.in change_master_to.sql.orig
          fi

          # Start a server to send backups when requested by peers.
          exec ncat --listen --keep-open --send-only --max-conns=1 3307 -c \
            &quot;xtrabackup --backup --slave-info --stream=xbstream --host=127.0.0.1 --user=root&quot;
        volumeMounts:
        - name: data
          mountPath: /var/lib/mysql
          subPath: mysql
        - name: conf
          mountPath: /etc/mysql/conf.d
        resources:
          requests:
            cpu: 100m
            memory: 100Mi
      volumes:
      - name: conf
        emptyDir: {}
      - name: config-map
        configMap:
          name: mysql
  volumeClaimTemplates:
  - metadata:
      name: data
    spec:
      accessModes: [&quot;ReadWriteOnce&quot;]
      resources:
        requests:
          storage: 10Gi
</code></pre>
