Horizontal Scaling for MySQL Clusters with KubeBlocks

This guide explains how to perform horizontal scaling (scale-out and scale-in) on a MySQL cluster managed by KubeBlocks. You'll learn how to use both OpsRequest and direct Cluster API updates to achieve this.

Prerequisites

Before proceeding, ensure the following:

• Environment Setup:
  • A Kubernetes cluster is up and running.
  • The kubectl CLI tool is configured to communicate with your cluster.
  • KubeBlocks CLI and KubeBlocks Operator are installed. Follow the installation instructions here.
• Namespace Preparation: To keep resources isolated, create a dedicated namespace for this tutorial:

kubectl create ns demo
namespace/demo created

Deploy a MySQL Semi-Synchronous Cluster

Deploy a 2-node MySQL cluster (1 primary, 1 replica) with semi-synchronous replication:

kubectl apply -f - <<EOF
apiVersion: apps.kubeblocks.io/v1
kind: Cluster
metadata:
  name: example-mysql-cluster
  namespace: demo
spec:
  clusterDef: mysql
  topology: semisync
  terminationPolicy: Delete
  componentSpecs:
    - name: mysql
      serviceVersion: 8.0.35
      replicas: 2
      resources:
        limits:
          cpu: '0.5'
          memory: 0.5Gi
        requests:
          cpu: '0.5'
          memory: 0.5Gi
      volumeClaimTemplates:
        - name: data
          spec:
            storageClassName: ""
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 20Gi
EOF

Verifying the Deployment

Monitor the cluster status until it transitions to the Running state:

kubectl get cluster example-mysql-cluster -n demo -w

Example Output:

NAME                     CLUSTER-DEFINITION   TERMINATION-POLICY   STATUS    AGE
example-mysql-cluster   mysql                Delete               Creating   8s
example-mysql-cluster   mysql                Delete               Running    87s

Once the cluster status becomes Running, your MySQL cluster is ready for use.

Scale-out (Add Replicas)

kubectl apply -f - <<EOF
apiVersion: operations.kubeblocks.io/v1alpha1
kind: OpsRequest
metadata:
  name: example-mysql-cluster-scale-out-ops
  namespace: demo
spec:
  # Specifies the name of the Cluster resource that this operation is targeting.
  clusterName: example-mysql-cluster
  type: HorizontalScaling
  # Lists HorizontalScaling objects, each specifying scaling requirements for a Component, including desired total replica counts, configurations for new instances, modifications for existing instances, and instance downscaling options
  horizontalScaling:
    # Specifies the name of the Component.
  - componentName: mysql
    # Specifies the replica changes for scaling in components
    scaleOut:
      # Specifies the replica changes for the component.
      # add one more replica to current component
      replicaChanges: 1
EOF

kubectl describe ops example-mysql-cluster-scale-out-ops -n demo

Status:
  Phase:            Succeed
  Progress:         1/1
  ...

kubectl patch cluster example-mysql-cluster -n demo --type=json -p='[{"op": "replace", "path": "/spec/componentSpecs/0/replicas", "value": 3}]'

Verify Scale-Out

After applying the operation, you will see a new pod created and the MySQL cluster status goes from Updating to Running, and the newly created pod has a new role secondary.

kubectl get pods -n demo -l app.kubernetes.io/instance=example-mysql-cluster

Example Output (3 Pods):

NAME                           READY   STATUS    RESTARTS   AGE
example-mysql-cluster-mysql-0   4/4     Running   0          4m30s
example-mysql-cluster-mysql-1   4/4     Running   0          4m30s
example-mysql-cluster-mysql-2   4/4     Running   0          49s

New replicas automatically join as secondary nodes.

kubectl get pods -n demo -l app.kubernetes.io/instance=example-mysql-cluster -o jsonpath='{range .items[*]}{.metadata.name}{"\t"}{.metadata.labels.kubeblocks\.io/role}{"\n"}{end}'

Example Output:

example-mysql-cluster-mysql-0	primary
example-mysql-cluster-mysql-1	secondary
example-mysql-cluster-mysql-2	secondary

Scale-in (Remove Replicas)

kubectl apply -f - <<EOF
apiVersion: operations.kubeblocks.io/v1alpha1
kind: OpsRequest
metadata:
  name: example-mysql-cluster-scale-in-ops
  namespace: demo
spec:
  # Specifies the name of the Cluster resource that this operation is targeting.
  clusterName: example-mysql-cluster
  type: HorizontalScaling
  # Lists HorizontalScaling objects, each specifying scaling requirements for a Component, including desired total replica counts, configurations for new instances, modifications for existing instances, and instance downscaling options
  horizontalScaling:
    # Specifies the name of the Component.
  - componentName: mysql
    # Specifies the replica changes for scaling in components
    scaleIn:
      # Specifies the replica changes for the component.
      # remove one replica from current component
      replicaChanges: 1
EOF

kubectl describe ops example-mysql-cluster-scale-in-ops -n demo

Status:
  Phase:            Succeed
  Progress:         1/1
  ...

kubectl patch cluster example-mysql-cluster -n demo --type=json -p='[{"op": "replace", "path": "/spec/componentSpecs/0/replicas", "value": 2}]'

Verify Scale-In

Example Output (2 Pods):

kubectl get pods -n demo -l app.kubernetes.io/instance=example-mysql-cluster
NAME                           READY   STATUS    RESTARTS   AGE
example-mysql-cluster-mysql-0   4/4     Running   0          10m
example-mysql-cluster-mysql-1   4/4     Running   0          10m

Summary

In this guide, you learned how to:

• Perform scale-out operations to add replicas to a MySQL cluster.
• Perform scale-in operations to remove replicas from a MySQL cluster.
• Use both OpsRequest and direct Cluster API updates for horizontal scaling.

KubeBlocks ensures seamless scaling with minimal disruption to your database operations. with minimal disruption to your database operations.