Vertical Scaling for MongoDB Replication Clusters with KubeBlocks

This guide demonstrates how to vertically scale a MongoDB ReplicaSet Cluster managed by KubeBlocks by adjusting compute resources (CPU and memory) while maintaining the same number of replicas.

Vertical scaling modifies compute resources (CPU and memory) for MongoDB instances while maintaining replica count. Key characteristics:

• Non-disruptive: When properly configured, maintains availability during scaling
• Granular: Adjust CPU, memory, or both independently
• Reversible: Scale up or down as needed

KubeBlocks orchestrates scaling with minimal impact:

1. Secondary replicas update first
2. Primary updates last after secondaries are healthy
3. Cluster status transitions from Updating to Running

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 MongoDB ReplicaSet Cluster

KubeBlocks uses a declarative approach for managing MongoDB Replication Clusters. Below is an example configuration for deploying a MongoDB ReplicaSet Cluster with one primary replica and two secondary replicas.

Apply the following YAML configuration to deploy the cluster:

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
metadata:
  name: mongo-cluster
  namespace: demo
spec:
  terminationPolicy: Delete
  clusterDef: mongodb
  topology: replicaset
  componentSpecs:
    - name: mongodb
      serviceVersion: "6.0.16"
      replicas: 3
      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

Verifying the Deployment

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

kubectl get cluster mongo-cluster -n demo -w

Expected Output:

kubectl get cluster mongo-cluster -n demo
NAME            CLUSTER-DEFINITION   TERMINATION-POLICY   STATUS     AGE
mongo-cluster   mongodb              Delete               Creating   49s
mongo-cluster   mongodb              Delete               Running    62s

Check the pod status and roles:

kubectl get pods -l app.kubernetes.io/instance=mongo-cluster -L  kubeblocks.io/role -n demo

Expected Output:

NAME                      READY   STATUS    RESTARTS   AGE   ROLE
mongo-cluster-mongodb-0   2/2     Running   0          78s   primary
mongo-cluster-mongodb-1   2/2     Running   0          63s   secondary
mongo-cluster-mongodb-2   2/2     Running   0          48s   secondary

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

Vertical Scale

Expected Workflow:

1. Secondary replicas are updated first (one at a time)
2. Primary is updated last after secondary replicas are healthy
3. Cluster status transitions from Updating to Running

apiVersion: operations.kubeblocks.io/v1alpha1
kind: OpsRequest
metadata:
  name: mongo-cluster-vscale-ops
  namespace: demo
spec:
  clusterName: mongo-cluster
  type: VerticalScaling
  verticalScaling:
  - componentName: mongodb
    requests:
      cpu: '1'
      memory: 1Gi
    limits:
      cpu: '1'
      memory: 1Gi

kubectl -n demo get ops mongo-cluster-vscale-ops -w

NAME                       TYPE              CLUSTER         STATUS    PROGRESS   AGE
mongo-cluster-vscale-ops   VerticalScaling   mongo-cluster   Running   0/3        32s
mongo-cluster-vscale-ops   VerticalScaling   mongo-cluster   Running   1/3        55s
mongo-cluster-vscale-ops   VerticalScaling   mongo-cluster   Running   2/3        82s
mongo-cluster-vscale-ops   VerticalScaling   mongo-cluster   Running   3/3        2m13s

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
spec:
  componentSpecs:
    - name: mongodb
      replicas: 3
      resources:
        requests:
          cpu: "1"       # Update the resources to your need.
          memory: "1Gi"  # Update the resources to your need.
        limits:
          cpu: "1"       # Update the resources to your need.
          memory: "1Gi"  # Update the resources to your need.
  ...

Best Practices & Considerations

Planning:

• Scale during maintenance windows or low-traffic periods
• Verify Kubernetes cluster has sufficient resources
• Check for any ongoing operations before starting

Execution:

• Maintain balanced CPU-to-Memory ratios
• Set identical requests/limits for guaranteed QoS

Post-Scaling:

• Monitor resource utilization and application performance
• Consider adjusting MongoDB parameters if needed

Verification

Verify the updated resources by inspecting the cluster configuration or Pod details:

kbcli cluster describe mongo-cluster -n demo

Expected Output:

Resources Allocation:
COMPONENT   INSTANCE-TEMPLATE   CPU(REQUEST/LIMIT)   MEMORY(REQUEST/LIMIT)   STORAGE-SIZE   STORAGE-CLASS
mongodb                         1 / 1                1Gi / 1Gi               data:20Gi      <none>

Key Benefits of Vertical Scaling with KubeBlocks

• Seamless Scaling: Pods are recreated in a specific order to ensure minimal disruption.
• Dynamic Resource Adjustments: Easily scale CPU and memory based on workload requirements.
• Flexibility: Choose between OpsRequest for dynamic scaling or direct API updates for precise control.
• Improved Availability: The cluster remains operational during the scaling process, maintaining high availability.

Cleanup

To remove all created resources, delete the MongoDB ReplicaSet Cluster along with its namespace:

kubectl delete cluster mongo-cluster -n demo
kubectl delete ns demo

Summary

In this guide, you learned how to:

1. Deploy a MongoDB ReplicaSet Cluster managed by KubeBlocks.
2. Perform vertical scaling by increasing or decreasing resources for the mongodb component.
3. Use both OpsRequest and direct Cluster API updates to adjust resource allocations.

Vertical scaling is a powerful tool for optimizing resource utilization and adapting to changing workload demands, ensuring your MongoDB ReplicaSet Cluster remains performant and resilient.