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Version: release-0.9

Scale for a MongoDB cluster

You can scale a MongoDB cluster in two ways, vertical scaling and horizontal scaling.

Vertical scaling​

You can vertically scale a cluster by changing resource requirements and limits (CPU and storage). For example, you can change the resource class from 1C2G to 2C4G by performing vertical scaling.

Before you start​

Check whether the cluster status is Running. Otherwise, the following operations may fail.

kbcli cluster list mongodb-cluster

Steps​

  1. Change configuration.

    Configure the parameters --components, --memory, and --cpu and run the command.

    kbcli cluster vscale mongodb-cluster --components=mongodb --cpu=500m --memory=500Mi
    • --components describes the component name ready for vertical scaling.
    • --memory describes the requested and limited size of the component memory.
    • --cpu describes the requested and limited size of the component CPU.

  2. Validate the vertical scaling.

    kbcli cluster list mongodb-cluster
    >
    NAME              NAMESPACE   CLUSTER-DEFINITION   VERSION          TERMINATION-POLICY   STATUS    CREATED-TIME                 
    mongodb-cluster   default     mongodb              mongodb-5.0      WipeOut              Running   Apr 26,2023 11:50 UTC+0800
    • STATUS=Updating: it means the vertical scaling is in progress.
    • STATUS=Running: it means the vertical scaling operation has been applied.
    • STATUS=Abnormal: it means the vertical scaling is abnormal. The reason may be the normal instances number is less than the total instance number or the leader instance is running properly while others are abnormal. To solve the problem, you can check manually to see whether resources are sufficient. If AutoScaling is supported, the system recovers when there are enough resources, otherwise, you can create enough resources and check the result with kubectl describe command.
note

Vertical scaling does not synchronize parameters related to CPU and memory and it is required to manually call the OpsRequest of configuration to change parameters accordingly. Refer to Configuration for instructions.

  1. Check whether the corresponding resources change.

    kbcli cluster describe mongodb-cluster

Horizontal scaling​

Horizontal scaling changes the amount of pods. For example, you can scale out replicas from three to five.

From v0.9.0, besides replicas, KubeBlocks also supports scaling in and out instances, refer to Horizontal Scale in API docs for more details and examples.

Before you start​

Check whether the cluster STATUS is Running. Otherwise, the following operations may fail.

kbcli cluster list mongodb-cluster
>
NAME                NAMESPACE        CLUSTER-DEFINITION    VERSION          TERMINATION-POLICY        STATUS         CREATED-TIME
mongodb-cluster     default          mongodb               mongodb-5.0      Delete                    Running        April 26,2023 12:00 UTC+0800

Steps​

  1. Change configuration.

    Configure the parameters --components and --replicas, and run the command.

    kbcli cluster hscale mongodb-cluster \
    --components="mongodb" --replicas=2
    • --components describes the component name ready for horizontal scaling.
    • --replicas describes the replica amount of the specified components. Edit the amount based on your demands to scale in or out replicas.

  2. Validate the horizontal scaling operation.

    Check the cluster STATUS to identify the horizontal scaling status.

    kbcli cluster list mongodb-cluster
    • STATUS=Updating: it means horizontal scaling is in progress.
    • STATUS=Running: it means horizontal scaling has been applied.

  3. Check whether the corresponding resources change.

    kbcli cluster describe mongodb-cluster

Handle the snapshot exception​

If STATUS=ConditionsError occurs during the horizontal scaling process, you can find the cause from cluster.status.condition.message for troubleshooting. In the example below, a snapshot exception occurs.

Status:
  conditions: 
  - lastTransitionTime: "2023-04-08T04:20:26Z"
    message: VolumeSnapshot/mongodb-cluster-mongodb-scaling-dbqgp: Failed to set default snapshot
      class with error cannot find default snapshot class
    reason: ApplyResourcesFailed
    status: "False"
    type: ApplyResources

Reason

This exception occurs because the VolumeSnapshotClass is not configured. This exception can be fixed after configuring VolumeSnapshotClass, but the horizontal scaling cannot continue to run. It is because the wrong backup (volumesnapshot is generated by backup) and volumesnapshot generated before still exist. First, delete these two wrong resources and then KubeBlocks re-generates new resources.

Steps:

  1. Configure the VolumeSnapshotClass by running the command below.

    kubectl create -f - <<EOF
    apiVersion: snapshot.storage.k8s.io/v1
    kind: VolumeSnapshotClass
    metadata:
      name: csi-aws-vsc
      annotations:
        snapshot.storage.kubernetes.io/is-default-class: "true"
    driver: ebs.csi.aws.com
    deletionPolicy: Delete
    EOF

  2. Delete the wrong backup (volumesnapshot is generated by backup) and volumesnapshot resources.

    kubectl delete backup -l app.kubernetes.io/instance=mongodb-cluster

    kubectl delete volumesnapshot -l app.kubernetes.io/instance=mongodb-cluster

Result

The horizontal scaling continues after backup and volumesnapshot are deleted and the cluster restores to running status.