Scale 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

kbcli cluster list mycluster -n demo
>
NAME             NAMESPACE        CLUSTER-DEFINITION    VERSION            TERMINATION-POLICY        STATUS         CREATED-TIME
mycluster        demo             mongodb               mongodb-5.0        Delete                    Running        Apr 10,2023 16:20 UTC+0800

kubectl

kubectl get cluster mycluster -n demo
>
NAME        CLUSTER-DEFINITION   VERSION       TERMINATION-POLICY   STATUS    AGE
mycluster   mongodb              mongodb-5.0   Delete               Running   27m

Steps

kbcli

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

   kbcli cluster vscale mycluster -n demo --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.

   • View the OpsRequest progress.

     KubeBlocks outputs a command automatically for you to view the OpsRequest progress. The output includes the status of this OpsRequest and Pods. When the status is Succeed, this OpsRequest is completed.

     kbcli cluster describe-ops mycluster-verticalscaling-g67k9 -n demo

   • Check the cluster status.

     kbcli cluster list mycluster -n demo
     >
     NAME        NAMESPACE   CLUSTER-DEFINITION   VERSION          TERMINATION-POLICY   STATUS    CREATED-TIME                 
     mycluster   demo        mongodb              mongodb-5.0      Delete               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.

3. After the OpsRequest status is Succeed or the cluster status is Running again, check whether the corresponding resources change.

   kbcli cluster describe mycluster -n demo

OpsRequest

1. Apply an OpsRequest to the specified cluster. Configure the parameters according to your needs.

   kubectl apply -f - <<EOF
   apiVersion: apps.kubeblocks.io/v1alpha1
   kind: OpsRequest
   metadata:
     name: ops-vertical-scaling
     namespace: demo
   spec:
     clusterName: mycluster
     type: VerticalScaling 
     verticalScaling:
     - componentName: mongodb
       requests:
         memory: "2Gi"
         cpu: "1"
       limits:
         memory: "4Gi"
         cpu: "2"
   EOF

2. Check the operation status to validate the vertical scaling.

   kubectl get ops -n demo
   >
   NAMESPACE   NAME                   TYPE              CLUSTER     STATUS    PROGRESS   AGE
   demo        ops-vertical-scaling   VerticalScaling   mycluster   Succeed   3/3        6m

   If an error occurs to the vertical scaling operation, you can troubleshoot with kubectl describe ops -n demo command to view the events of this operation.

3. Check whether the corresponding resources change.

   kubectl describe cluster mycluster -n demo
   >
   ......
   Component Specs:
    Component Def Ref:  mongodb
    Enabled Logs:
      running
    DisableExporter:   true
    Name:      mongodb
    Replicas:  1
    Resources:
      Limits:
        Cpu:     2
        Memory:  4Gi
      Requests:
        Cpu:     1
        Memory:  2Gi

Edit cluster YAML file

1. Change the configuration of spec.components.resources in the YAML file.

   spec.components.resources controls the requirement and limit of resources and changing them triggers a vertical scaling.

   kubectl edit cluster mycluster -n demo
   >
   ......
   spec:
     affinity:
       podAntiAffinity: Preferred
       topologyKeys:
       - kubernetes.io/hostname
     clusterDefinitionRef: mongodb
     clusterVersionRef: mongodb-5.0
     componentSpecs:
     - componentDefRef: mongodb
       enabledLogs:
       - running
       disableExporter: true
       name: mongodb
       replicas: 2
       resources:
         limits:
           cpu: "2"
           memory: 4Gi
         requests:
           cpu: "1"
           memory: 2Gi

2. Check whether the corresponding resources change.

   kubectl describe cluster mycluster -n demo
   >
   ......
   Component Specs:
    Component Def Ref:  mongodb
    Enabled Logs:
      running
    DisableExporter:   true
    Name:      mongodb
    Replicas:  1
    Resources:
      Limits:
        Cpu:     2
        Memory:  4Gi
      Requests:
        Cpu:     1
        Memory:  2Gi

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 the Horizontal Scale tutorial for more details and examples.

Before you start

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

kbcli

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

kubectl

kubectl get cluster mycluster -n demo
>
NAME        CLUSTER-DEFINITION   VERSION       TERMINATION-POLICY     STATUS    AGE
mycluster   mongodb              mongodb-5.0   Delete                 Running   47m

Steps

kbcli

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

   kbcli cluster hscale mycluster -n demo --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.

   • View the OpsRequest progress.

     KubeBlocks outputs a command automatically for you to view the OpsRequest progress. The output includes the status of this OpsRequest and Pods. When the status is Succeed, this OpsRequest is completed.

     kbcli cluster describe-ops mycluster-horizontalscaling-ffp9p -n demo

   • View the cluster satus.

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

3. After the OpsRequest status is Succeed or the cluster status is Running again, check whether the corresponding resources change.

   kbcli cluster describe mycluster -n demo

OpsRequest

1. Apply an OpsRequest to a specified cluster. Configure the parameters according to your needs.

   The example below means adding two replicas.

   kubectl apply -f - <<EOF
   apiVersion: apps.kubeblocks.io/v1alpha1
   kind: OpsRequest
   metadata:
     name: mongo-horizontalscaling
     namespace: default
   spec:
     clusterName: mycluster
     type: HorizontalScaling
     horizontalScaling:
     - componentName: mongodb
       scaleOut:
         replicaChanges: 2
   EOF

   If you want to scale in replicas, replace scaleOut with scaleIn.

   The example below means deleting two replicas.

   kubectl apply -f - <<EOF
   apiVersion: apps.kubeblocks.io/v1alpha1
   kind: OpsRequest
   metadata:
     name: mongo-horizontalscaling
     namespace: default
   spec:
     clusterName: mycluster
     type: HorizontalScaling
     horizontalScaling:
     - componentName: mongodb
       scaleIn:
         replicaChanges: 2
   EOF

2. Check the operation status to validate the horizontal scaling.

   kubectl get ops -n demo
   >
   NAMESPACE   NAME                     TYPE                CLUSTER     STATUS    PROGRESS   AGE
   demo        ops-horizontal-scaling   HorizontalScaling   mycluster   Succeed   3/3        6m

   If an error occurs, you can troubleshoot with kubectl describe ops -n demo command to view the events of this operation.

3. Check whether the corresponding resources change.

   kubectl describe cluster mycluster -n demo

Edit cluster YAML file

1. Change the configuration of spec.componentSpecs.replicas in the YAML file. spec.componentSpecs.replicas stands for the pod amount and changing this value triggers a horizontal scaling of a cluster.

   kubectl edit cluster mycluster -n demo
   >
   apiVersion: apps.kubeblocks.io/v1alpha1
   kind: Cluster
   metadata:
     name: mycluster
     namespace: demo
   spec:
     clusterDefinitionRef: mongo
     clusterVersionRef: mongodb-5.0
     componentSpecs:
     - name: mongo
       componentDefRef: mongo
       replicas: 4 # Change the amount
       volumeClaimTemplates:
       - name: data
         spec:
           accessModes:
             - ReadWriteOnce
           resources:
             requests:
               storage: 20Gi
    terminationPolicy: Delete

2. Check whether the corresponding resources change.

   kubectl describe cluster mycluster -n demo

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/mycluster-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=mycluster -n demo
   
   kubectl delete volumesnapshot -l app.kubernetes.io/instance=mycluster -n demo

Result

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