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New to KubeDB? Please start here.

Horizontal Scale ClickHouse Topology Cluster

This guide will show you how to use KubeDB Ops-manager operator to scale the ClickHouse cluster.

Before You Begin

  • At first, you need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using kind.

  • Install KubeDB Provisioner and Ops-manager operator in your cluster following the steps here.

  • You should be familiar with the following KubeDB concepts:

To keep everything isolated, we are going to use a separate namespace called demo throughout this tutorial.

$ kubectl create ns demo
namespace/demo created

Note: YAML files used in this tutorial are stored in docs/examples/clickhouse directory of kubedb/docs repository.

Apply Horizontal Scaling on ClickHouse Cluster

Here, we are going to deploy a ClickHouse cluster using a supported version by KubeDB operator. Then we are going to apply horizontal scaling on it.

Prepare ClickHouse cluster

Now, we are going to deploy a ClickHouse cluster with version 24.4.1.

Deploy ClickHouse Cluster

In this section, we are going to deploy a ClickHouse cluster. Then, in the next section we will scale the cluster using ClickHouseOpsRequest CRD. Below is the YAML of the ClickHouse CR that we are going to create,

apiVersion: kubedb.com/v1alpha2
kind: ClickHouse
metadata:
  name: clickhouse-prod
  namespace: demo
spec:
  version: 24.4.1
  clusterTopology:
    clickHouseKeeper:
      externallyManaged: false
      spec:
        replicas: 3
        storage:
          accessModes:
            - ReadWriteOnce
          resources:
            requests:
              storage: 1Gi
    cluster:
        name: appscode-cluster
        shards: 2
        replicas: 2
        podTemplate:
          spec:
            containers:
              - name: clickhouse
                resources:
                  limits:
                    memory: 4Gi
                  requests:
                    cpu: 500m
                    memory: 2Gi
            initContainers:
              - name: clickhouse-init
                resources:
                  limits:
                    memory: 1Gi
                  requests:
                    cpu: 500m
                    memory: 1Gi
        storage:
          accessModes:
            - ReadWriteOnce
          resources:
            requests:
              storage: 1Gi
  deletionPolicy: WipeOut

Let’s create the ClickHouse CR we have shown above,

$ kubectl create -f https://github.com/kubedb/docs/raw/v2025.8.31/docs/examples/clickhouse/scaling/clickhouse-cluster.yaml
clickhouse.kubedb.com/clickhouse-prod created

Now, wait until clickhouse-prod has status Ready. i.e,

➤ kubectl get clickhouse -n demo -w
NAME              TYPE                  VERSION   STATUS         AGE
clickhouse-prod   kubedb.com/v1alpha2   24.4.1    Provisioning   4s
clickhouse-prod   kubedb.com/v1alpha2   24.4.1    Provisioning   50s
.
.
clickhouse-prod   kubedb.com/v1alpha2   24.4.1    Ready          2m5s

Let’s check the number of replicas has from clickhouse object, number of pods the petset have,

➤ kubectl get petset -n demo clickhouse-prod-appscode-cluster-shard-0 -o json | jq '.spec.replicas'
2

We can see from commands that the cluster has 2 replicas for shard-0 as we have defined in the yaml.

We are now ready to apply the ClickHouseOpsRequest CR to scale this cluster.

Scale Up Replicas

Here, we are going to scale up the replicas of the cluster to meet the desired number of replicas after scaling.

Create ClickHouseOpsRequest

In order to scale up the replicas of the cluster, we have to create a ClickHouseOpsRequest CR with our desired replicas. Below is the YAML of the ClickHouseOpsRequest CR that we are going to create,

apiVersion: ops.kubedb.com/v1alpha1
kind: ClickHouseOpsRequest
metadata:
  name: chops-scale-horizontal-up
  namespace: demo
spec:
  type: HorizontalScaling
  databaseRef:
    name: clickhouse-prod
  horizontalScaling:
    replicas: 4

Here,

  • spec.databaseRef.name specifies that we are performing horizontal scaling operation on clickhouse-prod cluster.
  • spec.type specifies that we are performing HorizontalScaling on clickhouse.
  • spec.horizontalScaling.cluster[index].replicas specifies the desired replicas after scaling for clickhouse cluster.

Let’s create the ClickHouseOpsRequest CR we have shown above,

$ kubectl apply -f https://github.com/kubedb/docs/raw/v2025.8.31/docs/examples/clickhouse/scaling/horizontal-scaling/chops-horizontal-scaling-up.yaml
clickhouseopsrequest.ops.kubedb.com/chops-scale-horizontal-up created

Verify cluster replicas scaled up successfully

If everything goes well, KubeDB Ops-manager operator will update the replicas of ClickHouse object and related PetSets and Pods.

Let’s wait for ClickHouseOpsRequest to be Successful. Run the following command to watch ClickHouseOpsRequest CR,

➤ kubectl get clickhouseopsrequest -n demo
NAME                        TYPE                STATUS       AGE
chops-scale-horizontal-up   HorizontalScaling   Successful   3m16s

We can see from the above output that the ClickHouseOpsRequest has succeeded. If we describe the ClickHouseOpsRequest we will get an overview of the steps that were followed to scale the cluster.

➤ kubectl describe clickhouseopsrequest -n demo chops-scale-horizontal-up 
Name:         chops-scale-horizontal-up
Namespace:    demo
Labels:       <none>
Annotations:  <none>
API Version:  ops.kubedb.com/v1alpha1
Kind:         ClickHouseOpsRequest
Metadata:
  Creation Timestamp:  2025-08-25T12:23:20Z
  Generation:          1
  Resource Version:    820391
  UID:                 bfb2f335-01c0-4710-8784-c78f92386e76
Spec:
  Apply:  IfReady
  Database Ref:
    Name:  clickhouse-prod
  Horizontal Scaling:
    Cluster:
      Cluster Name:  appscode-cluster
      Replicas:      4
  Type:              HorizontalScaling
Status:
  Conditions:
    Last Transition Time:  2025-08-25T12:23:20Z
    Message:               ClickHouse ops-request has started to horizontally scaling the nodes
    Observed Generation:   1
    Reason:                HorizontalScaling
    Status:                True
    Type:                  HorizontalScaling
    Last Transition Time:  2025-08-25T12:25:28Z
    Message:               Successfully Scaled Up Node
    Observed Generation:   1
    Reason:                HorizontalScaleUp
    Status:                True
    Type:                  HorizontalScaleUp
    Last Transition Time:  2025-08-25T12:23:28Z
    Message:               patch petset; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  PatchPetset
    Last Transition Time:  2025-08-25T12:25:18Z
    Message:               is pod running; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  IsPodRunning
    Last Transition Time:  2025-08-25T12:25:23Z
    Message:               client failure; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  ClientFailure
    Last Transition Time:  2025-08-25T12:23:48Z
    Message:               is node in cluster; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  IsNodeInCluster
    Last Transition Time:  2025-08-25T12:25:33Z
    Message:               clickhouse-prod-appscode-cluster-shard-1 already has desired replicas
    Observed Generation:   1
    Reason:                HorizontalScale
    Status:                True
    Type:                  HorizontalScale
    Last Transition Time:  2025-08-25T12:25:33Z
    Message:               successfully reconciled the ClickHouse with modified node
    Observed Generation:   1
    Reason:                UpdatePetSets
    Status:                True
    Type:                  UpdatePetSets
    Last Transition Time:  2025-08-25T12:25:33Z
    Message:               reconcile; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  Reconcile
    Last Transition Time:  2025-08-25T12:25:33Z
    Message:               Successfully completed horizontally scale ClickHouse cluster
    Observed Generation:   1
    Reason:                Successful
    Status:                True
    Type:                  Successful
  Observed Generation:     1
  Phase:                   Successful
Events:
  Type     Reason                                    Age    From                         Message
  ----     ------                                    ----   ----                         -------
  Normal   Starting                                  3m48s  KubeDB Ops-manager Operator  Start processing for ClickHouseOpsRequest: demo/chops-scale-horizontal-up
  Normal   Starting                                  3m48s  KubeDB Ops-manager Operator  Pausing ClickHouse databse: demo/clickhouse-prod
  Normal   Successful                                3m48s  KubeDB Ops-manager Operator  Successfully paused ClickHouse database: demo/clickhouse-prod for ClickHouseOpsRequest: chops-scale-horizontal-up
  Warning  patch petset; ConditionStatus:True        3m40s  KubeDB Ops-manager Operator  patch petset; ConditionStatus:True
  Warning  is pod running; ConditionStatus:False     3m35s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      3m25s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      3m25s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:False     3m25s  KubeDB Ops-manager Operator  client failure; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      3m20s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      3m20s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:True      3m20s  KubeDB Ops-manager Operator  client failure; ConditionStatus:True
  Warning  is node in cluster; ConditionStatus:True  3m20s  KubeDB Ops-manager Operator  is node in cluster; ConditionStatus:True
  Warning  patch petset; ConditionStatus:True        3m15s  KubeDB Ops-manager Operator  patch petset; ConditionStatus:True
  Warning  is pod running; ConditionStatus:False     3m10s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      2m55s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      2m55s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:False     2m55s  KubeDB Ops-manager Operator  client failure; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      2m50s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      2m50s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:True      2m50s  KubeDB Ops-manager Operator  client failure; ConditionStatus:True
  Warning  is node in cluster; ConditionStatus:True  2m50s  KubeDB Ops-manager Operator  is node in cluster; ConditionStatus:True
  Normal   HorizontalScaleUp                         2m45s  KubeDB Ops-manager Operator  Successfully Scaled Up Node
  Warning  patch petset; ConditionStatus:True        2m40s  KubeDB Ops-manager Operator  patch petset; ConditionStatus:True
  Warning  is pod running; ConditionStatus:False     2m35s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      2m20s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      2m20s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:False     2m20s  KubeDB Ops-manager Operator  client failure; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      2m15s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      2m15s  KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:True      2m15s  KubeDB Ops-manager Operator  client failure; ConditionStatus:True
  Warning  is node in cluster; ConditionStatus:True  2m15s  KubeDB Ops-manager Operator  is node in cluster; ConditionStatus:True
  Warning  patch petset; ConditionStatus:True        2m10s  KubeDB Ops-manager Operator  patch petset; ConditionStatus:True
  Warning  is pod running; ConditionStatus:False     2m5s   KubeDB Ops-manager Operator  is pod running; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      110s   KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      110s   KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:False     110s   KubeDB Ops-manager Operator  client failure; ConditionStatus:False
  Warning  is pod running; ConditionStatus:True      105s   KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  is pod running; ConditionStatus:True      105s   KubeDB Ops-manager Operator  is pod running; ConditionStatus:True
  Warning  client failure; ConditionStatus:True      105s   KubeDB Ops-manager Operator  client failure; ConditionStatus:True
  Warning  is node in cluster; ConditionStatus:True  105s   KubeDB Ops-manager Operator  is node in cluster; ConditionStatus:True
  Normal   HorizontalScaleUp                         100s   KubeDB Ops-manager Operator  Successfully Scaled Up Node
  Warning  reconcile; ConditionStatus:True           95s    KubeDB Ops-manager Operator  reconcile; ConditionStatus:True
  Warning  reconcile; ConditionStatus:True           95s    KubeDB Ops-manager Operator  reconcile; ConditionStatus:True
  Warning  reconcile; ConditionStatus:True           95s    KubeDB Ops-manager Operator  reconcile; ConditionStatus:True
  Normal   UpdatePetSets                             95s    KubeDB Ops-manager Operator  successfully reconciled the ClickHouse with modified node
  Normal   Starting                                  95s    KubeDB Ops-manager Operator  Resuming ClickHouse database: demo/clickhouse-prod
  Normal   Successful                                95s    KubeDB Ops-manager Operator  Successfully resumed ClickHouse database: demo/clickhouse-prod for ClickHouseOpsRequest: chops-scale-horizontal-up

Now, we are going to verify the number of replicas this cluster has from the ClickHouse object, number of pods the petset have,

➤ kubectl get petset -n demo clickhouse-prod-appscode-cluster-shard-0 -o json | jq '.spec.replicas'
4

Scale Down Replicas

Here, we are going to scale down the replicas of the clickhouse cluster to meet the desired number of replicas after scaling.

Create ClickHouseOpsRequest

In order to scale down the replicas of the clickhouse cluster, we have to create a ClickHouseOpsRequest CR with our desired replicas. Below is the YAML of the ClickHouseOpsRequest CR that we are going to create,

apiVersion: ops.kubedb.com/v1alpha1
kind: ClickHouseOpsRequest
metadata:
  name: chops-scale-horizontal-down
  namespace: demo
spec:
  type: HorizontalScaling
  databaseRef:
    name: clickhouse-prod
  horizontalScaling:
    replicas: 3

Here,

  • spec.databaseRef.name specifies that we are performing horizontal scaling down operation on clickhouse-prod cluster.
  • spec.type specifies that we are performing HorizontalScaling on clickhouse.
  • spec.horizontalScaling.cluster[index].replicas specifies the desired replicas after scaling for the clickhouse nodes.

Let’s create the ClickHouseOpsRequest CR we have shown above,

$ kubectl apply -f https://github.com/kubedb/docs/raw/v2025.8.31/docs/examples/clickhouse/scaling/horizontal-scaling/chops-horizontal-scaling-down.yaml
clickhouseopsrequest.ops.kubedb.com/chops-scale-horizontal-down created

Verify clickhouse cluster replicas scaled down successfully

If everything goes well, KubeDB Ops-manager operator will update the replicas of ClickHouse object and related PetSets and Pods.

Let’s wait for ClickHouseOpsRequest to be Successful. Run the following command to watch ClickHouseOpsRequest CR,

➤ kubectl get clickhouseopsrequest -n demo chops-scale-horizontal-down
NAME                          TYPE                STATUS       AGE
chops-scale-horizontal-down   HorizontalScaling   Successful   60s

We can see from the above output that the ClickHouseOpsRequest has succeeded. If we describe the ClickHouseOpsRequest we will get an overview of the steps that were followed to scale the cluster.

➤ kubectl describe clickhouseopsrequest -n demo chops-scale-horizontal-down 
Name:         chops-scale-horizontal-down
Namespace:    demo
Labels:       <none>
Annotations:  <none>
API Version:  ops.kubedb.com/v1alpha1
Kind:         ClickHouseOpsRequest
Metadata:
  Creation Timestamp:  2025-08-25T12:30:09Z
  Generation:          1
  Resource Version:    821351
  UID:                 09e2b059-8ea7-4683-a5e5-c83ed14283cd
Spec:
  Apply:  IfReady
  Database Ref:
    Name:  clickhouse-prod
  Horizontal Scaling:
    Cluster:
      Cluster Name:  appscode-cluster
      Replicas:      3
  Type:              HorizontalScaling
Status:
  Conditions:
    Last Transition Time:  2025-08-25T12:30:09Z
    Message:               ClickHouse ops-request has started to horizontally scaling the nodes
    Observed Generation:   1
    Reason:                HorizontalScaling
    Status:                True
    Type:                  HorizontalScaling
    Last Transition Time:  2025-08-25T12:30:52Z
    Message:               Successfully Scaled Down Node
    Observed Generation:   1
    Reason:                HorizontalScaleDown
    Status:                True
    Type:                  HorizontalScaleDown
    Last Transition Time:  2025-08-25T12:30:17Z
    Message:               reassign partitions; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  ReassignPartitions
    Last Transition Time:  2025-08-25T12:30:17Z
    Message:               is pet set patched; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  IsPetSetPatched
    Last Transition Time:  2025-08-25T12:30:58Z
    Message:               clickhouse-prod-appscode-cluster-shard-1 already has desired replicas
    Observed Generation:   1
    Reason:                HorizontalScale
    Status:                True
    Type:                  HorizontalScale
    Last Transition Time:  2025-08-25T12:30:22Z
    Message:               get pod; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  GetPod
    Last Transition Time:  2025-08-25T12:30:22Z
    Message:               delete pvc; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  DeletePvc
    Last Transition Time:  2025-08-25T12:30:22Z
    Message:               get pvc; ConditionStatus:False
    Observed Generation:   1
    Status:                False
    Type:                  GetPvc
    Last Transition Time:  2025-08-25T12:30:58Z
    Message:               successfully reconciled the ClickHouse with modified node
    Observed Generation:   1
    Reason:                UpdatePetSets
    Status:                True
    Type:                  UpdatePetSets
    Last Transition Time:  2025-08-25T12:30:57Z
    Message:               reconcile; ConditionStatus:True
    Observed Generation:   1
    Status:                True
    Type:                  Reconcile
    Last Transition Time:  2025-08-25T12:30:58Z
    Message:               Successfully completed horizontally scale ClickHouse cluster
    Observed Generation:   1
    Reason:                Successful
    Status:                True
    Type:                  Successful
  Observed Generation:     1
  Phase:                   Successful
Events:
  Type     Reason                                     Age    From                         Message
  ----     ------                                     ----   ----                         -------
  Normal   Starting                                   4m33s  KubeDB Ops-manager Operator  Start processing for ClickHouseOpsRequest: demo/chops-scale-horizontal-down
  Normal   Starting                                   4m33s  KubeDB Ops-manager Operator  Pausing ClickHouse databse: demo/clickhouse-prod
  Normal   Successful                                 4m33s  KubeDB Ops-manager Operator  Successfully paused ClickHouse database: demo/clickhouse-prod for ClickHouseOpsRequest: chops-scale-horizontal-down
  Warning  reassign partitions; ConditionStatus:True  4m25s  KubeDB Ops-manager Operator  reassign partitions; ConditionStatus:True
  Warning  is pet set patched; ConditionStatus:True   4m25s  KubeDB Ops-manager Operator  is pet set patched; ConditionStatus:True
  Warning  get pod; ConditionStatus:True              4m20s  KubeDB Ops-manager Operator  get pod; ConditionStatus:True
  Warning  delete pvc; ConditionStatus:True           4m20s  KubeDB Ops-manager Operator  delete pvc; ConditionStatus:True
  Warning  get pvc; ConditionStatus:False             4m20s  KubeDB Ops-manager Operator  get pvc; ConditionStatus:False
  Warning  get pod; ConditionStatus:True              4m15s  KubeDB Ops-manager Operator  get pod; ConditionStatus:True
  Warning  delete pvc; ConditionStatus:True           4m15s  KubeDB Ops-manager Operator  delete pvc; ConditionStatus:True
  Warning  delete pvc; ConditionStatus:True           4m15s  KubeDB Ops-manager Operator  delete pvc; ConditionStatus:True
  Normal   HorizontalScaleDown                        4m10s  KubeDB Ops-manager Operator  Successfully Scaled Down Node
  Warning  reassign partitions; ConditionStatus:True  4m5s   KubeDB Ops-manager Operator  reassign partitions; ConditionStatus:True
  Warning  is pet set patched; ConditionStatus:True   4m5s   KubeDB Ops-manager Operator  is pet set patched; ConditionStatus:True
  Warning  get pod; ConditionStatus:True              4m     KubeDB Ops-manager Operator  get pod; ConditionStatus:True
  Warning  delete pvc; ConditionStatus:True           4m     KubeDB Ops-manager Operator  delete pvc; ConditionStatus:True
  Warning  get pvc; ConditionStatus:False             4m     KubeDB Ops-manager Operator  get pvc; ConditionStatus:False
  Warning  get pod; ConditionStatus:True              3m55s  KubeDB Ops-manager Operator  get pod; ConditionStatus:True
  Warning  delete pvc; ConditionStatus:True           3m55s  KubeDB Ops-manager Operator  delete pvc; ConditionStatus:True
  Warning  delete pvc; ConditionStatus:True           3m55s  KubeDB Ops-manager Operator  delete pvc; ConditionStatus:True
  Normal   HorizontalScaleDown                        3m50s  KubeDB Ops-manager Operator  Successfully Scaled Down Node
  Warning  reconcile; ConditionStatus:True            3m45s  KubeDB Ops-manager Operator  reconcile; ConditionStatus:True
  Warning  reconcile; ConditionStatus:True            3m45s  KubeDB Ops-manager Operator  reconcile; ConditionStatus:True
  Warning  reconcile; ConditionStatus:True            3m44s  KubeDB Ops-manager Operator  reconcile; ConditionStatus:True
  Normal   UpdatePetSets                              3m44s  KubeDB Ops-manager Operator  successfully reconciled the ClickHouse with modified node
  Normal   Starting                                   3m44s  KubeDB Ops-manager Operator  Resuming ClickHouse database: demo/clickhouse-prod
  Normal   Successful                                 3m44s  KubeDB Ops-manager Operator  Successfully resumed ClickHouse database: demo/clickhouse-prod for ClickHouseOpsRequest: chops-scale-horizontal-down

Now, we are going to verify the number of replicas this cluster has from the number of pods the petset have,

➤ kubectl get petset -n demo clickhouse-prod-appscode-cluster-shard-0 -o json | jq '.spec.replicas'
3

From all the above outputs we can see that the replicas of the clickhouse cluster is 3. That means we have successfully scaled down the replicas of the ClickHouse cluster.

Cleaning Up

To clean up the Kubernetes resources created by this tutorial, run:

kubectl delete clickhouse -n demo clickhouse-prod
kubectl delete clickhouseopsrequests -n demo chops-scale-horizontal-down chops-scale-horizontal-up
kubectl delete ns demo

Next Steps

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