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ClickHouse Horizontal Scaling

This guide will give an overview on how KubeDB Ops-manager operator scales up or down ClickHouse cluster replicas of various component such as Combined, Broker, Controller.

Before You Begin

• You should be familiar with the following KubeDB concepts:
  • ClickHouse
  • ClickHouseOpsRequest

How Horizontal Scaling Process Works

The following diagram shows how KubeDB Ops-manager operator scales up or down ClickHouse database components. Open the image in a new tab to see the enlarged version.

  

The Horizontal scaling process consists of the following steps:

1. At first, a user creates a ClickHouse Custom Resource (CR).

2. KubeDB Provisioner operator watches the ClickHouse CR.

3. When the operator finds a ClickHouse CR, it creates required number of PetSets and related necessary stuff like secrets, services, etc.

4. Then, in order to scale the various components (ie. ReplicaSet, Shard, ConfigServer etc.) of the ClickHouse cluster, the user creates a ClickHouseOpsRequest CR with desired information.

5. KubeDB Ops-manager operator watches the ClickHouseOpsRequest CR.

6. When it finds a ClickHouseOpsRequest CR, it halts the ClickHouse object which is referred from the ClickHouseOpsRequest. So, the KubeDB Provisioner operator doesn’t perform any operations on the ClickHouse object during the horizontal scaling process.

7. Then the KubeDB Ops-manager operator will scale the related PetSet Pods to reach the expected number of replicas defined in the ClickHouseOpsRequest CR.

8. After the successfully scaling the replicas of the related PetSet Pods, the KubeDB Ops-manager operator updates the number of replicas in the ClickHouse object to reflect the updated state.

9. After the successful scaling of the ClickHouse replicas, the KubeDB Ops-manager operator resumes the ClickHouse object so that the KubeDB Provisioner operator resumes its usual operations.

In the next docs, we are going to show a step by step guide on horizontal scaling of ClickHouse cluster using ClickHouseOpsRequest CRD.