Load balancing for an ApsaraDB for ClickHouse cluster - ApsaraDB for ClickHouse

An ApsaraDB for ClickHouse cluster mounts a Classic Load Balancer (CLB) instance for you by default when you use the cluster to process large-scale data and want to ensure efficient load balancing. After the ApsaraDB for ClickHouse cluster mounts the CLB instance, the CLB instance implements automatic failover for the cluster, performs health checks, and distributes traffic to normal nodes to ensure the high availability of the cluster. Mounting a CLB instance is a paid feature and will be billed in ApsaraDB for ClickHouse. If you do not need this feature, you can release the CLB instance. To ensure service reliability, we recommend that you do not release the CLB instance.

Precautions

You cannot release or mount a CLB instance as a Resource Access Management (RAM) user.
By default, a CLB instance is mounted to an ApsaraDB for ClickHouse cluster that was created after December 1, 2021, and you are charged for the CLB instance. For more information, see Billing details about relevant services.
After you release the CLB instance mounted to an ApsaraDB for ClickHouse cluster, you are not charged for the CLB instance from the next hour of the release.
If you release a CLB instance or mount a CLB instance again, the number of the port used to connect to the ApsaraDB for ClickHouse cluster over a virtual private cloud (VPC) changes. After the CLB instance is released or the CLB instance is mounted again, change the number of the port used to connect to the program.
Note
You can view the number of the port that is used to connect to the ApsaraDB for ClickHouse cluster over the VPC in the Network Information section of the Cluster Information page.

Overview

CLB distributes inbound network traffic across multiple Elastic Compute Service (ECS) instances that act as backend servers based on forwarding rules. CLB helps improve the performance and availability of your applications. For more information about CLB, see What is CLB?

CLB instance status	Description
Mounted	A CLB instance runs at the network layer. You can use a domain name to bind the CLB instance to the IP addresses of all nodes in an ApsaraDB for ClickHouse cluster to provide VPC connectivity. Note If you use the public endpoint to connect to an ApsaraDB for ClickHouse cluster after a CLB instance is mounted, the CLB instance in the public endpoint must be the same as that in the VPC. After a CLB instance is mounted to an ApsaraDB for ClickHouse cluster, the CLB instance implements automatic failover for the cluster. The CLB instance forwards requests to the nodes in the ApsaraDB for ClickHouse cluster and balances the request traffic among the nodes. The CLB instance checks the availability of backend nodes. If the CLB instance detects that a node is unavailable by checking the health status of all nodes, the CLB instance automatically isolates the unavailable node. This ensures that the request traffic is balanced among available nodes. Note If your ApsaraDB for ClickHouse cluster is of Single-replica Edition, the CLB instance does not route requests to abnormal nodes. However, due to abnormal nodes, your data is scattered on different nodes. Therefore, the data that you obtain may be incomplete.

Released	After you release a CLB instance, VPC connectivity is disabled, and simple load balancing is performed on inbound traffic based on the public domain name. After a CLB instance is released, the CLB instance no longer checks the health status of the nodes at the request routing layer for the ApsaraDB for ClickHouse cluster. As a result, unavailable nodes may fail to be detected, and normal requests may be routed to the unavailable nodes. This causes the failures of some read and write requests.

Release a CLB instance

Log on to the ApsaraDB for ClickHouse console.
In the top navigation bar, select the region where the cluster is deployed.
On the Clusters page, click the Default Instances tab.
Click the ID of the cluster. The Cluster Information page appears.
In the Network Information section, click Release CLB Instance next to VPC Endpoint.
In the message that appears, read the content and click OK.
When the cluster state changes from The CLB instance is being released to Running, the CLB instance is released.
Important
When you release a CLB instance, the number of the port that is used to connect to the ApsaraDB for ClickHouse cluster over the VPC changes, and then clients cannot connect to the database service. After the CLB instance is released, you must change the number of the port used to connect to the program. You can view the number of the port that is used to connect to the ApsaraDB for ClickHouse cluster over the VPC in the Network Information section of the Cluster Information page.

Mount a CLB instance

Log on to the ApsaraDB for ClickHouse console.
In the top navigation bar, select the region where the cluster is deployed.
On the Clusters page, click the Default Instances tab.
Click the ID of the cluster. The Cluster Information page appears.
In the Network Information section, click Mount CLB Instance next to VPC Endpoint.
In the message that appears, read the content and click OK.
When the cluster state changes from The CLB instance is being mounted to Running, the CLB instance is mounted.
Important
When you mount a CLB instance, the number of the port that is used to connect to the ApsaraDB for ClickHouse cluster over the VPC changes, and then clients cannot connect to the database service. After the CLB instance is mounted, you must change the number of the port used to connect to the program. You can view the number of the port that is used to connect to the ApsaraDB for ClickHouse cluster over the VPC in the Network Information section of the Cluster Information page.