Adding Read Replicas to a DB Instance

Scenarios

Read replicas of a DB instance are used to enhance instance capabilities and reduce the read pressure on the primary node. After a DB instance is created, you can add read replicas.

There are synchronous and asynchronous read replicas.

• Synchronous read replicas: Their failover priority is 1 and specifications are the same as those of the primary node. To avoid failover failures caused by inconsistent specifications between the primary node and read replicas, a DB instance must have a synchronous read replica, and a multi-AZ DB instance must have a synchronous read replica in a different AZ from the primary node.
• Asynchronous read replicas: Their failover priority is not 1 and specifications are different from those of the primary node.

For more information about read replicas, see Introducing Read Replicas.

Deployment Relationships Between the Primary Node and Read Replicas

• If you select single-AZ deployment, read replicas are deployed in the same AZ as the primary node.
• If you select multi-AZ deployment, read replicas are evenly deployed in different AZs to ensure high reliability.

Constraints

• Each yearly/monthly or pay-per-use DB instance has a maximum of 15 read replicas.
• Each serverless DB instance has a maximum of 7 read replicas.
• If all synchronous read replicas are unavailable during a failover, an asynchronous read replica will be promoted to primary.

Procedure

1. Log in to the management console.
2. Click in the upper left corner and select a region and project.
3. Click in the upper left corner of the page and choose Databases > GaussDB(for MySQL).
4. On the Instances page, locate the instance you want to add read replicas to and choose More > Create Read Replica in the Operation column.
5. On the displayed page, set required parameters.

   Table 1 Parameter description

   Parameter	Description
   Billing Mode	Pay-per-use DB instance: Pay-per-use and serverless read replicas can be added. Yearly/Monthly DB instance: Yearly/Monthly, pay-per-use, and serverless read replicas can be added. Serverless DB instance: Only serverless read replicas can be added.
   Failover Priority	Failover priority ranges from 1 for the first priority to 16 for the last priority. This priority determines the order in which read replicas are promoted when recovering from a primary node failure. Read replicas with the same priority have a same probability of being promoted to the new primary node. You can configure a failover priority for up to 9 read replicas, and the default priority for the remaining read replicas is -1, indicating these read replicas cannot be promoted to primary. You can change the failover priority of a read replica. NOTE: Serverless DB instance: The failover priority for the primary node can only be 1, while that for a newly added read replica can be 1 to 15. Yearly/Monthly DB instance: When a pay-per-use or serverless read replica is added, the failover priority is -1 by default and cannot be changed. Pay-per-use DB instance: When a serverless read replica is added, the failover priority is -1 by default and cannot be changed.
   AZ	GaussDB(for MySQL) multi-AZ instances allow you to select an AZ when creating a read replica. If no AZs are specified, the created read replicas are evenly distributed in each AZ. If too many nodes are created in a specified AZ, the read replicas may fail to be created due to insufficient resources. NOTE: To specify AZs, submit an application by choosing Service Tickets > Create Service Ticket in the upper right corner of the management console. Serverless DB instances do not allow you to specify AZs for read replicas.
   Instance Specifications	This parameter is only available for primary/standby DB instances. If the failover priority is set to 1, the specifications of read replicas must be the same as those of the primary node.
   Quantity	A DB instance can contain up to 15 read replicas.

6. For a yearly/monthly instance, click Next and select a payment mode.
7. For a pay-per-use instance, click Next.
8. Check the read replica settings.
   • If you need to modify the settings, click Previous.
   • If you do not need to modify the settings, click Submit.

9. View the new read replicas in the Node List area of the Basic Information page. You can also promote a read replica to primary or delete a read replica.

APIs

• Creating a Read Replica
• Deleting or Unsubscribing from a Read Replica