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.

Read replicas can be synchronous or asynchronous.

• Synchronous read replica: a read replica whose 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 replica: a read replica whose failover priority is not 1 and specifications are different from those of the primary node.

For more information, 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 a service ticket 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 when creating read replicas.
   Instance Specifications	Only primary/standby DB instances allow you to select specifications when creating a read replica. If the specifications of a read replica are different from those of the primary node, the failover priority cannot be set to 1.
   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