You can purchase more read-only nodes after you have created a cluster.

Each node in a PolarDB cluster has a failover priority. The system performs the following steps to automatically elect a primary node:

Find all the read-only nodes that can be elected as the primary node.

Select one or more read-only nodes that have the highest failover priority.

If the failover to the first node fails due to reasons, such as network issues and abnormal replication status, the system tries to switch to another node until the failover succeeds.

https://www.alibabacloud.com/help/doc-detail/141229.htm?spm=a2c63.p38356.b99.113.36df231dK2DVX0