ApsaraDB for MongoDB automatically configures replica sets for instances. You can operate the primary and secondary nodes. Advanced functions such as disaster recovery and failover are available after the instance is created. You are not aware of these functions when you use instances.
Scale out nodes in a replica set instance
ApsaraDB for MongoDB allows you to scale out the number of nodes to five or seven. You can increase the number of secondary nodes as needed. For more information, see Change the number of nodes for a replica set instance.
For example, in scenarios with more read operations and fewer write operations, such as reading websites and order query systems, or with business spikes, you can add or delete secondary nodes based on your business needs to improve the read performance.