ApsaraDB for MongoDB - Replica Set Instances Support Read-only Nodes
Target customers: users who require read/write splitting or data analysis. Features released: ApsaraDB for MongoDB replica set instances support read-only nodes to relieve the read operation pressure on primary and secondary nodes. When you create an instance, you can create up to five read-only nodes. When the instance is running, you can add or delete read-only nodes. This feature is launched in all regions and zones that support ApsaraDB for MongoDB replica set instances and available for the instances that run MongoDB 3.4, 4.0, and 4.2.