An ApsaraDB for MongoDB replica set instance consists of three nodes by default. ApsaraDB for MongoDB provides connection strings for you to connect to the primary node and a secondary node. The other secondary node is hidden as a backup to ensure high availability. If a node is faulty, the high availability system of ApsaraDB for MongoDB automatically triggers a primary/secondary failover to ensue the availability of the instance. You also can manually trigger a primary/secondary failover for an ApsaraDB for MongoDB instance in scenarios such as routine disaster recovery drills.
After you log on to the ApsaraDB for MongoDB console or call the SwitchDBInstanceHA operation to trigger a primary/secondary failover for a replica set instance, ApsaraDB for MongoDB interchanges the roles of the primary and secondary nodes.
- You can trigger a primary/secondary failover only for replica set and sharded cluster instances, but not for standalone instances due to their single-node architecture.
- After you trigger a primary/secondary failover for an instance, a transient connection error of up to 30 seconds will occur to the instance. Ensure that your applications can automatically re-establish a connection.
- You can trigger a primary/secondary failover only for instances in the running state.
- Log on to the ApsaraDB for MongoDB console.
- In the upper-left corner of the page, select the resource group and the region of the target instance.
- In the left-side navigation pane, click Replica Set Instances.
- Find the target instance and click its ID.
- In the Node List section, click Failover, as shown in the following figure.
- In the Failover dialog box, select Effective At and click Submit.Note Valid values for the Effective At parameter in the Failover dialog box:
- Effective Immediately: specifies that the system runs the failover task immediately.
- Effective Within Maintenance Window: specifies that the system runs the failover task within the specified maintenance period. For more information, see Specify a maintenance period.
- The instance status changes to HA Switching. The failover is successful when the instance status changes back to Running.
The failover takes about one minute. Then the instance returns to normal.Note If you have connected to the connection string of the primary node for an instance, you are connecting to a secondary node after a failover and you have no write permissions on the instance. In this case, you must connect to the connection string of the new primary node and obtain read and write permissions. For more information, see Overview of replica set instance connections.