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In a database cluster, standby nodes maintain a copy of the primary node. If the primary node fails, a standby node is automatically promoted to replace it. You can optionally send read traffic to standby nodes. See Use Standby Nodes for Reads.

MongoDB clusters can only have two standby nodes at this time. This is necessary for a high availability cluster configuration. See MongoDB replica set documentation.

You can add standby nodes during cluster creation in the cluster configuration section of the create page.

You can also add standby nodes to an existing database cluster. From the Databases page, click the cluster to go to its Overview page, then click the Settings tab.

On the Settings page, in the Cluster configuration section, click Edit. Open the Standby Nodes drop-down and choose the number of standby nodes.

When you’re done, click Save to immediately provision the standby nodes. The time to complete varies depending on the size of the primary node and its data, but we recommend allowing at least 5 minutes.

Use Standby Nodes for Reads

Standby nodes primarily provide high availability. If the primary node fails, a standby node is automatically promoted to replace it.

You can also send read traffic to standby nodes to improve cluster performance. However, doing so can overwhelm standby nodes and leave them unable to replace the primary during a failure.

To send read traffic to standby nodes, see Connect to Standby Nodes as Read-Only Nodes.