Hidden replica set members are used for special tasks. They are invisible to clients, will not show up in the db.isMaster() mongo shell command and similar administrative commands, and, for all purposes, will not be taken into account by clients (that is, read preference options).

They can vote for elections, but will never become a primary server. A hidden replica set member will only sync up to the primary server, and doesn't take reads from the clients. As such, it has the same write load as the primary server (for replication purposes), but no read load on its own.

Due to the previously mentioned characteristics, reporting is the most common application of a hidden member. We can connect directly to this member and use it as the data source of truth for OLAP.

To set up a hidden replica set member, we follow a similar procedure to priority to 0. After we have connected to our primary via the mongo shell, we get the configuration object, identify the member in the members sub-document that corresponds to the member we want to set as hidden, and subsequently set its priority to 0 and its hidden attribute to true. Finally, we have to apply the new configuration by calling rs.reconfig(config_object) with config_object that we used as a parameter:

> cfg = rs.conf()
> cfg.members[0].priority = 0
> cfg.members[0].hidden = true
> rs.reconfig(cfg)

A hidden replica set member can also be used for backup purposes. However, as you will see in the next section, we may want to use other options, either at the physical level or to replicate data at the logical level. In those cases, consider using a delayed replica set instead.