Hidden members are part of a replica set but cannot become primary and are invisible to client applications. Hidden members may vote in elections. For more information on hidden members and their uses, see Hidden Replica Set Members.
Considerations
The most common use of hidden nodes is to support backups.
You can also use hidden nodes to support delayed members. However, if you only need to prevent a member from becoming primary, configure a priority 0 member.
If the settings.chainingAllowed setting allows secondary members to sync from other secondaries, MongoDB prefers non-hidden members over hidden members as sync targets. MongoDB only chooses hidden members as a last resort. To override this behavior and sync from a specific, hidden member, use the replSetSyncFrom database command.
Examples
Member Configuration Document
To configure a secondary member as hidden, set its members[n].priority value to 0 and set its members[n].hidden value to true in its member configuration:
{
"_id" : <num>
"host" : <hostname:port>,
"priority" : 0, "hidden" : true
}
Configuration Procedure
The following example hides the secondary member currently at the index 0 in the members array. To configure a hidden member, use the following sequence of operations in a mongosh session that is connected to the primary, specifying the member to configure by its array index in the members array:
cfg = rs.conf()
cfg.members[0].priority = 0
cfg.members[0].hidden = true
rs.reconfig(cfg)
After re-configuring the set, this secondary member has a priority of 0 so that it cannot become primary and is hidden. The other members in the set will not advertise the hidden member in the hello command or db.hello() method output.
When updating the replica configuration object, access the replica set members in the members array with the array index. The array index begins with 0. Do not confuse this index value with the value of the members[n]._id field in each document in the members array.
Warning
- The
rs.reconfig() shell method can force the current primary to step down, which causes an election. When the primary steps down, the mongod closes all client connections. While this typically takes 10-20 seconds, try to make these changes during scheduled maintenance periods.
- Avoid reconfiguring replica sets that contain members of different MongoDB versions as validation rules may differ across MongoDB versions.
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