Navigation

Hidden Replica Set Members

On this page

  • Behavior
  • Further Reading

A hidden member maintains a copy of the primary's data set but is invisible to client applications. Hidden members are good for workloads with different usage patterns from the other members in the replica set. Hidden members must always be priority 0 members and so cannot become primary. The db.hello() method does not display hidden members. Hidden members, however, may vote in elections.

In the following five-member replica set, all four secondary members have copies of the primary's data set, but one of the secondary members is hidden.

Diagram of a 5 member replica set with a hidden priority 0 member.

Clients will not distribute reads with the appropriate read preference to hidden members. As a result, these members receive no traffic other than basic replication. Use hidden members for dedicated tasks such as reporting and backups. Delayed members should be hidden.

In a sharded cluster, mongos do not interact with hidden members.

Hidden members may vote in replica set elections. If you stop a voting hidden member, ensure that the set has an active majority or the primary will step down.

For the purposes of backups,

Hidden replica set members can acknowledge write operations issued with w: <number>. For write operations issued with w : "majority", however, hidden members must also be voting members (i.e. members[n].votes greater than 0) to acknowledge the "majority" write operation. Non-voting replica set members (i.e. members[n].votes is 0) cannot contribute to acknowledging write operations with majority write concern.

For more information about backing up MongoDB databases, see MongoDB Backup Methods. To configure a hidden member, see Configure a Hidden Replica Set Member.

Give Feedback

On this page

  • Behavior
  • Further Reading