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Hidden Replica Set Members

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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.isMaster() 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.

Behavior

Read Operations

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.

Voting

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,

  • If using the MMAPv1 storage engine, you can avoid stopping a hidden member with the db.fsyncLock() and db.fsyncUnlock() operations to flush all writes and lock the mongod instance for the duration of the backup operation.

  • Changed in version 3.2: db.fsyncLock() can ensure that the data files do not change for MongoDB instances using either the MMAPv1 or the WiredTiger storage engines, thus providing consistency for the purposes of creating backups.

    In previous MongoDB versions, db.fsyncLock() cannot guarantee a consistent set of files for low-level backups (e.g. via file copy cp, scp, tar) for WiredTiger.

Further Reading

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