MongoDB Documentation, Release 3.0.4 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, 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. Further Reading For more information about backing up MongoDB databases, see MongoDB Backup Methods (page 184). To configure a hidden member, see Configure a Hidden Replica Set Member (page 623). Delayed members contain copies of a replica set’s data set. However, a delayed member’s data set reflects an earlier, or delayed, state of the set. For example, if the current time is 09:52 and a member has a delay of an hour, the delayed member has no operation more recent than 08:52. Because delayed members are a “rolling backup” or a running “historical” snapshot of the data set, they may help you recover from various kinds of human error. For example, a delayed member can make it possible to recover from unsuccessful application upgrades and operator errors including dropped databases and collections. Requirements Delayed members: Behavior Delayed members apply operations from the oplog on a delay. When choosing the amount of delay, consider that the amount of delay: Sharding In sharded clusters, delayed members have limited utility when the balancer is enabled. Because delayed members replicate chunk migrations with a delay, the state of delayed members in a sharded cluster are not useful for recovering to a previous state of the sharded cluster if any migrations occur during the delay window. Example In the following 5-member replica set, the primary and all secondaries have copies of the data set. One member applies operations with a delay of 3600 seconds, or an hour. This delayed member is also hidden and is a priority 0 member . Chapter 9. Replication 570 Configuration A delayed member has its priority equal to 0 , hidden equal to true , and its slaveDelay equal to the number of seconds of delay: • must be is equal to or greater than your maintenance windows. • must be smaller than the capacity of the oplog. For more information on oplog size, see Oplog Size (page 593). • do vote in elections for primary. • Should be hidden (page 569) members. Always prevent applications from seeing and querying delayed mem- bers. • Must be priority 0 (page 567) members. Set the priority to 0 to prevent a delayed member from becoming primary. Delayed Replica Set Members Considerations