Hidden Replica Set Members Behavior MongoDB Documentation, Release 3.0.4 Priority 0 Members as Standbys A priority 0 member can function as a standby. In some replica sets, it might not be possible to add a new member in a reasonable amount of time. A standby member keeps a current copy of the data to be able to replace an unavailable member. In many cases, you need not set standby to priority 0 . However, in sets with varied hardware or geographic distribution (page 579), a priority 0 standby ensures that only qualified members become primary. If your set already has seven voting members, also configure the member as non-voting (page 583). Priority 0 Members and Failover When configuring a priority 0 member, consider potential failover patterns, including all possible network partitions. Always ensure that your main data center contains both a quorum of voting members and contains members that are eligible to be primary. Configuration To configure a priority 0 member, see Prevent Secondary from Becoming Primary (page 621). 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 (page 567) and so cannot become primary . The db.isMaster() method does not display hidden members. Hidden members, however, may vote in elections (page 580). 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. Read Operations Clients will not distribute reads with the appropriate read preference (page 588) to hidden mem- bers. 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 (page 570) should be hidden. In a sharded cluster, mongos do not interact with hidden members. 569 9.2. Replication Concepts A priority 0 standby may also be valuable for some members of a set with different hardware or workload profiles. In these cases, deploy a member with priority 0 so it can’t become primary. Also consider using an hidden member (page 569) for this purpose.