Replica Set High Availability¶
Changed in version 3.2: MongoDB introduces a version 1 of the replication protocol (protocolVersion: 1) to reduce replica set failover time and accelerates the detection of multiple simultaneous primaries. New replica sets will, by default, use protocolVersion: 1. Previous versions of MongoDB use version 0 of the protocol. To upgrade existing replica sets to use protocolVersion: 1, see Upgrade a Replica Set to 3.2.
Replica set members keep the same data set but are otherwise independent. If the primary becomes unavailable, an eligible secondary holds an election to elect itself as a new primary. In some situations, the failover process may undertake a rollback. 
|||Replica sets remove “rollback” data when needed without intervention. Administrators must apply or discard rollback data manually.|