Replica sets provide high availability using automatic failover. Failover allows a secondary members to become primary if primary is unavailable. Failover, in most situations does not require manual intervention.
Replica set members keep the same data set but are otherwise independent. If the primary becomes unavailable, the replica set holds an election to select a new primary. In some situations, the failover process may require a rollback. 
The deployment of a replica set affects the outcome of failover situations. To support effective failover, ensure that one facility can elect a primary if needed. Choose the facility that hosts the core application systems to host the majority of the replica set. Place a majority of voting members and all the members that can become primary in this facility. Otherwise, network partitions could prevent the set from being able to form a majority.
|||Replica sets remove “rollback” data when needed without intervention. Administrators must apply or discard rollback data manually.|
The replica set recovers from the loss of a primary by holding an election. Consider the following: