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Replica Set High Availability

Replica sets provide high availability using automatic failover. Failover allows a secondary member to become primary if the current primary becomes unavailable.

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.

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. [1]

[1]Replica sets remove “rollback” data when needed without intervention. Administrators must apply or discard rollback data manually.