What is Replica Set?
A replica set in MongoDB is a group of mongod processes that maintain the same data set.
Each replica set node must belong to one, and only one, replica set. Replica set nodes cannot belong to more than one replica set.
A replica set contains several data bearing nodes and optionally one arbiter node. Of the data bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.