-> Replication is the process of synchronizing data across multiple servers. Replication provides redundancy and increased data availability with multiple copies of data on different database servers. Replication protects database from the loss of a single server. Replication also allows you to recover from hardware failure and service interruptions.
-> To keep your data safe.
-> High (24*7) availability of data.
-> Disaster recovery
-> Read scaling (extra copies to read from)
-> MongoDB achieves replication by the use of replica set. A replica set is a group of mongod instances that host the same dataset. In a replica set, one node is primary node that receives all write operations. All other instances, such as secondaries, apply operations from the primary so that they have the same dataset. Replica set can have only one primary node.