MongoDB Replication

Keeping copies of the same data on several servers, and keeping those copies up to date, is called replication. If one server fails, the data still exists on the others, and another server can start accepting writes in its place.

MongoDB organizes this as a group of servers called a replica set. All members of the group hold the same data. One member, the primary, accepts every write. The other data-bearing members, the secondaries, copy those writes from the primary and can serve reads.