Keep multiple copies of your MongoDB data so your application can survive a server failure.
MongoDB replication keeps multiple copies of the same data across different servers. A replica set normally has a primary that accepts writes and secondary members that replicate the primary's data.
If the primary becomes unavailable, MongoDB can elect another eligible member as the new primary. Replication improves availability and can also support certain read-scaling patterns, but it does not mean every read or write is automatically distributed across all servers.
What you'll walk away knowing