Security is one of the cornerstones of any enterprise-level system. Not always would you find a system in a completely safe and secure environment to allow unauthenticated user access to it. Apart from test environments, almost every production environment requires proper access rights and perhaps audit of the system access too. Mongo security has multiple aspects:
Access rights for the end users accessing the system. There would be multiple roles such as admin, read-only users, and read and write non-administrative users.
Authentication of the nodes that are added to the replica set. In a replica set, one should only be allowed to add authenticated systems. The integrity of the system would be compromised if any unauthenticated node is added to the replica set.
Encryption of the data that is transmitted across the wire between the nodes of the replica sets or even the client and the server (or the mongos process in case of sharded setup).
In this and the next recipe...