From the perspective of a client application, whether a MongoDB instance is running as a single server (i.e. "standalone") or a replica set is transparent. However, MongoDB provides additional read and write configurations for replica sets.
Note
Sharded clusters where the shards are also replica sets provide the same operational semantics with regards to write and read operations.
- Write concern describes the level of acknowledgment requested from MongoDB for write operations.
- Read preference specifies where (i.e. which members of the replica set) the drivers should direct the read operations.
- Describes the mechanics of read preference.