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Replica Set Read and Write Semantics

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.


Sharded clusters where the shards are also replica sets provide the same operational semantics with regards to write and read operations.

Write Concern for Replica Sets
Write concern describes the level of acknowledgement requested from MongoDB for write operations.
Read Preference
Read preference specifies where (i.e. which members of the replica set) the drivers should direct the read operations.
Server Selection Algorithm
Describes the mechanics of read preference.