Read 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.

By default, in MongoDB, read operations to a replica set return results from the primary.

Users may configure read preference on a per-connection basis to prefer that the read operations return results from the secondary members. If clients configure the read preference to permit secondary reads, read operations can return data from secondary members that have not replicated more recent write operations.

This behavior is sometimes characterized as eventual consistency because the secondary member’s state will eventually reflect the primary’s state and MongoDB cannot guarantee strict consistency for read operations from secondary members.

Note