(mongodb >=1.0.0)
MongoDB\Driver\ReadPreference::RP_PRIMARYAll operations read from the current replica set primary. This is the default read preference for MongoDB.
MongoDB\Driver\ReadPreference::RP_PRIMARY_PREFERREDIn most situations, operations read from the primary but if it is unavailable, operations read from secondary members.
MongoDB\Driver\ReadPreference::RP_SECONDARYAll operations read from the secondary members of the replica set.
MongoDB\Driver\ReadPreference::RP_SECONDARY_PREFERREDIn most situations, operations read from secondary members but if no secondary members are available, operations read from the primary.
MongoDB\Driver\ReadPreference::RP_NEARESTOperations read from member of the replica set with the least network latency, irrespective of the member's type.