(mongodb >=1.0.0)
MongoDB\Driver\ReadPreference::RP_PRIMARY
All operations read from the current replica set primary. This is the default read preference for MongoDB.
MongoDB\Driver\ReadPreference::RP_PRIMARY_PREFERRED
In most situations, operations read from the primary but if it is unavailable, operations read from secondary members.
MongoDB\Driver\ReadPreference::RP_SECONDARY
All operations read from the secondary members of the replica set.
MongoDB\Driver\ReadPreference::RP_SECONDARY_PREFERRED
In most situations, operations read from secondary members but if no secondary members are available, operations read from the primary.
MongoDB\Driver\ReadPreference::RP_NEAREST
Operations read from member of the replica set with the least network latency, irrespective of the member's type.