All operations read from the current replica set primary. This is the default read preference for MongoDB.
In most situations, operations read from the primary but if it is unavailable, operations read from secondary members.
All operations read from the secondary members of the replica set.
In most situations, operations read from secondary members but if no secondary members are available, operations read from the primary.
Operations read from member of the replica set with the least network latency, irrespective of the member's type.