说明

This finds the address of the secondary currently being used for reads. It is a read-only method: it does not change anything about the internal state of the object.

When you create a connection to the database, the driver will not immediately decide on a secondary to use. Thus, after you connect, this function will return NULL even if there are secondaries available. When you first do a query with slaveOkay set, at that point the driver will choose a secondary for this connection. At that point, this function will return the chosen secondary.

See the query section of this manual for information on distributing reads to secondaries.