Mongo.setReadPref(mode, tagSet) primaries and secondaries are treated equivalently. 读优先级策略

https://docs.mongodb.com/manual/reference/method/Mongo.setReadPref/#Mongo.setReadPref

Mongo.setReadPref(mode, tagSet)

Call the setReadPref() method on a Mongo connection object to control how the client will route all queries to members of the replica set.

Parameter	Type	Description
mode	string	One of the following read preference modes: primary, primaryPreferred,secondary, secondaryPreferred, or nearest.
tagSet	array	Optional. A tag set used to specify custom read preference modes. For details, see Tag Sets.

https://docs.mongodb.com/manual/reference/read-preference/#nearest

The driver reads from a member whose network latency falls within the acceptable latency window. Reads in the nearest mode do not consider whether a member is a primary or secondary when routing read operations: primaries and secondaries are treated equivalently. The read preference member selectiondocumentation describes the process in detail.