Mastering MongoDB 4.x - Second Edition

Read preference

By default, all writes and reads go/come from the primary server. Secondary servers replicate data, but are not used for querying.

In some cases, it may be beneficial to change this and start to take reads from secondaries.

The MongoDB official drivers support five levels of read preference:

Read Preference Mode	Description
primary	This is the default mode, where reads come from the primary server of the replica set.
primaryPreferred	With this mode, applications will read from the primary, unless it is unavailable, in which case reads will come from secondary members.
secondary	Reads come exclusively from secondary servers.
secondaryPreferred	With this mode, applications will read from secondary ...