mongoc_read_mode_t - Man Page

typedef enum {
   MONGOC_READ_PRIMARY = (1 << 0),
   MONGOC_READ_SECONDARY = (1 << 1),
   MONGOC_READ_PRIMARY_PREFERRED = (1 << 2) | MONGOC_READ_PRIMARY,
   MONGOC_READ_SECONDARY_PREFERRED = (1 << 2) | MONGOC_READ_SECONDARY,
   MONGOC_READ_NEAREST = (1 << 3) | MONGOC_READ_SECONDARY,
} mongoc_read_mode_t;

This enum describes how reads should be dispatched. The default is MONGOC_READ_PRIMARY.

Please see the MongoDB website for a description of Read Preferences.

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