A read preference abstraction


mongoc_read_prefs_t provides an abstraction on top of the MongoDB connection read prefences. It allows for hinting to the driver which nodes in a replica set should be accessed first.

You can specify a read preference mode on connection objects, database objects, collection objects, or per-operation. Generally, it makes the most sense to stick with the global default, MONGOC_READ_PRIMARY. All of the other modes come with caveats that won't be covered in great detail here.

Read Modes


Default mode. All operations read from the current replica set primary.


All operations read from among the nearest secondary members of the replica set.


In most situations, operations read from the primary but if it is unavailable, operations read from secondary members.


In most situations, operations read from among the nearest secondary members, but if no secondaries are available, operations read from the primary.


Operations read from among the nearest members of the replica set, irrespective of the member’s type.

Tag Sets

Tag sets allow you to specify custom read preferences and write concerns so that your application can target operations to specific members.

Custom read preferences and write concerns evaluate tags sets in different ways: read preferences consider the value of a tag when selecting a member to read from. while write concerns ignore the value of a tag to when selecting a member except to consider whether or not the value is unique.

You can specify tag sets with the following read preference modes:

  • primaryPreferred

  • secondary

  • secondaryPreferred

  • nearest

Tags are not compatible with MONGOC_READ_PRIMARY and, in general, only apply when selecting a secondary member of a set for a read operation. However, the nearest read mode, when combined with a tag set will select the nearest member that matches the specified tag set, which may be a primary or secondary.

All interfaces use the same member selection logic to choose the member to which to direct read operations, basing the choice on read preference mode and tag sets.