NAME

       mongoc_read_prefs_t - A read preference abstraction

SYNOPSIS

       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

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

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

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

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

       MONGOC_READ_NEAREST
              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:

       \[bu]
         primaryPreferred

       \[bu]
         secondary

       \[bu]
         secondaryPreferred

       \[bu]
         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.

COLOPHON

       This     page     is     part     of    MongoDB    C    Driver.     Please    report    any    bugs    at
       https://jira.mongodb.org/browse/CDRIVER.

MongoDB C Driver                                   2016‐01‐18                             MONGOC_READ_PREFS_T(3)