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.