Provided by: libmongoc-doc_1.22.1-1_all 
NAME
mongoc_read_prefs_t - mongoc_read_prefs_t
A read preference abstraction
SYNOPSIS
mongoc_read_prefs_t provides an abstraction on top of the MongoDB connection read
preferences. It allows for hinting to the driver which nodes in a replica set should be
accessed first and how.
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 mode, 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 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.
Tag sets are represented as a comma-separated list of colon-separated key-value pairs when
provided as a connection string, e.g. dc:ny,rack:1.
To specify a list of tag sets, using multiple readPreferenceTags, e.g.
readPreferenceTags=dc:ny,rack:1;readPreferenceTags=dc:ny;readPreferenceTags=
Note the empty value for the last one, which means "match any secondary as a last resort".
Order matters when using multiple readPreferenceTags.
Tag Sets can also be configured using mongoc_read_prefs_set_tags().
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.
MAX STALENESS
When connected to replica set running MongoDB 3.4 or later, the driver estimates the
staleness of each secondary based on lastWriteDate values provided in server hello
responses.
Max Staleness is the maximum replication lag in seconds (wall clock time) that a secondary
can suffer and still be eligible for reads. The default is MONGOC_NO_MAX_STALENESS, which
disables staleness checks. Otherwise, it must be a positive integer at least
MONGOC_SMALLEST_MAX_STALENESS_SECONDS (90 seconds).
Max Staleness is also supported by sharded clusters of replica sets if all servers run
MongoDB 3.4 or later.
HEDGED READS
When connecting to a sharded cluster running MongoDB 4.4 or later, reads can be sent in
parallel to the two "best" hosts. Once one result returns, any other outstanding
operations that were part of the hedged read are cancelled.
When the read preference mode is MONGOC_READ_NEAREST and the sharded cluster is running
MongoDB 4.4 or later, hedged reads are enabled by default. Additionally, hedged reads may
be explicitly enabled or disabled by calling mongoc_read_prefs_set_hedge() with a BSON
document, e.g.
{
enabled: true
}
Appropriate values for the enabled key are true or false.
AUTHOR
MongoDB, Inc
COPYRIGHT
2017-present, MongoDB, Inc