NAME

       MongoDB::ReadPreference - Encapsulate and validate read preferences

VERSION

       version v1.8.1

SYNOPSIS

           use MongoDB::ReadPreference;

           $rp = MongoDB::ReadPreference->new(); # mode: primary

           $rp = MongoDB::ReadPreference->new(
               mode     => 'primaryPreferred',
               tag_sets => [ { dc => 'useast' }, {} ],
           );

DESCRIPTION

       A read preference indicates which servers should be used for read operations.

       For core documentation on read preference see
       <http://docs.mongodb.org/manual/core/read-preference/>.

USAGE

       Read preferences work via two attributes: "mode" and "tag_sets".  The "mode" parameter
       controls the types of servers that are candidates for a read operation as well as the
       logic for applying the "tag_sets" attribute to further restrict the list.

       The following terminology is used in describing read preferences:

       •   candidates – based on "mode", servers that could be suitable, based on "tag_sets" and
           other logic

       •   eligible – these are candidates that match "tag_sets"

       •   suitable – servers that meet all criteria for a read operation

   Read preference modes
       primary

       Only an available primary is suitable.  "tag_sets" do not apply and must not be provided
       or an exception is thrown.

       secondary

       All secondaries (and only secondaries) are candidates, but only eligible candidates (i.e.
       after applying "tag_sets") are suitable.

       primaryPreferred

       Try to find a server using mode "primary" (with no "tag_sets").  If that fails, try to
       find one using mode "secondary" and the "tag_sets" attribute.

       secondaryPreferred

       Try to find a server using mode "secondary" and the "tag_sets" attribute.  If that fails,
       try to find a server using mode "primary" (with no "tag_sets").

       nearest

       The primary and all secondaries are candidates, but only eligible candidates (i.e. after
       applying "tag_sets" to all candidates) are suitable.

       NOTE: in retrospect, the name "nearest" is misleading, as it implies a choice based on
       lowest absolute latency or geographic proximity, neither which are true.

       The "nearest" mode merely includes both primaries and secondaries without any preference
       between the two.  All are filtered on "tag_sets".  Because of filtering, servers might not
       be "closest" in any sense.  And if multiple servers are suitable, one is randomly chosen
       based on the rules for server selection, which again might not be the closest in absolute
       latency terms.

   Tag set matching
       The "tag_sets" parameter is a list of tag sets (i.e. key/value pairs) to try in order.
       The first tag set in the list to match any candidate server is used as the filter for all
       candidate servers.  Any subsequent tag sets are ignored.

       A read preference tag set ("T") matches a server tag set ("S") – or equivalently a server
       tag set ("S") matches a read preference tag set ("T") — if "T" is a subset of "S" (i.e. "T
       ⊆ S").

       For example, the read preference tag set "{ dc => 'ny', rack => 2 }" matches a secondary
       server with tag set "{ dc => 'ny', rack => 2, size => 'large' }".

       A tag set that is an empty document – "{}" – matches any server, because the empty tag set
       is a subset of any tag set.

ATTRIBUTES

   mode
       The read preference mode determines which server types are candidates for a read
       operation.  Valid values are:

       •   primary

       •   primaryPreferred

       •   secondary

       •   secondaryPreferred

       •   nearest

   tag_sets
       The "tag_sets" parameter is an ordered list of tag sets used to restrict the eligibility
       of servers, such as for data center awareness.

       The application of "tag_sets" varies depending on the "mode" parameter.  If the "mode" is
       'primary', then "tag_sets" must not be supplied.

   max_staleness_seconds
       The "max_staleness_seconds" parameter represents the maximum replication lag in seconds
       (wall clock time) that a secondary can suffer and still be eligible for reads. The default
       is -1, which disables staleness checks.

       If the "mode" is 'primary', then "max_staleness_seconds" must not be supplied.

AUTHORS

       •   David Golden <david@mongodb.com>

       •   Rassi <rassi@mongodb.com>

       •   Mike Friedman <friedo@friedo.com>

       •   Kristina Chodorow <k.chodorow@gmail.com>

       •   Florian Ragwitz <rafl@debian.org>

COPYRIGHT AND LICENSE

       This software is Copyright (c) 2018 by MongoDB, Inc.

       This is free software, licensed under:

         The Apache License, Version 2.0, January 2004