phpman > perldoc > MongoDB::ReadPreference(3pm)

NAME
    MongoDB::ReadPreference - Encapsulate and validate read preferences

VERSION
    version v2.2.2

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 AT mongodb.com>

    *   Rassi <rassi AT mongodb.com>

    *   Mike Friedman <friedo AT friedo.com>

    *   Kristina Chodorow <k.chodorow AT gmail.com>

    *   Florian Ragwitz <rafl AT debian.org>

COPYRIGHT AND LICENSE
    This software is Copyright (c) 2020 by MongoDB, Inc.

    This is free software, licensed under:

      The Apache License, Version 2.0, January 2004