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@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 is free software, licensed under:

  The Apache License, Version 2.0, January 2004