MongoDB::ReadPreference(3) Encapsulate and validate read preferences


version v1.2.3


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


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

For core documentation on read preference see <>.


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


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


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


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.


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").


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.



The read preference mode determines which server types are candidates for a read operation. Valid values are:
  • primary
  • primaryPreferred
  • secondary
  • secondaryPreferred
  • nearest


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.



This software is Copyright (c) 2016 by MongoDB, Inc..

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004