MongoDB::ReadPreference - Encapsulate and validate read preferences


version v2.2.2


    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.


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.


  • David Golden <>

  • Rassi <>

  • Mike Friedman <>

  • Kristina Chodorow <>

  • Florian Ragwitz <>


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

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004