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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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            <title>[SERVER-10686] Selection of replicas in mongos not conforming to documentation</title>
                <link>https://jira.mongodb.org/browse/SERVER-10686</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The mechanism in mongos to select a replica with secondary read preference does not do what the documentation says it does.&lt;/p&gt;

&lt;p&gt;At &lt;a href=&quot;http://docs.mongodb.org/manual/core/read-preference-mechanics/#member-selection&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://docs.mongodb.org/manual/core/read-preference-mechanics/#member-selection&lt;/a&gt; it says that the algorithm takes the nearest replica (lowest ping time), and then computes a set of other replicas that are no further than the lowest ping time plus the value of --localThreshold.  It then chooses one of these replicas at random.&lt;/p&gt;

&lt;p&gt;Looking at the code in dbclient_rs.cpp, it takes --localThreshold as an absolute value and returns the first node that has a ping time less than that.  If there is no such node, it returns a random node that matches the tags.&lt;/p&gt;

&lt;p&gt;For comparison, I checked the Java driver and found that it implements what the documentation says.&lt;/p&gt;</description>
                <environment></environment>
        <key id="88670">SERVER-10686</key>
            <summary>Selection of replicas in mongos not conforming to documentation</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="3">Duplicate</resolution>
                                        <assignee username="randolph@mongodb.com">Randolph Tan</assignee>
                                    <reporter username="andre.spiegel@mongodb.com">Andre Spiegel</reporter>
                        <labels>
                    </labels>
                <created>Thu, 5 Sep 2013 17:27:46 +0000</created>
                <updated>Wed, 10 Dec 2014 23:05:07 +0000</updated>
                            <resolved>Fri, 24 Jan 2014 20:53:18 +0000</resolved>
                                    <version>2.5.2</version>
                                                    <component>Internal Client</component>
                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="487539" author="richard@10gen.com" created="Fri, 24 Jan 2014 20:52:49 +0000"  >&lt;p&gt;Marking as duplicate per &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=redbeard0531&quot; class=&quot;user-hover&quot; rel=&quot;redbeard0531&quot;&gt;redbeard0531&lt;/a&gt;&apos;s direction.&lt;/p&gt;</comment>
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