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    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-19467] Race in sh.startBalancer()</title>
                <link>https://jira.mongodb.org/browse/SERVER-19467</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;&lt;tt&gt;sh.startBalancer()&lt;/tt&gt; calls updates the balancer document in config.settings, followed by watching for a change in the timestamp of the balancer lock.  The sequence is:&lt;/p&gt;

&lt;ol&gt;
	&lt;li&gt;setBalancerState to true&lt;/li&gt;
	&lt;li&gt;Get balancer lock document&lt;/li&gt;
	&lt;li&gt;Extract the timestamp&lt;/li&gt;
	&lt;li&gt;assert.soon (up to 30s by default) watching for the timestamp to change&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;&lt;b&gt;If the balancer manages to start and take the lock between #1 and #2, and starts doing a non-trivial chunk migration, then the timeout will occur.&lt;/b&gt;  By contrast, if the balancer is slower the lock is taken after #2, then this will not happen, even if there is a non-trivial chunk migration.&lt;/p&gt;

&lt;p&gt;The impact of this is low, although the apparent failure of &lt;tt&gt;sh.startBalancer()&lt;/tt&gt; (despite the balancer clearly working) is often confusing (and there are other reasons it can happen).&lt;/p&gt;

&lt;p&gt;&lt;b&gt;Better would be to grab the lock document &lt;em&gt;before&lt;/em&gt; enabling the balancer, and then pass it through to the (eventual) &lt;tt&gt;assert.soon&lt;/tt&gt;.&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;&lt;tt&gt;sh.stopBalancer()&lt;/tt&gt; doesn&apos;t have this problem, because it waits for the balancer lock state to go to false.&lt;/p&gt;</description>
                <environment></environment>
        <key id="217348">SERVER-19467</key>
            <summary>Race in sh.startBalancer()</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="2">Won&apos;t Fix</resolution>
                                        <assignee username="backlog-server-sharding">[DO NOT USE] Backlog - Sharding Team</assignee>
                                    <reporter username="kevin.pulo@mongodb.com">Kevin Pulo</reporter>
                        <labels>
                            <label>PM229</label>
                            <label>balancer</label>
                    </labels>
                <created>Fri, 17 Jul 2015 04:09:23 +0000</created>
                <updated>Tue, 6 Dec 2022 04:47:56 +0000</updated>
                            <resolved>Wed, 30 Nov 2016 20:46:45 +0000</resolved>
                                                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
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