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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <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-74202] Update balancer to use client local Prng for thread-safe shuffling</title>
                <link>https://jira.mongodb.org/browse/SERVER-74202</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;As part of introducing the defragmentation policy, we introduced new threads in the balancer. These different threads are using the same &lt;a href=&quot;https://github.com/mongodb/mongo/blob/543ad92616e96d54894f84dafb4c2904c1a032c3/src/mongo/db/s/balancer/balancer.cpp#L278&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;random_device&lt;/a&gt; to shuffle different vectors, causing data races.&lt;/p&gt;

&lt;p&gt;Currently, we are using &lt;tt&gt;std::random_device&lt;/tt&gt; in:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/df67dd6db8dfa969930fa09a730b12d8220e8a3e/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp#L361&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;balancer_chunk_selection_policy_impl.cpp&lt;/a&gt;&lt;/li&gt;
	&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/df67dd6db8dfa969930fa09a730b12d8220e8a3e/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp#L426&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;balancer_chunk_selection_policy_impl.cpp&lt;/a&gt;&lt;/li&gt;
	&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c636ba3a9914fd3eddc905d8b1dfc50668e239b4/src/mongo/db/s/balancer/balancer_defragmentation_policy_impl.cpp#L1514&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;balancer_defragmentation_policy_impl.cpp&lt;/a&gt;&lt;/li&gt;
	&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/c636ba3a9914fd3eddc905d8b1dfc50668e239b4/src/mongo/db/s/balancer/balancer.cpp#L549&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;balancer.cpp&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;We instead need to use the Client local Prng when using &lt;tt&gt;std::shuffle&lt;/tt&gt; as:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Client * client = ...;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;std::shuffle(begin, end, client-&amp;gt;getPrng().urbg());&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;With this implementation, we will not worry about thread-safety because the RNG is thread-local.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2270382">SERVER-74202</key>
            <summary>Update balancer to use client local Prng for thread-safe shuffling</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="13201">Fixed</resolution>
                                        <assignee username="pol.pinol@mongodb.com">Pol Pinol</assignee>
                                    <reporter username="pol.pinol@mongodb.com">Pol Pinol</reporter>
                        <labels>
                            <label>sharding-wfbf-day</label>
                    </labels>
                <created>Tue, 21 Feb 2023 11:49:01 +0000</created>
                <updated>Sun, 29 Oct 2023 21:25:58 +0000</updated>
                            <resolved>Mon, 17 Apr 2023 09:38:32 +0000</resolved>
                                                    <fixVersion>7.0.0-rc0</fixVersion>
                    <fixVersion>7.1.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="5358206" author="xgen-internal-githook" created="Wed, 19 Apr 2023 09:03:17 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Pol Pinol Castuera&apos;, &apos;email&apos;: &apos;pol.pinol@mongodb.com&apos;, &apos;username&apos;: &apos;PolPinol&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-74202&quot; title=&quot;Update balancer to use client local Prng for thread-safe shuffling&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-74202&quot;&gt;&lt;del&gt;SERVER-74202&lt;/del&gt;&lt;/a&gt; Update balancer to use client local Prng for thread-safe shuffling&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 6a90a4d95b2f3c1f2619d332dd9d73fb39fe6167)&lt;br/&gt;
Branch: v7.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/69eb1ea37aee4c18edc7324a8f42992fc1f21eb1&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/69eb1ea37aee4c18edc7324a8f42992fc1f21eb1&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5351609" author="xgen-internal-githook" created="Mon, 17 Apr 2023 09:08:29 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Pol Pinol Castuera&apos;, &apos;email&apos;: &apos;pol.pinol@mongodb.com&apos;, &apos;username&apos;: &apos;PolPinol&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-74202&quot; title=&quot;Update balancer to use client local Prng for thread-safe shuffling&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-74202&quot;&gt;&lt;del&gt;SERVER-74202&lt;/del&gt;&lt;/a&gt; Update balancer to use client local Prng for thread-safe shuffling&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/6a90a4d95b2f3c1f2619d332dd9d73fb39fe6167&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/6a90a4d95b2f3c1f2619d332dd9d73fb39fe6167&lt;/a&gt;&lt;/p&gt;</comment>
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