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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>[DOCS-13583] Investigate changes in SERVER-45624: Pre-split and distribute chunks of sessions collection</title>
                <link>https://jira.mongodb.org/browse/DOCS-13583</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;h2&gt;&lt;a name=&quot;Description&quot;&gt;&lt;/a&gt;Description&lt;/h2&gt;
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&lt;p&gt;This ticket introduces a mongod startup+runtime server parameter called &quot;minNumChunksForSessionsCollection&quot; (default: 1024, min: 1, max: 1,000,000). On a config server&apos;s primary, this parameter corresponds to the the target minimum number of config.system.sessions chunks for the balancer. Based on the comments in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-45624&quot; title=&quot;Pre-split and distribute chunks of sessions collection&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-45624&quot;&gt;&lt;del&gt;SERVER-45624&lt;/del&gt;&lt;/a&gt;, Atlas users do not have the privileges to do any administrative changes to the sessions collection so it is up to the DOCS team to decide whether this should be documented.&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;
&lt;h2&gt;&lt;a name=&quot;DescriptionofLinkedTicket&quot;&gt;&lt;/a&gt;Description of Linked Ticket&lt;/h2&gt;

&lt;p&gt;Sessions records are small in size, so usually the sessions collection doesn&apos;t get automatically splitted by the auto-splitter and thus it is not balanced automatically. This can cause an high load on the primary shard of the sessions collection.&lt;/p&gt;

&lt;p&gt;Some context:&lt;br/&gt;
 The session collection is sharded by default on the `_id` field and a session record looks like this one:&lt;/p&gt;
&lt;p/&gt;
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&lt;p/&gt;
&lt;p&gt;As you can see the `_id` field is an object itself and the internal `id` is of type UUID. In particular we use &lt;a href=&quot;https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;UUIDv4&lt;/a&gt; that is a 128-bit randomly generated number. Since some of the internal bits are used to store the version and the variant, it is not possible to use normal integer arithmetics on this type. Fortunately we can still compare two UUID and sort them, this is necessary to use them as split points.&lt;/p&gt;

&lt;p&gt;I ran a &lt;a href=&quot;https://gist.github.com/toto-dev/c44dd19bb5f2d2ca46c8aa75d779f974#file-uuid-js&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;syntethic benchmark&lt;/a&gt; to see how sessions get distributed on a partitioned collection. I partitioned the collection in 10 equally sized chunks and assigned each one to a different shard. I tried first to simulate 1000 sessions and then 100.000.&lt;/p&gt;

&lt;p&gt;&lt;img src=&quot;https://jira.mongodb.org/images/icons/attach/noimage.png&quot; imagetext=&quot;Sessions distribution on shards_1k_sessions.jpeg&quot; align=&quot;absmiddle&quot; border=&quot;0&quot; /&gt;&lt;br/&gt;
 &lt;img src=&quot;https://jira.mongodb.org/images/icons/attach/noimage.png&quot; imagetext=&quot;Sessions distribution on shards_100k_sessions.jpeg&quot; align=&quot;absmiddle&quot; border=&quot;0&quot; /&gt;&lt;/p&gt;

&lt;p&gt;As you can see from the graphs the truly random distribution of the generated UUID made possible to distribute them fairly among the shards.&lt;/p&gt;

&lt;p&gt;I&apos;ve also created a &lt;a href=&quot;https://gist.github.com/toto-dev/c44dd19bb5f2d2ca46c8aa75d779f974#file-split_sessions-js&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;js script&lt;/a&gt; that if executed on a mongos will partition the session collection and it will distribute one chunk to every shard.&lt;/p&gt;
&lt;h2&gt;&lt;a name=&quot;Scopeofchanges&quot;&gt;&lt;/a&gt;Scope of changes&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;ImpacttoOtherDocs&quot;&gt;&lt;/a&gt;Impact to Other Docs&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;MVP%28WorkandDate%29&quot;&gt;&lt;/a&gt;MVP (Work and Date)&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;Resources%28ScopeorDesignDocs%2CInvision%2Cetc.%29&quot;&gt;&lt;/a&gt;Resources (Scope or Design Docs, Invision, etc.)&lt;/h2&gt;</description>
                <environment></environment>
        <key id="1310842">DOCS-13583</key>
            <summary>Investigate changes in SERVER-45624: Pre-split and distribute chunks of sessions collection</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</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="kay.kim@mongodb.com">Kay Kim</assignee>
                                    <reporter username="backlog-server-pm">Backlog - Core Eng Program Management Team</reporter>
                        <labels>
                            <label>docs-sharding</label>
                    </labels>
                <created>Fri, 10 Apr 2020 13:47:21 +0000</created>
                <updated>Mon, 13 Nov 2023 18:07:01 +0000</updated>
                            <resolved>Mon, 18 May 2020 17:30:34 +0000</resolved>
                                                    <fixVersion>4.4.0-rc1</fixVersion>
                    <fixVersion>4.2.7</fixVersion>
                    <fixVersion>4.7.0</fixVersion>
                    <fixVersion>3.6.21</fixVersion>
                    <fixVersion>4.0.22</fixVersion>
                    <fixVersion>Server_Docs_20231030</fixVersion>
                    <fixVersion>Server_Docs_20231106</fixVersion>
                    <fixVersion>Server_Docs_20231105</fixVersion>
                    <fixVersion>Server_Docs_20231113</fixVersion>
                                    <component>manual</component>
                        <due>Fri, 15 May 2020 00:00:00 +0000</due>
                            <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="3106688" author="xgen-internal-githook" created="Tue, 26 May 2020 13:18:22 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Kay Kim&apos;, &apos;email&apos;: &apos;kay.kim@10gen.com&apos;, &apos;username&apos;: &apos;kay-kim&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13583&quot; title=&quot;Investigate changes in SERVER-45624: Pre-split and distribute chunks of sessions collection&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13583&quot;&gt;&lt;del&gt;DOCS-13583&lt;/del&gt;&lt;/a&gt;: 4.2.7 system.sessions autosplit&lt;br/&gt;
Branch: v4.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/850d2f3f1daed8195fba4582ea5884e26106c138&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/850d2f3f1daed8195fba4582ea5884e26106c138&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3093267" author="xgen-internal-githook" created="Mon, 18 May 2020 17:29:36 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Kay Kim&apos;, &apos;email&apos;: &apos;kay.kim@10gen.com&apos;, &apos;username&apos;: &apos;kay-kim&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13583&quot; title=&quot;Investigate changes in SERVER-45624: Pre-split and distribute chunks of sessions collection&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13583&quot;&gt;&lt;del&gt;DOCS-13583&lt;/del&gt;&lt;/a&gt;: 4.2.7 system.sessions autosplit&lt;br/&gt;
Branch: v4.2.7&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/68c7e9dfa13b39d7cd5400769fa18140c46e4105&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/68c7e9dfa13b39d7cd5400769fa18140c46e4105&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3093167" author="xgen-internal-githook" created="Mon, 18 May 2020 17:28:04 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Kay Kim&apos;, &apos;email&apos;: &apos;kay.kim@10gen.com&apos;, &apos;username&apos;: &apos;kay-kim&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13583&quot; title=&quot;Investigate changes in SERVER-45624: Pre-split and distribute chunks of sessions collection&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13583&quot;&gt;&lt;del&gt;DOCS-13583&lt;/del&gt;&lt;/a&gt;: system.sessions autosplit&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/50b79f02874fe80a031acba155a7afc646b428b8&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/50b79f02874fe80a031acba155a7afc646b428b8&lt;/a&gt;&lt;/p&gt;</comment>
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