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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>
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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[DOCS-13229] Conflicting info about Aggregation pipelines and sharded collections</title>
                <link>https://jira.mongodb.org/browse/DOCS-13229</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;
&lt;p&gt;&#160;&lt;br/&gt;
The docs for &quot;Aggregation Pipeline and Sharded Collections&quot; at&#160;&lt;a href=&quot;https://docs.mongodb.com/manual/core/aggregation-pipeline-sharded-collections/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/core/aggregation-pipeline-sharded-collections/&lt;/a&gt;&#160;has a behavior section that says &quot;For aggregation operations that must run on multiple shards, if the operations do not require running on the database&#8217;s primary shard, these operations will route the results to a random shard to merge the results to avoid overloading the primary shard for that database. The $out stage and the $lookup stage require running on the database&#8217;s primary shard.&quot;&#160;&#160;(starting 3.2)&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;But the docs for mongos&#160;&lt;a href=&quot;https://docs.mongodb.com/manual/core/sharded-cluster-query-router/#routing-and-results-process&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/core/sharded-cluster-query-router/#routing-and-results-process&lt;/a&gt;&#160;say &quot;Changed in version 3.6: For aggregation operations that run on multiple shards, if the operations do not require running on the database&#8217;s primary shard, these operations may route the results back to the mongos where the results are then merged.&quot;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;I asked in slack and Asya confirmed that the latter is the appropriate version - with the clarification that &quot;may&quot; is:&lt;/p&gt;

&lt;p&gt;If the operations do not require running on the database&#8217;s primary shard, and need to be merged on a shard, they are merged on a random shard which may include the primary.&#160;&lt;/p&gt;

&lt;p&gt;If the operations do not require running on the database&#8217;s primary shard, and DO NOT need to be merged on a shard, they are merged on the mongos.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;&#160;&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="1005011">DOCS-13229</key>
            <summary>Conflicting info about Aggregation pipelines and sharded collections</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="2" iconUrl="https://jira.mongodb.org/images/icons/priorities/critical.svg">Critical - P2</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="andrew.feierabend@mongodb.com">Andrew Feierabend</assignee>
                                    <reporter username="sheeri.cabral">Sheeri Cabral</reporter>
                        <labels>
                    </labels>
                <created>Fri, 15 Nov 2019 20:52:34 +0000</created>
                <updated>Mon, 30 Oct 2023 19:33:07 +0000</updated>
                            <resolved>Tue, 2 Mar 2021 21:54:50 +0000</resolved>
                                                    <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>manual</component>
                    <component>Server</component>
                        <due>Fri, 19 Feb 2021 00:00:00 +0000</due>
                            <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="3643197" author="xgen-internal-githook" created="Tue, 2 Mar 2021 21:53:21 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Andrew Feierabend&apos;, &apos;email&apos;: &apos;andrew.feierabend@mongodb.com&apos;, &apos;username&apos;: &apos;andf-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13229&quot; title=&quot;Conflicting info about Aggregation pipelines and sharded collections&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13229&quot;&gt;&lt;del&gt;DOCS-13229&lt;/del&gt;&lt;/a&gt; correct merge behavior sharded cluster&lt;br/&gt;
Branch: v3.6&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/1bea077ddc9c25ba065e5c18cd86de7c9b32bdca&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/1bea077ddc9c25ba065e5c18cd86de7c9b32bdca&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3643194" author="xgen-internal-githook" created="Tue, 2 Mar 2021 21:52:25 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Andrew Feierabend&apos;, &apos;email&apos;: &apos;andrew.feierabend@mongodb.com&apos;, &apos;username&apos;: &apos;andf-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13229&quot; title=&quot;Conflicting info about Aggregation pipelines and sharded collections&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13229&quot;&gt;&lt;del&gt;DOCS-13229&lt;/del&gt;&lt;/a&gt; correct merge behavior sharded cluster&lt;br/&gt;
Branch: v4.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/ff4b0eb04d789ff95e2d301a36bde55a3e810aec&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/ff4b0eb04d789ff95e2d301a36bde55a3e810aec&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3643188" author="xgen-internal-githook" created="Tue, 2 Mar 2021 21:51:26 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Andrew Feierabend&apos;, &apos;email&apos;: &apos;andrew.feierabend@mongodb.com&apos;, &apos;username&apos;: &apos;andf-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13229&quot; title=&quot;Conflicting info about Aggregation pipelines and sharded collections&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13229&quot;&gt;&lt;del&gt;DOCS-13229&lt;/del&gt;&lt;/a&gt; correct merge behavior sharded cluster&lt;br/&gt;
Branch: v4.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/43065f2af8d4bf34d9aed9b4877c116a3049c5ad&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/43065f2af8d4bf34d9aed9b4877c116a3049c5ad&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3643176" author="xgen-internal-githook" created="Tue, 2 Mar 2021 21:47:26 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Andrew Feierabend&apos;, &apos;email&apos;: &apos;andrew.feierabend@mongodb.com&apos;, &apos;username&apos;: &apos;andf-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13229&quot; title=&quot;Conflicting info about Aggregation pipelines and sharded collections&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13229&quot;&gt;&lt;del&gt;DOCS-13229&lt;/del&gt;&lt;/a&gt; correct merge behavior sharded cluster&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/fb8bd8cc913285ee56d0d53efc4061ed16aa121f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/fb8bd8cc913285ee56d0d53efc4061ed16aa121f&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3643172" author="xgen-internal-githook" created="Tue, 2 Mar 2021 21:46:27 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Andrew Feierabend&apos;, &apos;email&apos;: &apos;andrew.feierabend@mongodb.com&apos;, &apos;username&apos;: &apos;andf-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-13229&quot; title=&quot;Conflicting info about Aggregation pipelines and sharded collections&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-13229&quot;&gt;&lt;del&gt;DOCS-13229&lt;/del&gt;&lt;/a&gt; correct merge behavior sharded cluster&lt;br/&gt;
Branch: v5.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/b5294b77761cc6726cd0c3b45f21020dd58fad91&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/b5294b77761cc6726cd0c3b45f21020dd58fad91&lt;/a&gt;&lt;/p&gt;</comment>
                    </comments>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="267572">DOCS-7237</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="514224">DOCS-11469</issuekey>
        </issuelink>
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