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    <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-12271] Docs for SERVER-32308: Add the ability for a $lookup stage to execute on mongos against a sharded foreign collection</title>
                <link>https://jira.mongodb.org/browse/DOCS-12271</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;blockquote&gt;
&lt;h3&gt;&lt;a name=&quot;Description%3A&quot;&gt;&lt;/a&gt;Description: &lt;/h3&gt;

&lt;p&gt;Allows for localField/foreignField $lookup into a sharded collection.&lt;/p&gt;

&lt;h3&gt;&lt;a name=&quot;EngineeringTicketDescription%3A&quot;&gt;&lt;/a&gt;Engineering Ticket Description:&lt;/h3&gt;

&lt;p&gt;In order to support &lt;tt&gt;$changeStreams&lt;/tt&gt; &quot;updateLookup&quot; functionality (&lt;a href=&quot;https://docs.mongodb.com/manual/tutorial/change-streams-example/#lookup-full-document-for-update-operations&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;see the documentation here&lt;/a&gt;), the aggregation subsystem now has machinery for $lookup-like query execution on mongos.&lt;/p&gt;

&lt;p&gt;We should extend this functionality to work for the &lt;tt&gt;$lookup&lt;/tt&gt; stage itself:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;When the foreign collection is sharded, split the pipeline such that the &lt;tt&gt;$lookup&lt;/tt&gt; is part of the merging pipeline. Execute the merging pipeline on mongos, performing lookups across the network to the shards as needed.&lt;/li&gt;
	&lt;li&gt;If the foreign collection is not sharded, send the merging part of the pipeline to the primary shard and perform local lookups, just as we do in all existing versions of MongoDB.&lt;/li&gt;
	&lt;li&gt;If the merging part of the pipeline containing the &lt;tt&gt;$lookup&lt;/tt&gt; cannot execute on mongos, raise an error to the application.&lt;/li&gt;
	&lt;li&gt;Support only &lt;tt&gt;localField&lt;/tt&gt;/&lt;tt&gt;foreignField&lt;/tt&gt; &lt;tt&gt;$lookup&lt;/tt&gt; for now; adding support for &lt;tt&gt;$lookup&lt;/tt&gt; with a sub-pipeline into a sharded foreign collection is future work.&lt;/li&gt;
&lt;/ul&gt;
&lt;/blockquote&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="653057">DOCS-12271</key>
            <summary>Docs for SERVER-32308: Add the ability for a $lookup stage to execute on mongos against a sharded foreign 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="13203">Gone away</resolution>
                                        <assignee username="kay.kim@mongodb.com">Kay Kim</assignee>
                                    <reporter username="kay.kim@mongodb.com">Kay Kim</reporter>
                        <labels>
                    </labels>
                <created>Thu, 13 Dec 2018 16:17:17 +0000</created>
                <updated>Mon, 13 Nov 2023 18:27:56 +0000</updated>
                            <resolved>Tue, 2 Jul 2019 23:00:13 +0000</resolved>
                                                    <fixVersion>4.1.7</fixVersion>
                    <fixVersion>Server_Docs_20231030</fixVersion>
                    <fixVersion>Server_Docs_20231106</fixVersion>
                    <fixVersion>Server_Docs_20231105</fixVersion>
                    <fixVersion>Server_Docs_20231113</fixVersion>
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