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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-28627] replace MapReduceFinishCommand&apos;s use of ParallelSortClusteredCursor with establishCursors()/ARM</title>
                <link>https://jira.mongodb.org/browse/SERVER-28627</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;MapReduceFinishCommand both &lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/commands/mr.cpp#L1835-L1844&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;establishes cursors and merges them&lt;/a&gt; using ParallelSortClusteredCursor.&lt;/p&gt;

&lt;p&gt;We should be able to replace the establishing cursors part with the new establishCursors() function, but I&apos;m not sure what the best way to replace the merging functionality is.&lt;/p&gt;

&lt;p&gt;My first thought is naturally to use the ARM, but currently the ARM is embedded in ClusterClientCursor, and ClusterClientCursor doesn&apos;t exist on shards.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch&quot; class=&quot;user-hover&quot; rel=&quot;david.storch&quot;&gt;david.storch&lt;/a&gt;, is there another way we might be able to merge the cursor streams on a shard (DocumentSourceMerge?)? Does the Query team still have plans to make the ARM available on shards?&lt;/p&gt;</description>
                <environment></environment>
        <key id="370892">SERVER-28627</key>
            <summary>replace MapReduceFinishCommand&apos;s use of ParallelSortClusteredCursor with establishCursors()/ARM</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="2">Won&apos;t Fix</resolution>
                                        <assignee username="backlog-server-sharding">[DO NOT USE] Backlog - Sharding Team</assignee>
                                    <reporter username="esha.maharishi@mongodb.com">Esha Maharishi</reporter>
                        <labels>
                    </labels>
                <created>Tue, 4 Apr 2017 21:53:18 +0000</created>
                <updated>Tue, 6 Dec 2022 04:04:11 +0000</updated>
                            <resolved>Mon, 23 Sep 2019 16:06:38 +0000</resolved>
                                    <version>3.5.5</version>
                                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="1541432" author="david.storch" created="Wed, 5 Apr 2017 14:43:39 +0000"  >&lt;p&gt;Sounds good, feel free to put something on my calendar for next week.&lt;/p&gt;</comment>
                            <comment id="1541420" author="esha.maharishi@10gen.com" created="Wed, 5 Apr 2017 14:36:19 +0000"  >&lt;p&gt;Hmm, okay.&lt;/p&gt;

&lt;p&gt;The intent of getting rid of ParallelSortClusteredCursor is to remove all uses of old shard versioning logic (things that use ShardConnection/setShardVersion).&lt;/p&gt;

&lt;p&gt;However, I don&apos;t think mapReduce actually needs the shard versioning logic, since the chunks shouldn&apos;t be moved.&lt;/p&gt;

&lt;p&gt;Given that there isn&apos;t an obvious alternative, I&apos;ll think about whether we can rip the shard versioning logic out and convert ParallelSortClusteredCursor into a simple cursor-merging component for shards. I don&apos;t love this solution, though, since it means find, agg, and mapReduce will each have their own cursor-merging utilities.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch&quot; class=&quot;user-hover&quot; rel=&quot;david.storch&quot;&gt;david.storch&lt;/a&gt;, let&apos;s chat early next week about a final decision?&lt;/p&gt;</comment>
                            <comment id="1541314" author="david.storch" created="Wed, 5 Apr 2017 13:28:32 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=esha.maharishi&quot; class=&quot;user-hover&quot; rel=&quot;esha.maharishi&quot;&gt;esha.maharishi&lt;/a&gt;, as far as I know there are three cursor merging implementations:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;ParallelSortClusteredCursor&lt;/li&gt;
	&lt;li&gt;AsyncResultsMerger&lt;/li&gt;
	&lt;li&gt;DocumentSourceMergeCursors&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;The query team does not currently have plans to make the ARM available on shards or to abstract the ARM in such a way that makes it useful outside the mongos find/aggregate paths. DocumentSourceMergeCursors is used on mongod, but it probably isn&apos;t what you want either, since it&apos;s pretty specific to the aggregation subsystem.&lt;/p&gt;

&lt;p&gt;What&apos;s the goal of getting rid of ParallelSortClusteredCursor in the mapReduce sharded finish logic? Is it mainly just so you can delete ParallelSortClusteredCursor, or was there something related to killing off the old shard versioning logic as well?&lt;/p&gt;</comment>
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