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    <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-27239] Remove the usage of distributed collection lock during map/reduce</title>
                <link>https://jira.mongodb.org/browse/SERVER-27239</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Map/reduce with a &lt;a href=&quot;https://docs.mongodb.com/v3.2/core/map-reduce-sharded-collections/#sharded-collection-as-output&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;sharded output collection&lt;/a&gt; performs an optimization where the final reduce step is omitted and the previous reduce step is done in parallel on all shards. This is done by creating an empty sharded collection and spreading the chunks so they are co-located with the data to be reduced based on the shard key, writing all output locally to an empty temporary collection and then renaming the temporary collection to the name of the output collection.&lt;/p&gt;

&lt;p&gt;This process only works if no chunks of the output collection move around while the output is being written and is protected through the usage of the collection distributed lock.&lt;/p&gt;

&lt;p&gt;This task is to get rid of this reliance on the collection distributed lock.&lt;/p&gt;</description>
                <environment></environment>
        <key id="335326">SERVER-27239</key>
            <summary>Remove the usage of distributed collection lock during map/reduce</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="9">Done</resolution>
                                        <assignee username="backlog-server-sharding">[DO NOT USE] Backlog - Sharding Team</assignee>
                                    <reporter username="kaloian.manassiev@mongodb.com">Kaloian Manassiev</reporter>
                        <labels>
                            <label>remove-distributed-lock-fallout</label>
                    </labels>
                <created>Wed, 30 Nov 2016 22:14:38 +0000</created>
                <updated>Tue, 6 Dec 2022 04:10:36 +0000</updated>
                            <resolved>Sat, 24 Jul 2021 06:41:40 +0000</resolved>
                                                                    <component>MapReduce</component>
                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="3961262" author="kaloian.manassiev" created="Sat, 24 Jul 2021 06:41:40 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=asya&quot; class=&quot;user-hover&quot; rel=&quot;asya&quot;&gt;asya&lt;/a&gt;, this is correct, but not because it&apos;s aggregation, but because it never creates a new sharded collection. Closing as Gone Away.&lt;/p&gt;</comment>
                            <comment id="3959839" author="asya" created="Fri, 23 Jul 2021 14:40:56 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=kaloian.manassiev&quot; class=&quot;user-hover&quot; rel=&quot;kaloian.manassiev&quot;&gt;kaloian.manassiev&lt;/a&gt; this can probably be closed since MR is now aggregation?&lt;/p&gt;</comment>
                            <comment id="2087467" author="renctan" created="Tue, 11 Dec 2018 19:52:15 +0000"  >&lt;p&gt;I also realized that the dist lock also serves another purpose besides making sure that chunks don&apos;t move around. It serializes concurrent mapReduce outputting to the same sharded collection. Because of how merge uses a temp collection to perform the reduce and drop the original collection, mapReduce outputting to the same collection will step on each other&apos;s toes if they are allowed to run at the same time.&lt;/p&gt;</comment>
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