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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-56763] Validate collection epoch when not holding a DB lock for $merge</title>
                <link>https://jira.mongodb.org/browse/SERVER-56763</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When fixing &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-54507&quot; title=&quot;Can&amp;#39;t execute $merge if sharding catalog cache is empty&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-54507&quot;&gt;&lt;del&gt;SERVER-54507&lt;/del&gt;&lt;/a&gt; we discussed a possible future optimization in preparing to execute $merge. The idea is to check the targetCollectionVersion epoch against matching later on when you are not holding the DB lock, or call&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;ShardServerProcessInterface::checkRoutingInfoEpochOrThrow()&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;not under a DB lock right before execution of the query on the leaf nodes of the merge topology.&lt;/p&gt;

&lt;p&gt;Why would this be better?&#160;&lt;/p&gt;

&lt;p&gt;The short answer:&lt;br/&gt;
Because the shard is serving as a router and not as a shard, so this epoch check also doesn&apos;t matter theoretically.&lt;br/&gt;
The long answer:&lt;br/&gt;
Because the current check only makes sure that this MongoD (pretending to be a router) knows at least as much as the router which sent the merge command. However,&#160;in the grand scheme of things both can be wrong and agree on the same wrong thing.&#160;&#160;It&apos;s more correct to have the leaf nodes of the merge topology do the epoch check or to call&#160;&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;ShardServerProcessInterface::checkRoutingInfoEpochOrThrow()&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;Which avoids the case where the leaf nodes that do the data reads know about a dropped collection and the mongos doesn&apos;t at the time it sends the targetCollectionVersion to the mongod acting as the router.&lt;/p&gt;

&lt;p&gt;Note that there could still be a pathological case where the merge topology has 2 leaf nodes and one is reached much earlier than the second, and the first one processes Petabytes of data when the collection is dropped on the second leaf. The only theoretical way to get around this is to probably open cursors on all shards that will participate in the merge plan, but that would be possibly infeasible.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1706905">SERVER-56763</key>
            <summary>Validate collection epoch when not holding a DB lock for $merge</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="nicholas.zolnierz@mongodb.com">Nicholas Zolnierz</assignee>
                                    <reporter username="eric.cox@mongodb.com">Eric Cox</reporter>
                        <labels>
                    </labels>
                <created>Fri, 7 May 2021 17:35:41 +0000</created>
                <updated>Sun, 29 Oct 2023 21:53:53 +0000</updated>
                            <resolved>Thu, 15 Jul 2021 22:29:21 +0000</resolved>
                                                    <fixVersion>5.0.3</fixVersion>
                    <fixVersion>5.1.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="4108352" author="JIRAUSER1259052" created="Wed, 6 Oct 2021 19:04:38 +0000"  >&lt;p&gt;Updating the fixversion since branching activities occurred yesterday. This ticket will be in rc0 when it&#8217;s been triggered. For more active release information, please keep an eye on #server-release. Thank you!&lt;/p&gt;</comment>
                            <comment id="3981115" author="xgen-internal-githook" created="Wed, 4 Aug 2021 17:54:38 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Nick Zolnierz&apos;, &apos;email&apos;: &apos;nicholas.zolnierz@mongodb.com&apos;, &apos;username&apos;: &apos;nzolnierzmdb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-56763&quot; title=&quot;Validate collection epoch when not holding a DB lock for $merge&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-56763&quot;&gt;&lt;del&gt;SERVER-56763&lt;/del&gt;&lt;/a&gt; Move $merge target collection epoch validation outside of&lt;br/&gt;
parsing&lt;/p&gt;

&lt;p&gt;This allows us to consult and refresh the catalog cache without holding locks&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 23ecc48f89f4ec03d7b42e637c5969802efdb261)&lt;br/&gt;
Branch: v5.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/c1a2864de00a38c9c800218ca44c31088927de8b&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/c1a2864de00a38c9c800218ca44c31088927de8b&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3948276" author="kaloian.manassiev" created="Sat, 17 Jul 2021 06:09:38 +0000"  >&lt;p&gt;I am not familiar with what is the purpose of the collection Epoch change, but wanted to clarify that any version/epoch changes without lock performed by a node that&apos;s acting as a shard (i.e., returning some data) are incorrect because nothing guarantees that this epoch will not change immediately afterwards. Furthermore, the check in this change is done from the cache which assumes it is done while acting as a router.&lt;/p&gt;

&lt;p&gt;Which is the role of the code which executes this check (i.e., will it return any data locally or is it just passing it on to another node and prepared to receive a StaleShardVersion) ?&lt;/p&gt;</comment>
                            <comment id="3944905" author="xgen-internal-githook" created="Thu, 15 Jul 2021 22:28:53 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Nick Zolnierz&apos;, &apos;email&apos;: &apos;nicholas.zolnierz@mongodb.com&apos;, &apos;username&apos;: &apos;nzolnierzmdb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-56763&quot; title=&quot;Validate collection epoch when not holding a DB lock for $merge&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-56763&quot;&gt;&lt;del&gt;SERVER-56763&lt;/del&gt;&lt;/a&gt; Move $merge target collection epoch validation outside of&lt;br/&gt;
parsing&lt;/p&gt;

&lt;p&gt;This allows us to consult and refresh the catalog cache without holding locks&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/23ecc48f89f4ec03d7b42e637c5969802efdb261&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/23ecc48f89f4ec03d7b42e637c5969802efdb261&lt;/a&gt;&lt;/p&gt;</comment>
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