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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-59904] The ShardServerCatalogCacheLoader ended up having two different chunks cache</title>
                <link>https://jira.mongodb.org/browse/SERVER-59904</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;As part of PM-2384, we introduced two ways of creating the chunks cache on shards: based on the name of the original collection or based on the UUID of that collection.&#160; An invariant of that project, which we don&apos;t explicitly check although we might want to do it, is that only one chunks cache exists at the same time. However, this is not always true (from BF-22521):&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;P1(Primary): it starts a refresh of a certain nss. Despite the fact that in the config server we find that this collection supports long names and the previous info we had about it didn&apos;t, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/bd87264196b631f9bc7d9960f94a246a2a93dc3f/src/mongo/db/s/shard_server_catalog_cache_loader.cpp#L812-L813&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;we don&apos;t actually rename the chunks cache because the epochs are different&lt;/a&gt; (which makes a lot of sense).&lt;/li&gt;
	&lt;li&gt;P1(Primary): The background thread &lt;a href=&quot;https://github.com/mongodb/mongo/blob/bd87264196b631f9bc7d9960f94a246a2a93dc3f/src/mongo/db/s/shard_server_catalog_cache_loader.cpp#L86&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;starts removing the entry on &lt;tt&gt;config.cache.collection&lt;/tt&gt; and the &lt;tt&gt;config.cache.chunks.nss&lt;/tt&gt;&lt;/a&gt;. A bit after we dropped the chunks cache this node stepsdown. The drop of the chunks cache is rollbacked but not the deletion of the entry on &lt;tt&gt;config.cache.collections&lt;/tt&gt; because it was majority committed.&lt;/li&gt;
	&lt;li&gt;P2 (New primary): it starts a refresh of the same nss. It doesn&apos;t see anything about that name on &lt;tt&gt;config.cache.collections&lt;/tt&gt; since the previous operation was majority committed. &lt;tt&gt;maxLoaderVersion&lt;/tt&gt; is &lt;tt&gt;UNSHARDED&lt;/tt&gt;.&lt;/li&gt;
	&lt;li&gt;P2 (New primary): it finds valid data on the config server. It creates a task that it is executed by the background thread.&lt;/li&gt;
	&lt;li&gt;P2 (New primary): the background thread &lt;a href=&quot;https://github.com/mongodb/mongo/blob/bd87264196b631f9bc7d9960f94a246a2a93dc3f/src/mongo/db/s/shard_server_catalog_cache_loader.cpp#L84-L85&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;is going to persist the valid info&lt;/a&gt; we got from the config server. Since &lt;tt&gt;maxLoaderVersion&lt;/tt&gt; is &lt;tt&gt;UNSHARDED&lt;/tt&gt; we believe that we don&apos;t have information about the previous collection so we don&apos;t drop &lt;tt&gt;config.cache.chunks.nss&lt;/tt&gt;. &lt;a href=&quot;https://github.com/mongodb/mongo/blob/bd87264196b631f9bc7d9960f94a246a2a93dc3f/src/mongo/db/s/shard_server_catalog_cache_loader.cpp#L132-L137&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;A few statements later we create &lt;tt&gt;config.cache.chunks.uuid&lt;/tt&gt;&lt;/a&gt;, having two chunk caches for the same name.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;A few questions we have to solve:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Would it be enough to &lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/s/shard_server_catalog_cache_loader.cpp#L729-L731&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;drop the target collection here&lt;/a&gt;?&lt;/li&gt;
	&lt;li&gt;Can we avoid leaving garbage?&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment></environment>
        <key id="1872470">SERVER-59904</key>
            <summary>The ShardServerCatalogCacheLoader ended up having two different chunks cache</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="antonio.fuschetto@mongodb.com">Antonio Fuschetto</assignee>
                                    <reporter username="sergi.mateo-bellido@mongodb.com">Sergi Mateo Bellido</reporter>
                        <labels>
                            <label>SSCCL-BUG</label>
                    </labels>
                <created>Mon, 13 Sep 2021 11:47:08 +0000</created>
                <updated>Sun, 29 Oct 2023 21:48:41 +0000</updated>
                            <resolved>Mon, 27 Sep 2021 12:10:37 +0000</resolved>
                                                    <fixVersion>5.1.0-rc0</fixVersion>
                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="4106538" author="JIRAUSER1259052" created="Wed, 6 Oct 2021 17:27:42 +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="4085932" author="xgen-internal-githook" created="Mon, 27 Sep 2021 11:25:59 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Antonio Fuschetto&apos;, &apos;email&apos;: &apos;antonio.fuschetto@mongodb.com&apos;, &apos;username&apos;: &apos;afuschetto&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-59904&quot; title=&quot;The ShardServerCatalogCacheLoader ended up having two different chunks cache&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-59904&quot;&gt;&lt;del&gt;SERVER-59904&lt;/del&gt;&lt;/a&gt; The ShardServerCatalogCacheLoader ended up having two different chunks cache&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/b9bf8bc726148c6c755e8fc045ace6ba235faf2a&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/b9bf8bc726148c6c755e8fc045ace6ba235faf2a&lt;/a&gt;&lt;/p&gt;</comment>
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