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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>[SERVER-33247] Repair replicated and sharded collection UUIDs</title>
                <link>https://jira.mongodb.org/browse/SERVER-33247</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In v3.6, --repair sort of handles lack of UUIDs in some collections by placing the node in kUpgradingTo36 FCV mode.&lt;/p&gt;

&lt;p&gt;In v4.0, however, UUIDs should always be present, so there is no mode we can put it in where it is acceptable for UUIDs not to be present. Therefore, we shall fix up the UUIDs on --repair if any are missing. &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-33151&quot; title=&quot;Refuse to start up mongod 4.0 unless all collections have UUIDs&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-33151&quot;&gt;&lt;del&gt;SERVER-33151&lt;/del&gt;&lt;/a&gt; makes startup require all collections to have UUIDs. We can easily generate UUIDs on startup for non-replicated collections without complication (this is done by &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-33246&quot; title=&quot;Assign UUIDs to non-replicated collections on startup if any are missing UUIDs&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-33246&quot;&gt;&lt;del&gt;SERVER-33246&lt;/del&gt;&lt;/a&gt;). However, replicated collections require a cross replica set repair solution; and sharded collection require a cross cluster repair solution. That is what this ticket is for.&lt;/p&gt;

&lt;p&gt;The sharding catalog also persists UUIDs in the config.collections collection on config servers, and config.cache.collections collection on shards. These may have to be considered as well.&lt;/p&gt;</description>
                <environment></environment>
        <key id="495969">SERVER-33247</key>
            <summary>Repair replicated and sharded collection UUIDs</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="xiangyu.yao@mongodb.com">Xiangyu Yao</assignee>
                                    <reporter username="dianna.hohensee@mongodb.com">Dianna Hohensee</reporter>
                        <labels>
                    </labels>
                <created>Fri, 9 Feb 2018 21:05:54 +0000</created>
                <updated>Mon, 9 Jul 2018 12:43:32 +0000</updated>
                            <resolved>Fri, 29 Jun 2018 18:13:18 +0000</resolved>
                                                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="1941704" author="dianna.hohensee" created="Mon, 9 Jul 2018 12:43:32 +0000"  >&lt;p&gt;Sounds good. Thanks for the explanation.&lt;/p&gt;</comment>
                            <comment id="1940672" author="xiangyu.yao" created="Fri, 6 Jul 2018 16:12:52 +0000"  >&lt;blockquote&gt;
&lt;p&gt;how would the UUID be fixed in standalone mode?&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;It would require the server to downgrade to v3.6 and use v3.6 mechanism to fix the UUIDs.&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;And can initial sync be provoked when up to date data-wise, and will it get a UUID for a namespace it already has?&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;Initial sync method requires users to manually wipe out all the data and re-sync from a node that has UUIDs.&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;Can the UUID be dictated, so a sharded cluster can be repaired? Writes to the config server&apos;s sharding metadata would be necessary as well.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;I think it would be a huge issue if there is any UUID inconsistency between shards,  between config servers or between shards and config servers. Given that there could be all sorts of different scenarios and the rarity of the problem, a manual fix is better than a one command fix.&lt;/p&gt;

&lt;p&gt;The overall idea is that this scenario should be rare and if it happened, manual fixes should be involved. Even fixing UUIDs on standalone by &quot;--repair&quot; would be dangerous because the usage is not what users expect: users might see it as a way to fix a node which has some UUIDs missing but if other nodes in the replica set have the UUIDs, an initial sync is actually the right solution.&lt;/p&gt;</comment>
                            <comment id="1936411" author="dianna.hohensee" created="Mon, 2 Jul 2018 13:25:42 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=xiangyu.yao&quot; class=&quot;user-hover&quot; rel=&quot;xiangyu.yao&quot;&gt;xiangyu.yao&lt;/a&gt;, how would the UUID be fixed in standalone mode? And can initial sync be provoked when up to date data-wise, and will it get a UUID for a namespace it already has? Can the UUID be dictated, so a sharded cluster can be repaired? Writes to the config server&apos;s sharding metadata would be necessary as well.&lt;/p&gt;

&lt;p&gt;Should this manual recovery process be documented, and an appropriate error message given to users when UUIDs are missing? The mongod server won&apos;t start up without all UUIDs, I believe, so the fix is somewhat complicated.&lt;/p&gt;</comment>
                            <comment id="1935187" author="xiangyu.yao" created="Fri, 29 Jun 2018 18:12:59 +0000"  >&lt;p&gt;In the distributed environment, if a node has UUIDs missing, it should do an initial sync rather than repair. If all the nodes in the replica set don&apos;t have the UUID for a collection, we should fix one node in standalone mode and let other nodes initial sync from it.&lt;br/&gt;
I&apos;m closing this ticket as Won&apos;t Fix.&lt;/p&gt;</comment>
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