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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-34554] Database drops do not have a total ordering in a change stream</title>
                <link>https://jira.mongodb.org/browse/SERVER-34554</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In a sharded cluster it may be possible for two database drops to happen on two different shards at the same time. Since a database does not have a UUID or a documentKey, the resume token for the drop will simply contain a timestamp. This means that is possible for two distinct changes to have the same resume token.&lt;/p&gt;

&lt;p&gt;A database drop will invalidate the stream, so this likely isn&apos;t an issue, but if we allow an actual notification of this event without an invalidation, then a client who tries to resume with one of the two identical resume tokens might see a drop twice or not at all.&lt;/p&gt;</description>
                <environment></environment>
        <key id="530584">SERVER-34554</key>
            <summary>Database drops do not have a total ordering in a change stream</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="charlie.swanson@mongodb.com">Charlie Swanson</reporter>
                        <labels>
                            <label>change-streams-improvements</label>
                            <label>query-44-grooming</label>
                    </labels>
                <created>Wed, 18 Apr 2018 20:27:48 +0000</created>
                <updated>Tue, 6 Dec 2022 03:31:56 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>10</watches>
                                                                                                                <comments>
                            <comment id="1934390" author="nicholas.zolnierz" created="Thu, 28 Jun 2018 22:11:22 +0000"  >&lt;p&gt;Bumping out of the Metadata Notifications epic, but planning to circle back as part of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-34932&quot; title=&quot;[change streams] Adjust resume token (if necessary) for sharded transactions&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-34932&quot;&gt;&lt;del&gt;SERVER-34932&lt;/del&gt;&lt;/a&gt; since there may be related resume token work for sharded transactions. &lt;/p&gt;</comment>
                            <comment id="1874423" author="spencer" created="Wed, 25 Apr 2018 21:34:32 +0000"  >&lt;p&gt;Also is it weird that now a drop of a sharded collection can give a notification for each shard?  Should we be de-duping them somehow? &lt;br/&gt;
 We avoided this issue in 3.6 by invalidating the stream after the first drop is seen.&lt;/p&gt;</comment>
                            <comment id="1874179" author="charlie.swanson" created="Wed, 25 Apr 2018 18:54:34 +0000"  >&lt;p&gt;Yes, they each represent the same event, so the impact is probably very low if it exists at all. But it&apos;s still technically a gap in the global ordering.&lt;/p&gt;</comment>
                            <comment id="1874170" author="asya" created="Wed, 25 Apr 2018 18:49:14 +0000"  >&lt;p&gt;But those represent the exact same event, right? (collection drop globally which just happened to have been executed on every shard, is that right?)&lt;/p&gt;</comment>
                            <comment id="1873635" author="charlie.swanson" created="Wed, 25 Apr 2018 13:27:41 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=spencer&quot; class=&quot;user-hover&quot; rel=&quot;spencer&quot;&gt;spencer&lt;/a&gt; also points out that this is a problem for drops of a sharded collection, for which there will be multiple entries all with the same UUID, and potentially the same timestamp.&lt;/p&gt;</comment>
                            <comment id="1871732" author="asya" created="Mon, 23 Apr 2018 18:33:45 +0000"  >&lt;p&gt;Right, it also seems to be a non-problem since before dropping database all collection drops will show up in the stream.   So it reduces the importance of guarantying that a database drop is seen exactly once.&lt;/p&gt;</comment>
                            <comment id="1868163" author="charlie.swanson" created="Wed, 18 Apr 2018 20:39:24 +0000"  >&lt;p&gt;cc &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=schwerin&quot; class=&quot;user-hover&quot; rel=&quot;schwerin&quot;&gt;schwerin&lt;/a&gt; and &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=spencer&quot; class=&quot;user-hover&quot; rel=&quot;spencer&quot;&gt;spencer&lt;/a&gt; per our in-person discussion we believe that this might not be a problem in practice because the config servers are responsible for driving a database drop, and so there may be an imposed ordering that would guarantee no two drops can have the same cluster time?&lt;/p&gt;

&lt;p&gt;This needs further investigation.&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="542153">SERVER-34932</issuekey>
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