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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-33090] $changeStream can fail if a shard was removed</title>
                <link>https://jira.mongodb.org/browse/SERVER-33090</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;because mongos currently does not allow remote queries on shards that were removed. It is also unclear from the user point of view when is it safe to physically take out the shard that was removed and not make the change stream lose track of some of the writes.&lt;/p&gt;

&lt;p&gt;Possible fix is to make it such that removeShard and changeStream work together:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;removeShard will insert an &apos;end of life&apos; sentinel oplog entry&lt;/li&gt;
	&lt;li&gt;changeStream sees the sentinel and kills itself&lt;/li&gt;
	&lt;li&gt;removeShard will not return success until there are no more change stream cursors in the shard&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;In this scheme, the user will know that it is safe to completely decommission the shard once removeShard completes.&lt;/p&gt;</description>
                <environment></environment>
        <key id="491683">SERVER-33090</key>
            <summary>$changeStream can fail if a shard was removed</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="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="randolph@mongodb.com">Randolph Tan</reporter>
                        <labels>
                            <label>change-streams-improvements</label>
                            <label>query-44-grooming</label>
                    </labels>
                <created>Fri, 2 Feb 2018 20:48:49 +0000</created>
                <updated>Mon, 30 Jan 2023 17:11:31 +0000</updated>
                                            <version>3.6.2</version>
                                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="1822006" author="renctan" created="Fri, 2 Mar 2018 22:54:07 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=charlie.swanson&quot; class=&quot;user-hover&quot; rel=&quot;charlie.swanson&quot;&gt;charlie.swanson&lt;/a&gt; The reason why it happens occasionally is because of the timing of when mongos realize that the shard was removed. The refresh currently happens every 30 seconds and if the refresh just happened right after the shard has been removed and right before the remote command is about to be sent, you will get this error. I have attached a diff that can consistently reproduce this failure.&lt;/p&gt;</comment>
                            <comment id="1821093" author="kaloian.manassiev" created="Fri, 2 Mar 2018 14:57:02 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=renctan&quot; class=&quot;user-hover&quot; rel=&quot;renctan&quot;&gt;renctan&lt;/a&gt;, can you please elaborate here what the problem is and if appropriate put it back on the query team&apos;s backlog?&lt;/p&gt;</comment>
                            <comment id="1814352" author="ian@10gen.com" created="Fri, 23 Feb 2018 16:26:56 +0000"  >&lt;p&gt;Closing as Incomplete and moving the related Build Failure back to Open.&lt;/p&gt;</comment>
                            <comment id="1807939" author="charlie.swanson" created="Fri, 16 Feb 2018 14:06:26 +0000"  >&lt;p&gt;Ping &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=renctan&quot; class=&quot;user-hover&quot; rel=&quot;renctan&quot;&gt;renctan&lt;/a&gt;.&lt;/p&gt;</comment>
                            <comment id="1802901" author="charlie.swanson" created="Mon, 12 Feb 2018 16:09:41 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=renctan&quot; class=&quot;user-hover&quot; rel=&quot;renctan&quot;&gt;renctan&lt;/a&gt; I don&apos;t think we know enough here, and I&apos;m confusing myself trying to figure out how all this networking stuff happens.&lt;/p&gt;

&lt;p&gt;The assertion you point to indicates that the AsyncResultsMerger could not perform a getMore on an existing cursor. I don&apos;t understand why this is the case. The error message there looks like it comes from &lt;a href=&quot;https://github.com/mongodb/mongo/blob/b86dc5ef90ab546c3dd57a8a81f828422b40164a/src/mongo/s/client/sharding_network_connection_hook.cpp#L57&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt;. From what I can understand, it looks like we are prevented from opening a connection to a shard that has been removed from the cluster. This broadly would make sense, but in this case we have already opened a cursor on that shard and we would like to finish iterating it before the host is stopped.&lt;/p&gt;

&lt;p&gt;Do you have any insight into why this won&apos;t happen on every run, but only occasionally? One guess is that usually the connection stays open, but things may have been slow enough here that it got dropped before we sent the next getMore?&lt;/p&gt;</comment>
                    </comments>
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                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="508687">SERVER-33778</issuekey>
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                                        <issuelink>
            <issuekey id="971041">SERVER-44039</issuekey>
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                <attachments>
                            <attachment id="180777" name="change_stream.diff" size="1185" author="randolph@mongodb.com" created="Fri, 2 Mar 2018 22:54:59 +0000"/>
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