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    <title>MongoDB Jira</title>
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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-66335] Make change streams robust to future no-op oplog collisions</title>
                <link>https://jira.mongodb.org/browse/SERVER-66335</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;After PM-1950, change streams consumes various no-op oplog messages to detect the completion of certain events, such as &lt;tt&gt;shardCollection&lt;/tt&gt; or &lt;tt&gt;refineCollectionShardKey&lt;/tt&gt;. These are written as command-style objects in the &lt;tt&gt;o2&lt;/tt&gt; field of the oplog entry, e.g.&lt;/p&gt;

&lt;p/&gt;
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					&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;{ ... o2: {shardCollection: &quot;db.coll&quot;, shardKey: {a: 1}}}&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;The change stream filter matches these events by checking for the existence of the command name in the &lt;tt&gt;o2&lt;/tt&gt; field. This means that, if future authors of no-op oplog entries include fields with these command names in their &lt;tt&gt;o2&lt;/tt&gt; entry, they will inadvertently cause change streams to erroneously match these no-op events.&lt;/p&gt;

&lt;p&gt;We should come up with a more robust way of preventing this, perhaps by adding a default-false parameter to the no-op oplog API which specifies whether or not the no-op message is intended for consumption by change streams, and if not &lt;tt&gt;tasserts&lt;/tt&gt; that the &lt;tt&gt;o2&lt;/tt&gt; message does not collide with any fields that are reserved by change streams.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2043108">SERVER-66335</key>
            <summary>Make change streams robust to future no-op oplog collisions</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="bernard.gorman@mongodb.com">Bernard Gorman</reporter>
                        <labels>
                    </labels>
                <created>Tue, 10 May 2022 09:07:21 +0000</created>
                <updated>Tue, 6 Dec 2022 00:07:48 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="4537695" author="arun.banala" created="Tue, 10 May 2022 11:10:40 +0000"  >&lt;p&gt;One way to achieve this is to add an Enum describing all the no-op oplog entries, and add a debug assertion in &lt;a href=&quot;https://github.com/mongodb/mongo/blob/c6e976d9f67a1b50826784e7a0eda9bfe212dd73/src/mongo/db/repl/oplog.cpp#L399-L409&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this&lt;/a&gt; function to validate that every no-op oplog entries belongs to the enum.&lt;/p&gt;</comment>
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