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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-34564] Potential oplog visibility bug replicating from a secondary</title>
                <link>https://jira.mongodb.org/browse/SERVER-34564</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;This only applies to development versions of 4.0 that allow chained replication where secondaries are running in a mode that allows readers while concurrently processing batches.&lt;/p&gt;

&lt;p&gt;There are two mechanisms that publish a new oplog visibility timestamp on secondaries during steady state replication. The first is &lt;a href=&quot;https://github.com/mongodb/mongo/blob/cf34f10d361240f4a94f5fae94cb55bc2c0acf9b/src/mongo/db/repl/sync_tail.cpp#L911-L912&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;forcefully setting the read timestamp to the end of a batch after it is applied&lt;/a&gt;. The second is a background thread that &lt;a href=&quot;https://github.com/mongodb/mongo/blob/cf34f10d361240f4a94f5fae94cb55bc2c0acf9b/src/mongo/db/storage/wiredtiger/wiredtiger_oplog_manager.cpp#L210-L230&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;queries WT for the &quot;all committed&quot; time and publishes it as the new read visibility&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;Secondary oplog application works in two phases. The first phase writes a batch of documents to the oplog asynchronously. This has the impact of causing the &quot;all committed&quot; time to jump around as opposed to only incrementing, which is what oplog visibility expects. Without a concurrent reader, this is fine, a reader can only come in when those oplog holes become plugged.&lt;/p&gt;

&lt;p&gt;As a last detail, I heard it was expected for secondary oplog readers to use the &quot;last applied&quot; time to cap oplog visibility, which would be correct AFAIK. However, if &lt;a href=&quot;https://github.com/mongodb/mongo/blob/cf34f10d361240f4a94f5fae94cb55bc2c0acf9b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp#L1565-L1567&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;_isOplogReader&lt;/tt&gt; is set&lt;/a&gt;, the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/cf34f10d361240f4a94f5fae94cb55bc2c0acf9b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp#L346-L356&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;oplog visibility timestamp gets priority over the last applied&lt;/a&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="530967">SERVER-34564</key>
            <summary>Potential oplog visibility bug replicating from a secondary</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="2" iconUrl="https://jira.mongodb.org/images/icons/priorities/critical.svg">Critical - P2</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="3">Duplicate</resolution>
                                        <assignee username="backlog-server-execution">Backlog - Storage Execution Team</assignee>
                                    <reporter username="daniel.gottlieb@mongodb.com">Daniel Gottlieb</reporter>
                        <labels>
                    </labels>
                <created>Thu, 19 Apr 2018 13:59:40 +0000</created>
                <updated>Tue, 6 Dec 2022 03:31:54 +0000</updated>
                            <resolved>Fri, 20 Apr 2018 14:07:53 +0000</resolved>
                                                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>9</watches>
                                                                                                                <comments>
                            <comment id="1868742" author="daniel.gottlieb@10gen.com" created="Thu, 19 Apr 2018 14:01:47 +0000"  >&lt;p&gt;I forgot to mention, this is only hypothesized. Our testing likely doesn&apos;t exercise this too well. After talking with people, there doesn&apos;t seem to be a mechanism that prevents this. I&apos;ll (eventually) work on making a repro unless someone beats me to it.&lt;/p&gt;</comment>
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