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    <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-34172] Turn primary index build ghost writes into noop oplog writes.</title>
                <link>https://jira.mongodb.org/browse/SERVER-34172</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;There are two metadata writes associated with an index build, the start and the completion (or failure). Primaries &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/commands/create_indexes.cpp#L402-L407&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;naturally timestamp a successful completion as that is in the same transaction that writes an oplog entry&lt;/a&gt;. On secondaries, the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/repl/oplog.cpp#L1587-L1588&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;beginning of an index build is naturally timestamped as that is associated with processing an oplog entry&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;For primaries, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/catalog/index_create_impl.cpp#L320-L324&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;beginning&lt;/a&gt; and &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/catalog/index_create_impl.cpp#L189-L193&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;failing&lt;/a&gt; an index build need to also be timestamped. This is currently accomplished by looking at the logical clock and assigning that value as the timestamp. This has the race condition that the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L1072-L1084&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;stable timestamp&lt;/a&gt; may race ahead after reading the logical clock, but before setting the timestamp on the index metadata write.&lt;/p&gt;

&lt;p&gt;Instead, if the write goes through the oplog as a no-op entry, the write will be timestamped without the possibility of a race (the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/repl/oplog.cpp#L156-L165&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;logical clock is read and the timestamp is set under a mutex&lt;/a&gt;. Why this prevents the stable timestamp from racing ahead is beyond what I&apos;d like to explain here, but am happy to in person).&lt;/p&gt;

&lt;p&gt;Note, it is legal for secondaries to look at the logical clock and use that to timestamp the metadata update on &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e6503239af/src/mongo/db/catalog/index_create_impl.cpp#L561-L565&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;index completion&lt;/a&gt;. The index build is either in the foreground and no other operations are being processed, or the index build is in the background and acquiring a lock to perform the write prevents the replication from processing batches (via the Parallel Batch/aka peanut butter, lock).&lt;/p&gt;</description>
                <environment></environment>
        <key id="518219">SERVER-34172</key>
            <summary>Turn primary index build ghost writes into noop oplog writes.</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="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="13201">Fixed</resolution>
                                        <assignee username="daniel.gottlieb@mongodb.com">Daniel Gottlieb</assignee>
                                    <reporter username="daniel.gottlieb@mongodb.com">Daniel Gottlieb</reporter>
                        <labels>
                            <label>rollback-non-functional</label>
                    </labels>
                <created>Wed, 28 Mar 2018 19:12:17 +0000</created>
                <updated>Sun, 29 Oct 2023 22:33:16 +0000</updated>
                            <resolved>Sat, 31 Mar 2018 02:40:37 +0000</resolved>
                                                    <fixVersion>3.7.4</fixVersion>
                                    <component>Replication</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="1850874" author="xgen-internal-githook" created="Sat, 31 Mar 2018 02:40:07 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;email&apos;: &apos;daniel.gottlieb@mongodb.com&apos;, &apos;name&apos;: &apos;Daniel Gottlieb&apos;, &apos;username&apos;: &apos;dgottlieb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-34172&quot; title=&quot;Turn primary index build ghost writes into noop oplog writes.&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-34172&quot;&gt;&lt;del&gt;SERVER-34172&lt;/del&gt;&lt;/a&gt;: Use noop writes to timestamp index completion/failure on primaries.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/48c552d8b037fe3eb821290c3f924d413e44e7db&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/48c552d8b037fe3eb821290c3f924d413e44e7db&lt;/a&gt;&lt;/p&gt;</comment>
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                                        <issuelink>
            <issuekey id="1365487">SERVER-48465</issuekey>
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            <issuekey id="893076">SERVER-42799</issuekey>
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