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    <link>https://jira.mongodb.org</link>
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    <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-54760] (4.2) Ghost timestamps can cause concurrent causal snapshot reads to not read their own writes</title>
                <link>https://jira.mongodb.org/browse/SERVER-54760</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;We recently fixed an issue in 4.2 where the &quot;all durable&quot; timestamp can go backwards. However 4.2 maintains two variables (&lt;a href=&quot;https://github.com/mongodb/mongo/blob/46fc6fb233e85538a94dda6eea91cc2ac34cee15/src/mongo/db/storage/wiredtiger/wiredtiger_oplog_manager.cpp#L215&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;1&lt;/a&gt; and &lt;a href=&quot;https://github.com/mongodb/mongo/blob/46fc6fb233e85538a94dda6eea91cc2ac34cee15/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L2042-L2046&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;2&lt;/a&gt;) that each independently prevent their readers from seeing a value going backwards.&lt;/p&gt;

&lt;p&gt;These variables do not communicate with each other and are not synchronized in any way. Thus a read from a method that looks up against one value can see an &quot;all durable&quot; TS. Following that with a read on a different method can see a different &quot;all durable&quot; smaller than the previously observed TS (due to ghost timestamp writes [1]).&lt;/p&gt;

&lt;p&gt;A decreasing all_durable can be observed when a &lt;tt&gt;startTransaction&lt;/tt&gt; request has an &lt;tt&gt;afterClusterTime&lt;/tt&gt; and &lt;tt&gt;level: &quot;snapshot&quot;&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;The &lt;tt&gt;afterClusterTime&lt;/tt&gt; first &lt;a href=&quot;https://github.com/mongodb/mongo/blob/16ce9754595d39cd093e2a6adc78366db1d46f2b/src/mongo/db/repl/replication_coordinator_impl.cpp#L1506&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;waits for all earlier writes to complete&lt;/a&gt;. This logic  compares against &quot;variable 1&quot;.&lt;/p&gt;

&lt;p&gt;Then we open a WT snapshot using the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/16ce9754595d39cd093e2a6adc78366db1d46f2b/src/mongo/db/transaction_participant.cpp#L629-L630&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&quot;all durable&quot; read source&lt;/a&gt;. This code path compares against &quot;variable 2&quot;, breaking the guarantee necessary to read at or after the &lt;tt&gt;afterClusterTime&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;[1] WT&apos;s all_durable only goes backwards on the primary due to &quot;ghost timestamp&quot; writes. On 4.2, this is primarily from a &lt;a href=&quot;https://github.com/mongodb/mongo/blob/46fc6fb233e85538a94dda6eea91cc2ac34cee15/src/mongo/db/catalog/index_catalog_entry_impl.cpp#L333-L339&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;multikey write inside of a multi-statement transaction&lt;/a&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1632443">SERVER-54760</key>
            <summary>(4.2) Ghost timestamps can cause concurrent causal snapshot reads to not read their own 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>
                    </labels>
                <created>Wed, 24 Feb 2021 18:24:07 +0000</created>
                <updated>Sun, 29 Oct 2023 21:57:05 +0000</updated>
                            <resolved>Thu, 25 Feb 2021 01:31:34 +0000</resolved>
                                                    <fixVersion>4.2.13</fixVersion>
                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="3634162" author="xgen-internal-githook" created="Thu, 25 Feb 2021 01:27:32 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Daniel Gottlieb&apos;, &apos;email&apos;: &apos;daniel.gottlieb@mongodb.com&apos;, &apos;username&apos;: &apos;dgottlieb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-54760&quot; title=&quot;(4.2) Ghost timestamps can cause concurrent causal snapshot reads to not read their own writes&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-54760&quot;&gt;&lt;del&gt;SERVER-54760&lt;/del&gt;&lt;/a&gt;: Guarantee causal relationship between WTOplogManager::waitForAllEarlierOplogWritesToBeVisible and `kAllDurableSnapshot` read source.&lt;br/&gt;
Branch: v4.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/02087dc8d18138e9f9d555da21f6105046573ed1&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/02087dc8d18138e9f9d555da21f6105046573ed1&lt;/a&gt;&lt;/p&gt;</comment>
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